Dynamic linq predicates

This post is actually a repost of this great article A universal PredicateBuilder that I found very useful so I decided to share it here.

Sometimes I need to build dynamically the where part of a linq query and, unfortunately, there is no straight forward, out of the box solution inside the .NET framework for doing this.

For example, imagine that you have a list of houses that are for rent, and the user can filter this list depending on: country, region, area, price range, facilities and so on. So your linq query might look something like this:

using (var context = new DatabaseEntities())
{
	var list = context.Rentals.Where(item => 
				(string.IsNullOrEmpty(countryFilterValue) || item.Country == countryFilterValue) &&
				(string.IsNullOrEmpty(regionFilterValue) || item.Region == regionFilterValue) &&
				(string.IsNullOrEmpty(areaFilterValue) || item.Area == areaFilterValue)).ToList();
}

If you’re using linq to entities to query your DB, then the SQL that will be generated will most likely be inefficient. In our example, the generated SQL for the first condition will be more or less like this:

exec sp_executesql N'SELECT 
*
FROM [dbo].[Rentals] AS [Extent1]
WHERE  (@p__linq__0 IS NULL) OR (( CAST(LEN(@p__linq__0) AS int)) = 0) OR ([Extent1].[Country] = @p__linq__1)',N'@p__linq__0 nvarchar(4000),@p__linq__1 varchar(8000)',@p__linq__0=N'',@p__linq__1=''

In the where part you will have a lot of similar conditions (one for each filer). There could be a lot of filters but almost always the user will not complete all of them. So you will end up with an inefficient query that checks if an empty parameter is null or empty.

The solution is to build dynamically the where part of the linq query. Fortunately this is possible using the code that you can find in this post: A universal PredicateBuilder

Below I did a copy/paste of the original code so I can have it forever and ever with me if I need it ūüôā

/// <summary>
/// Enables the efficient, dynamic composition of query predicates.
/// </summary>
public static class PredicateBuilder
{
    /// <summary>
    /// Creates a predicate that evaluates to true.
    /// </summary>
    public static Expression<Func<T, bool>> True<T>() { return param => true; }
 
    /// <summary>
    /// Creates a predicate that evaluates to false.
    /// </summary>
    public static Expression<Func<T, bool>> False<T>() { return param => false; }
 
    /// <summary>
    /// Creates a predicate expression from the specified lambda expression.
    /// </summary>
    public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }
 
    /// <summary>
    /// Combines the first predicate with the second using the logical "and".
    /// </summary>
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.AndAlso);
    }
 
    /// <summary>
    /// Combines the first predicate with the second using the logical "or".
    /// </summary>
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.OrElse);
    }
 
    /// <summary>
    /// Negates the predicate.
    /// </summary>
    public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
    {
        var negated = Expression.Not(expression.Body);
        return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
    }
 
    /// <summary>
    /// Combines the first expression with the second using the specified merge function.
    /// </summary>
    static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        // zip parameters (map from parameters of second to parameters of first)
        var map = first.Parameters
            .Select((f, i) => new { f, s = second.Parameters[i] })
            .ToDictionary(p => p.s, p => p.f);
 
        // replace parameters in the second lambda expression with the parameters in the first
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
 
        // create a merged lambda expression with parameters from the first expression
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }
 
    class ParameterRebinder : ExpressionVisitor
    {
        readonly Dictionary<ParameterExpression, ParameterExpression> map;
 
        ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }
 
        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }
 
        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;
 
            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }
 
            return base.VisitParameter(p);
        }
    }
}

Now the where part can be created dynamically, including only the filters for which the user provided a value:

using (var context = new DatabaseEntities ())
{
	var predicate = PredicateBuilder.Create<Rentals>(item => true);

	if (!string.IsNullOrWhiteSpace(countryFilterValue))
		predicate = predicate.And(item => item.Country == countryFilterValue);
	if (!string.IsNullOrWhiteSpace(regionFilterValue))
		predicate = predicate.And(item => item.Region == regionFilterValue);
	if (!string.IsNullOrWhiteSpace(areaFilterValue))
		predicate = predicate.And(item => item.Area == areaFilterValue);

	var list = context.Rentals.Where(predicate).ToList();
}
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Manipulating excel files in C#

C# Excel

Thanks to stackoverflow I found out that Microsoft.Office.Interop.Excel is not supported in server-side code such as Microsoft Active Server Pages (ASP), ASP.NET, DCOM, or a Windows NT service.

As an alternative, you can use a free SDK from Microsoft – Open XML SDK 2.5 for Office. It provides strongly-typed classes that can be used to manipulate documents that adhere to the Office Open XML File Formats Specification.

How to use it:

  1. Download and install the SDK.
  2. In Solution Explorer, right-click on References ¬Ľ Add Reference. On the .NET tab select WindowsBase and DocumentFormat.OpenXml.
    • If DocumentFormat.OpenXml is not available in the list, copy the dll file from GAC.
    • For .NET framework version 4.0 it is located under the folder %windir%\Microsoft.NET\assembly\GAC_MSIL\DocumentFormat.OpenXml
    • Copy DocumentFormat.OpenXml.dll into your solution and add a reference to it.
  3. As an example I have created a simple method in C#, that you can use inside an asp.net web site to create an excel file in memory and then download it. The excel file created with this sample method contains one column with a header row and a content row. The header has a dark blue background and the text is white and appears in bold.
    Excel Example
public void DownloadExcelFile()
{
	var memoryStream = new MemoryStream();

	//Create a spreadsheet document in memory (by default the type is xlsx)
	using (var spreadsheetDocument = SpreadsheetDocument.Create(memoryStream, SpreadsheetDocumentType.Workbook))
	{
		//Add a WorkbookPart to the document. (The workbook element is the top level element. It contains elements and attributes that encompass the data content of the workbook.)
		var workbookpart = spreadsheetDocument.AddWorkbookPart();
		workbookpart.Workbook = new Workbook();
		workbookpart.Workbook.AppendChild(new FileVersion {ApplicationName = "Microsoft Office Excel"});

		//Add a WorksheetPart to the WorkbookPart.
		var worksheetPart = workbookpart.AddNewPart<WorksheetPart>();               
		worksheetPart.Worksheet = new Worksheet();

		//Set a custom width for the first column
		var columns = new Columns();
		columns.AppendChild(new Column { Min = 1, Max = 1, CustomWidth = true, Width = 15 });
		worksheetPart.Worksheet.AppendChild(columns);

		//Important! We must associate the sheet data after the columns otherwise the file will fail to open.
		var sheetData = new SheetData();
		worksheetPart.Worksheet.AppendChild(sheetData);

		//Add styles to format the header cells
		var workbookStylesPart = workbookpart.AddNewPart<WorkbookStylesPart>();
		workbookStylesPart.Stylesheet = new Stylesheet(
			new Fonts(
				new Font(), // Index 0 - default font
				new Font(   // Index 1 - The bold white font
					new Bold(),
					new Color {Rgb = new HexBinaryValue {Value = "FFFFFF"}}
					)
				),
			new Fills(
				new Fill(new PatternFill {PatternType = PatternValues.None}),                                                                            // Index 0 - default fill (should be always none)
				new Fill(new PatternFill { PatternType = PatternValues.Gray125 }),                                                                       // Index 1 - it's required and should always be gray 125
				new Fill(new PatternFill(new ForegroundColor { Rgb = new HexBinaryValue { Value = "000080" } }) { PatternType = PatternValues.Solid })   // Index 2 - dark blue fill
				),
			new Borders(
				new Border( // Index 0 - The default border
					new LeftBorder(),
					new RightBorder(),
					new TopBorder(),
					new BottomBorder(),
					new DiagonalBorder())
				),
			new CellFormats(
				new CellFormat {FontId = 0, FillId = 0, BorderId = 0}, // Index 0 - default cell style
				new CellFormat {FontId = 1, FillId = 2, BorderId = 0, ApplyFont = true, ApplyFill = true, } // Index 1 - Blue background cell with white bold text
				)
			);               

		//Add Sheets to the Workbook. (Sheets: represents the collection of worksheets in the workbook. The sheets are the central structure within a workbook, and contain the text, numbers, dates, formulas, and other elements of a workbook.)
		var sheets = spreadsheetDocument.WorkbookPart.Workbook.AppendChild(new Sheets());

		//Append a new worksheet and associate it with the workbook.
		sheets.AppendChild(new Sheet
			{
				Id = spreadsheetDocument.WorkbookPart.GetIdOfPart(worksheetPart),
				SheetId = 1,
				Name = "Sheet 1"
			});

		//Populate the sheet data
		var headerRow = sheetData.AppendChild(new Row());
		headerRow.AppendChild(new Cell
			{
				CellValue = new CellValue("Header text"),
				DataType = CellValues.String,
				StyleIndex = 1
			});

		var contentRow = sheetData.AppendChild(new Row());
		contentRow.AppendChild(new Cell
		{
			CellValue = new CellValue("Value"),
			DataType = CellValues.String
		});                               

		workbookpart.Workbook.Save();
	}
	
	Response.Clear();
	Response.CacheControl = "Private";
	Response.Cache.SetExpires(DateTime.Now.AddSeconds(30));
	Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
	Response.AddHeader("Content-Type", "application/octet-stream");
	Response.AppendHeader("content-disposition", string.Format("attachment; filename=\"test.xlsx\"; size={0}", memoryStream.Length));
	Response.Flush();            
	Response.BinaryWrite(memoryStream.ToArray());

	Response.Flush();
	Response.End(); 
}

Introduction to MVC

Contents:

  1. Basic concepts
  2. Advantages of MVC
  3. From HTTP to Routes
  4. A Hello World application
  5. Adding a master page
  6. How to create a ‘Contact us’ form

1. Basic concepts

The Microsoft ASP.NET Model-View-Controller (MVC) is a web development framework that comes as an alternative to the standard ASP.NET Web Forms model. This alternative is based on a common software architecture design pattern called MVC.

A design pattern is a general reusable solution to a commonly occurring problem and can be seen as a best practice in the field.

The main purpose of MVC is to separate an application into 3 distinct parts so that the complexity of each part is decreased. This idea is very similar to the divite et impera concept. In addition to this separation MVC defines also the interactions between these 3 components:

  • Model – contains the data and the business logic,
  • View – defines the user interface and displays the model data,
  • Controller – connects the view and the model and controls the input handling.

mvc In simple words, the application flow can be summarized like this: when an event occurs (e.g. a user requests a page), the controller is invoked and is its responsibility to understanding what kind of event occurred, what action to do (e.g. update some data in a model) and what view to render as a response to that event. Except for the very simple cases, the view needs to receive a model as an input in order to know what content to render. If this is the case, the controller is also responsible for instantiating the correct model and passing it to the view.

This separation into components is called in the literature separation of concerns and it helps to minimize the impact a modification in one component has on the other components. For example, if we need to change the way the data is displayed, it should be enough to change the view without having to modify the business logic inside the model.

2. Advantages of MVC

The main advantage that comes from using the MVC pattern is that the application is divided into components avoiding the mixing of the code into a single big monolithic application. Apart from this we have another set of advantages that come rather from the way the new Microsoft MVC framework is written than from the pure concept of MVC. After all MVC is only an architectural pattern, a way of structuring an application. Nothing more and nothing less!

As everybody already knows, with the Web Forms, Microsoft tried to wrap the HTML controls into .net objects that are able to automatically remember between post backs the values of their properties by making use of the view state. This has 2 negative impacts:

  1. the view state is ugly and unacceptably big for medium to large pages and
  2. the HTML that the .net controls render is usually hard to control and change. For example the id of the elements is unnecessarily long. This was fixed with the introduction of the static client id mode but still the name attribute of the controls keeps the old format.

With the MVC framework the view state is removed and we have complete control on the rendered HTML.

Another advantage that comes from the way in which the MVC web development framework is implemented and not from the MVC pattern itself is that the complicated Page Life Cycle set of events was removed and we now have a simplified execution pipeline. The main purpose of the Page Life Cycle was to hide the way in which the communication between the client and the server happens and automatically bind user interface events, like the click of a button, with server side methods that would handle that event, creating the impression that the web interaction is event-driven and similar to the way Windows applications work. Though this was a nice idea in the beginning, time proved that it adds a lot of complexity:

  • understanding the Page Life Cycle is not a trivial job, and
  • it tends to make the pages monolithic, for example, in order to execute a button’s click event handler first the page initialization and loading events are executed, making all those event methods tightly coupled.

So, with no .net wrapper controls, no view state and no page life cycle the MVC framework doesn’t try to hide the way HTTP anf HTML work and offers a more elegant and simple development framework. Now, since we no longer have the abstractions provided by the Web Forms it’s better to get started by making sure all the basic concepts about how the web actually works are clear.

3. From HTTP to Routes

The Hypertext Transfer Protocol (HTTP) is an application level response-request protocol that defines the set of rules used by 2 participants (a client and a server) in order to communicate. Or, a more memorable definition taken from this nice book RESTful Web Services: “HTTP is the one thing that all ‘animals’ on the programmable web have in common.”

When a page is requested, the browser sends a request to the web server. The first 2 lines of a request look like this:Fiddler
Note: For inspecting the request text I used Fiddler which it’s a very useful tool that can help you see all the HTTP(S) traffic between your computer and the Internet.

The first word in the request is the HTTP method (in the example: GET). It is followed by the Uniform Resource Identifier (URI) of the resource to be retrieved (http://wordpress.com/) and the HTTP version to be used for processing the command (HTTP/1.1). The second line (Host: wordpress.com) identifies the name of the website and is used by the servers that host multiple websites under a single IP address to understand what is the website the page belongs to.

So, a request message from a client to a server includes (within the first line of that message) the method to be applied to the resource, the identifier of the resource, and the protocol version to use.

The HTTP methods, also known as HTTP verbs, are tokens that indicate the action to be performed on a resource and are very important for us because we will use them together with the URIs to map an incoming browser request to a particular MVC controller action.

Below is a list of possible methods, as defined by the World Wide Web Consortium (W3C):

  1. OPTIONS – represents a request for information about the communication options available on the request/response chain identified by the request URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval. Responses to this method are not cacheable. Simply said, you can use OPTIONS to check if a server allows a particular command and avoid wasting network bandwidth trying to send an unsupported request.
  2. GET – means: retrieve whatever information is identified by the request URI.
  3. HEAD – is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method can be used for testing hypertext links for validity, accessibility, and recent modification. HEAD is typically used to verify that a resource didn’t change since the browser cached it.
  4. POST – requests that the origin server accepts the entity enclosed in the request as a new subordinate of the resource identified by the request URI. The actual function performed by the POST method is determined by the server and is usually dependent on the request URI. POST is designed to allow a uniform method to cover the following functions:
    • Annotation of existing resources;
    • Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;
    • Providing a block of data, such as the result of submitting a form, to a data-handling process;
    • Extending a database through an append operation.
  5. PUT – requests that the enclosed entity be stored under the supplied request URI. If the request URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the request URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. So, simply said, PUT allows a client to add a resource to the server at the specified URI. If the user has permissions, the server creates the file specified in the URI and copies the request body to the newly created file.
  6. DELETE – requests that the origin server deletes the resource identified by the request URI.
  7. TRACE – is used for testing or diagnostics information and allows a client to see what is being received at the other end of the request chain.
  8. CONNECT – is reserved to be used with a proxy that can dynamically switch to being a tunnel (e.g. SSL tunneling).

Notes:
– The HTTP methods are case-sensitive.
– As specified by the W3C the methods GET and HEAD MUST be supported by all general-purpose servers. All other methods are OPTIONAL; however, if they are implemented, they MUST be implemented with the semantics previously explained.

Now, after so much talking is time to move on and do something practical, and that obviously is a Hello World application.

4. A Hello World application

Let’s start with the classic ‘File’ -> ‘New’ -> ‘Project’ but this time we will create an ‘ASP.NET MVC 4 Web Application’ :D.
Create a new MVC project
The next step is to select a project template. Please choose the empty project template and then make sure that the selected view engine is Razor.
Select the project templateA view engine is a pluggable module that provides a way to insert dynamic blocks into a template and is able to render that template to HTML or whatever content type it is designed to emit. There are 2 available view engines:

  1. one is the old ASPX that uses code blocks separated by <% tags %>,
  2. the other one is Razor that uses the character @ in order to indicate the beginning of a code block and doesn’t require the explicit closing of the code block.

Razor is not a new language, and it’s actually very easy and intuitive, it helps to create a template that contains static HTML and C# code blocks that can render dynamic content. So, we will use Razor for creating the Views.

Looking at the structure of our new empty project we see that we have a dedicated folder for the controllers, one for the models and one for the views plus another folder called App_Start that contains a RouteConfig.cs file. This file is used to define the allowed URLs and the association between an URL and the controller that will process that request.
Defining the routesEach route must:

  • have an unique name, otherwise we will receive an error
  • and must define the structure of the allowed URLs. In the example, the URLs are composed by 3 parts: the controller name {controller}, the name of the method inside the controller that will handle the request {action} and an identifier {id}.

The last line defines the default values to be used in case a part of the URL is missing. For example, if the root of the web site is requested (http://site-name/) the default controller that will be called is ‘Home’ and the default action is ‘Index’. The last parameter is optional so it will be empty.

Now, let’s add a controller called ‘Home’ in order to see how the routing works. In Solution Explorer right click on the Controller folder -> Add -> Controller. Name the new controller ‘HomeController’. Note that the naming convention for controllers is: Name + Controller and using another name, for example only ‘Home’ will make the controller unreachable.
Add a new controllerLet’s replace the default implementation of the index method with the following:

        public string Index()
        {
            return "Hello world!";
        }

Now, if we run the site, we will see the text ‘Hello world!’. If we want to use also the {id} part of the URL we can change the Index action like this:

        public string Index(string id)
        {
            return string.IsNullOrEmpty(id) ? "Hello world!" : string.Format("Hello world! Your id is: {0}.", id);
        }

So calling http://site-name/home/index/Hey we will see the text: ‘Hello world! Your id is: Hey.’. Note that here we have another naming convention, that is, the name of the input parameter matches the 3rd part of our route {id}, if the name of the parameter would have been different the displayed text would be ‘Hello world!’.

Next thing to do is to add a view file for returning HTML content rather than a string. A fast way of doing this is to right click on the action name and select ‘Add View’:
Add a new view
Let’s replace the default content of the view with the following one:

@{
    ViewBag.Title = "Home";
}
<h2>Hello from the view.</h2>
@if(!string.IsNullOrEmpty(ViewBag.Id))
{
    <text>Your id is: @ViewBag.Id.</text>
}

<i>@ViewBag.Message</i>

and update the Index action method like this:

        public ActionResult Index(string id)
        {
            ViewData["Message"] = "This is a string created in the controller";
            ViewBag.Id = id;
            return View();
        }

The first thing to notice here is the ViewData which is a dictionary that we can use to pass data from the controller to the view. The ViewBag is a dynamic type created as a wrapper over the ViewData that allows us to use dynamic properties instead of strings for accessing the objects in the ViewData. As you can see in the last line of the view, we can access the value in ViewData[“Message”] by calling @ViewBag.Message.

The last line in the action method tells the rendering engine to return the view associated with this action. To find the correct view, another naming convention is applied. The implicit view should be located under the Views folder in a sub-folder with the same name as the controller (‘Home’) and the name of the file should match the name of the action method (‘Index.cshtml’). Note that it is possible to render a different view, for this we have to pass the name of the view as a parameter to the View method.

5. Adding a master page

Our next goal is to define a common layout for the entire site. To do this we have to add a file called _ViewStart.cshtml (or _ViewStart.vbhtml for VB) under the Views folder of the project.

@{
    Layout = "~/Views/Shared/_Layout.cshtml";
}

_ViewStart.cshtml indicates what file to use as a master page. It is recommended that we store the common layout files under the folder ‘~/Views/Shared’ and that we prefix the name of those files with an underscore, like _Layout.cshtml so, let’s stick to the recommendations and add the folder Shared and the file _Layout.cshtml with the following content:

<!DOCTYPE html>
<html>
    <head>
       <title>MVC - @ViewBag.Title</title>
        <style type="text/css">            
            body { margin:0; padding:0; }
            #header_container { background: black; color: white; height:60px; left:0; position:fixed; width:100%; top:0; }
            #header { line-height:60px; margin:0 auto; padding-left: 50px; font-weight: bold;font-size: 18pt;}
            #container { margin:0 auto; overflow:auto; padding:80px 0; width:940px; }
            .control-label{ width: 120px;display: inline-block;}
        </style>
    </head>
    <body>       
        <div id="header_container">
            <div id="header">
                <i>=(^.^)=</i>
            </div>
        </div>              
        <div id="container">
            <div id="content">
                @RenderBody()    
            </div>
        </div>
        @RenderSection("scripts", required: false)
    </body>
</html>

The code it’s pretty straight forward:
– @RenderBody() will render the content of a view that is not within any named sections,
– @RenderSection(“scripts”, required: false) – if the view contains a section called scripts it renders the content of that section.

To define the scripts section inside Index.cshtml just add at the end of the file the following:

@section scripts{
<script type="text/javascript">
    document.getElementById("header").innerHTML += ' MVC'
</script>
}

Now, that we have such a nice layout plus some text inserted via JavaScript, let’s add a ‘Contact us’ page that will allow our visitors to praise our great design. We will save all the positive comments in the DB so we can show to the managers how great we are!

6. How to create a ‘Contact us’ form

Contact Us Form
First thing first, let’s create a DB and use Entity Framework to access it. Our DB will have these 2 tables:
The DB structure I named the new DB [MvcDemo]. Below is the script for creating the tables:

USE [MvcDemo]
GO
/****** Object:  Table [dbo].[ContactReasons]    Script Date: 03/12/2013 21:51:40 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[ContactReasons](
	[ContactReasonId] [int] IDENTITY(1,1) NOT NULL,
	[ContactReasonText] [nvarchar](100) NOT NULL,
 CONSTRAINT [PK_ContactReasons] PRIMARY KEY CLUSTERED 
(
	[ContactReasonId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object:  Table [dbo].[Messages]    Script Date: 03/12/2013 21:51:40 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Messages](
	[MessageId] [int] IDENTITY(1,1) NOT NULL,
	[Name] [nvarchar](50) NOT NULL,
	[Email] [nvarchar](50) NOT NULL,
	[ContactReasonId] [int] NULL,
	[Subject] [nvarchar](100) NOT NULL,
	[Message] [text] NOT NULL,
 CONSTRAINT [PK_Messages] PRIMARY KEY CLUSTERED 
(
	[MessageId] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
/****** Object:  ForeignKey [FK_Messages_ContactReasons]    Script Date: 03/12/2013 21:51:40 ******/
ALTER TABLE [dbo].[Messages]  WITH CHECK ADD  CONSTRAINT [FK_Messages_ContactReasons] FOREIGN KEY([ContactReasonId])
REFERENCES [dbo].[ContactReasons] ([ContactReasonId])
GO
ALTER TABLE [dbo].[Messages] CHECK CONSTRAINT [FK_Messages_ContactReasons]
GO

USE [MvcDemo];
SET NOCOUNT ON;
SET XACT_ABORT ON;
GO
SET IDENTITY_INSERT [dbo].[ContactReasons] ON;
BEGIN TRANSACTION;
INSERT INTO [dbo].[ContactReasons]([ContactReasonId], [ContactReasonText])
SELECT 1, N'Positive remark' UNION ALL
SELECT 2, N'Negative remark'
COMMIT;
RAISERROR (N'[dbo].[ContactReasons]: Insert Batch: 1.....Done!', 10, 1) WITH NOWAIT;
GO
SET IDENTITY_INSERT [dbo].[ContactReasons] OFF;
GO

We will use Entity Framework to connect to the DB, so let’s create a new EF file ‘MvcDemoDb.edmx’ under the folder ‘~/Code/DataAccess’. This is how the solution looks now:
Project structure after adding EFNow, let’s add a new action method in the Home controller called ContactUs and a new view for this action. This time, when creating the view, tick the ‘Create a strongly-typed view’ check-box and enter as the model class: HelloWorld.Code.DataAccess.Message.
Create a contact us viewLet’s add inside the view the code below:

@model HelloWorld.Code.DataAccess.Message

@{
    ViewBag.Title = "Contact Us";
}

<h2>Contact Us</h2>
<hr/>
@using (Html.BeginForm("ContactUs", "Home", FormMethod.Post))
{
    @Html.LabelFor(model => model.Name, new { @class = "control-label" })
    @Html.TextBoxFor(model => model.Name)
    <br/>
    @Html.LabelFor(model => model.Email, new { @class = "control-label" })
    @Html.TextBoxFor(model => model.Email)
    <br/>
    @Html.LabelFor(model => model.ContactReasonId, new { @class = "control-label" })
    @Html.DropDownListFor(model => model.ContactReasonId, new List<SelectListItem>())
    <br/>
    @Html.LabelFor(model => model.Subject, new { @class = "control-label" })
    @Html.TextBoxFor(model => model.Subject)
    <br/>
    @Html.LabelFor(model => model.Message1, new { @class = "control-label" })
    @Html.TextBoxFor(model => model.Message1)
    <br/>
    <input type="submit" name="Save" value="Save" />
}

In the above code, the syntax Html.BeginForm is used to create a HTML form element. Here Html is a property of the view and it’s an instance of the HtmlHelper class. The HtmlHelper class provides methods that can be used to generate HTML elements programatically. All the methods of the HtmlHelper class generate HTML and return the result as a string. There are a lot of extension methods for the HtmlHelper class and, in the next posts, I will show how we can create our own custom extensions. So @Html.LabelFor(model => model.Name, new { @class = “control-label” }) will create a label element for the property Name of the model. With new { @class = “control-label” } we create a dictionary object that contains the HTML attributes to set to the element. In our example the rendered HTML will be:

 
<label class="control-label" for="Name">Name</label>

Try to enter some data and press the save button. You will notice that the values entered are not remembered after the post back. This is the expected behavior since we are not using the view state any more. Let’s have a look at the request with Fiddler and see what’s sent to the server.
Inspect the post data with FiddlerSo we have a POST to /Home/ContactUs that results in the action method ContactUs being called. In MVC we have a set of attributes that we can use to restrict what HTTP verbs an action method will handle. We can use [HttpGet] and [HttpPost] to indicate that an action method will be called only for GET/POST requests. Knowing this, we can modify our controller class and add a new custom action that will handle only the POST method, which, in our case, means that the user pressed the save button:

[HttpGet]
public ActionResult ContactUs()
{
	return View(new Code.DataAccess.Message());
}

[HttpPost]
public ActionResult ContactUs(Code.DataAccess.Message message)
{
	return View(message);
}

The new ContactUs method receives a message object that is filled automatically with the data the user inserted. This happens because we used in the view the HtmlHelper method @Html.TextBoxFor that created form elements with the same name as the model properties and now the MVC framework can automatically map the HTML form elements with the model properties based on the name. When rendering the response we will pass this message object back to the View and now we can notice that the data the user entered remains filled also after a post back.

The next thing that we can improve is the text that appears in the labels. For example we want to display ‘Contact reason’ instead of ‘ContactReasonId’. There is a little trick for doing this and it is called the MetadataTypeAttribute. We are going to add in our Models folder a new file and we will extent here the class HelloWorld.Code.DataAccess.Message. We can do this because the Message class is partial which means that we can split the definition of the class in more files. Please make sure that the class name and the namespace name are identical to the one generated by the entity framework model, otherwise you will be creating a new class instead of adding new elements to the existing partial class.

using System.ComponentModel.DataAnnotations;

namespace HelloWorld.Code.DataAccess
{
    [MetadataType(typeof(MessageMetaData))]
    public partial class Message
    {
    }

    public class MessageMetaData
    {
        [Display(Name = "E-mail")]
        public string Email { get; set; }

        [Display(Name = "Contact reason")]
        public int ContactReasonId { get; set; }

        [Display(Name = "Message")]
        public int Message1 { get; set; }
    }
}

The MessageMetaData defines additional attributes for our Message class. This type of helper class is called a buddy class. The display attribute sets the UI text to associate to a property and needs to be declared in a buddy class because, in this way, we don’t have to change the file generated by the EF that will be overridden the next time we will change the EF edmx file. Note that the properties defined in the MessageMetaData class are exactly the same as the ones defined in the Message class that was generated by the entity framework, otherwise the mapping would fail. If we see now the contact us page we can notice that the label text is changed. Next, we will populate the contact reason drop-down. First we define the collection in the model:

public IEnumerable<SelectListItem> ContactReasonsList
{
	get
	{
		using (var context = new MvcDemoEntities())
		{
			return (new[]
					{
						new SelectListItem
							{
								Selected = (ContactReasonId == 0),
								Text = "Select...",
								Value = string.Empty
							}
					}).Union(context.ContactReasons.
									 Select(c => new SelectListItem
									 {
										 Selected = (c.ContactReasonId == ContactReasonId),
										 Text = c.ContactReasonText,
										 Value = c.ContactReasonId.ToString(CultureInfo.InvariantCulture)
									 }));
		}                
	}
}

and then we use it in the view:

@Html.DropDownListFor(model => model.ContactReasonId, Model.ContactReasonsList)

The code above speaks for itself BUT it has a big problem: each time the view is rendered a call to the DB is made to retrieve the contact reasons. So we need to implement a basic caching mechanism. I’ll just put the code here, if you need more details check this post where you can find some explanations.

public static class CacheManager
{
	/// <summary>
	/// Get the list of contact reasons
	/// </summary>
	/// <param name="getFromCache"></param>
	/// <returns></returns>
	public static List<ContactReason> GetContactReasons(bool getFromCache)
	{
		return (List<ContactReason>)GetFromCache("ContactReasons", getFromCache, delegate
		{
			using (var context = new MvcDemoEntities())
			{
				return context.ContactReasons.OrderBy(c => c.ContactReasonText).ToList();
			}
		});
	}

	#region Private methods

	/// <summary>
	/// Helper method for adding/retrieving a value from/to the cache.
	/// </summary>
	/// <param name="cacheKey">The key of the cached item</param>
	/// <param name="f">If the key is not found in the cache execute the f() function to retrieve the object</param>
	/// <returns></returns>
	private static object GetFromCache(string cacheKey, Func<object> f)
	{
		return GetFromCache(cacheKey, true, f);
	}

	/// <summary>
	/// Helper method for adding/retrieving a value from/to the cache.
	/// </summary>
	/// <param name="cacheKey">The key of the cached item</param>
	/// <param name="getFromCache">True if a cached vaersion cand be returned. If false, f() will be used to get the data</param>
	/// <param name="f">If the key is not found in the cache execute the f() function to retrieve the object</param>
	/// <returns></returns>
	private static object GetFromCache(string cacheKey, bool getFromCache, Func<object> f)
	{
		object cachedItem = null;
		if (getFromCache)
			cachedItem = HttpRuntime.Cache[cacheKey];

		if (cachedItem == null)
		{
			cachedItem = f();
			if (cachedItem != null)
				HttpRuntime.Cache.Insert(cacheKey, cachedItem, null, DateTime.Now.AddSeconds(WebConfigSettings.CachingTime), TimeSpan.Zero);
		}

		return cachedItem;
	}

	#endregion
}

Below the modification to be done in the model:

public IEnumerable<SelectListItem> ContactReasonsList
{
	get
	{
		return (new[]
					{
						new SelectListItem
							{
								Selected = (ContactReasonId == 0),
								Text = "Select...",
								Value = string.Empty
							}
					}).Union(CacheManager.GetContactReasons(HttpContext.Current.Request.HttpMethod == "POST").
								 Select(c => new SelectListItem
								 {
									 Selected = (c.ContactReasonId == ContactReasonId),
									 Text = c.ContactReasonText,
									 Value = c.ContactReasonId.ToString(CultureInfo.InvariantCulture)
								 }));              
	}
}

And now the final touch, implementing the save with our preferred business logic (save in the DB only the positive comments). Of course, the business logic goes in the model:

public bool Save(out string message)
{
	try
	{
		using (var context = new MvcDemoEntities())
		{
			if (ContactReasonId == 1) //A positive remark
			{
				context.Messages.AddObject(this);
				message = "Your message was saved successfully. Thank you for your appreciation!";
				context.SaveChanges();
				return true;
			}                   
			message = "We are sorry you were disappointed.";
			return false;                                        
		}
	}
	catch (Exception exp)
	{
		message = "An unexpected error occured.";
		return false;
	}
}

then we call the save method from the controller

[HttpPost]
public ActionResult ContactUs(Code.DataAccess.Message message)
{
	string saveResultMessage;
	ViewBag.SaveResult = message.Save(out saveResultMessage);
	ViewBag.SaveResultMessage = saveResultMessage;
	return View(message);
}

and display the result in the view.

@if (ViewBag.SaveResult != null &amp;&amp; ViewBag.SaveResult)
{
    <span style="color:green;">@ViewBag.SaveResultMessage</span>
}
@if (ViewBag.SaveResult != null &amp;&amp; !ViewBag.SaveResult)
{
    <span style="color:red;">@ViewBag.SaveResultMessage</span>
}

This is it, now we have a completely functional ‘Contact Us’ and we can add a link to it in our default page:

@Html.ActionLink("Contact Us", "ContactUs", "Home")

Download the source code from CodePlex

Ajax like file upload

It is not possible to upload a file to the server through Ajax because the access to the local file system using JavaScript is forbidden but we can use a hidden IFrame in order to obtain an Ajax like behavior. This can be done in the following way:

  1. Create a form element that contains a file upload, an IFrame and a button.
  2. Set the name attribute of the child IFrame to a value (ex: uploadIframe)
  3. Set the target attribute of the containing form element to the name of the child iFrame (uploadIframe). The target attribute specifies where to open the action URL.
  4. In order to know the result of the upload, the IFrame will contain some JavaScript code that will set the result of the operation.

Below is a code example in asp.net MVC:

  • The main form:
    <form action='@Url.Action("UploadFile", "Home")' id="formContainer" method="post"  target="uploadIframe" enctype="multipart/form-data">
        <input type="file" name="fileUpload" id="fileUpload" />              
    </form>
    <input type="submit" value="Upload File" id="buttonUploadFile" name="buttonUploadFile"/>
    <p id="result"></p>
    
    @section scripts {
    <script type="text/javascript">
        $().ready(function () {
            $('#buttonUploadFile').click(function (event) {
                event.preventDefault();
                $('#result').text('Upload started');
                $('#formContainer').submit(); //submit the file to the server
            });
        });
    </script>
    }
    
  • The controller method that will upload the file:
    [HttpPost]
    public ActionResult UploadFile(HttpPostedFileBase fileUpload)
    {
    	try
    	{
    		if (fileUpload != null && fileUpload.ContentLength > 0)
    		{
    			var fileName = Path.GetFileName(fileUpload.FileName);
    			var path = Path.Combine(Server.MapPath("~/uploads"), fileName);
    			fileUpload.SaveAs(path);
    			ViewBag.FileUploadResult = "Success";
    		}
    		else
    			ViewBag.FileUploadResult = "No file to upload";
    	}
    	catch (Exception ex)
    	{
    		ViewBag.FileUploadResult = string.Format("Error: {0}", ex.Message);
    	}
    
    	return View();
    }
    
  • The content of the IFrame page (contains only a script that sets the result of the upload):
    @section scripts {
        <script type="text/javascript">
            $().ready(function () {
                //set the result in the parent page
                window.parent.$('#result').text('@ViewBag.FileUploadResult');
        });
    </script>
    }
    

Download the source code from CodePlex

Passing methods as parameters with Func<T, TResult>

Today I found this nice feature that I like a lot: Func<T, TResult>. Basically this makes it possible to pass a method as a parameter without explicitly declaring a custom delegate.

I used it a lot in a helper class that caches the data retrieved from DB. Below I’ll leave some examples:

/// <summary>
/// Helper method for adding/retrieving a value from/to the cache.
/// </summary>
/// <param name="cacheKey">The key of the cached item</param>
/// <param name="getFromCache">True if a cached vaersion cand be returned. If false, f() will be used to get the data</param>
/// <param name="f">If the key is not found in the cache execute the f() function to retrieve the object</param>
/// <returns></returns>
private static object GetFromCache(string cacheKey, bool getFromCache, Func<object> f)
{
	object cachedItem = null;
	if (getFromCache)
		cachedItem = HttpRuntime.Cache[cacheKey];

	if (cachedItem == null)
	{
		cachedItem = f();
		if (cachedItem != null)
			HttpRuntime.Cache.Insert(cacheKey, cachedItem, null, DateTime.Now.AddSeconds(WebConfigSettings.CachingTime), TimeSpan.Zero);
	}

	return cachedItem;
}

Now, in order to get the data from the cache, if it exists, or otherwise retrieve it from DB, it’s enough to pass a key and an anonymous method that will be used in case the data is not in the cache:

public static List GetListFromDB()
{
	return (List)GetFromCache("GetListFromDB", () => new List {"aaa", "bbb", "ccc"} ); 
	//Not exactly from DB, but you get the point
}

That’s it! Simple and elegant. ūüôā