ASP.NET 4.5 Model Binding: Creating Custom Value Provider

With ASP.NET 4.5, ASP.NET introduced model binding for web forms as well. Model binding helps to simplify code focused data access logic within web forms. Check out this video by Scott Hanselman to know more on ASP.NET 4.5 Web Forms Model Binding.

Here in this post, we will see how we can create our own custom value provider (read my post on how I used value provider for encrypted query string in MVC application). We will also examine inbuilt class of ASP.NET 4.5 model binding framework which can be useful in creating custom value provider more easily with focused approach.

Before we look into actual interfaces and classes, let us examine few basics of model binding framework. All model binding framework and corresponding classes are resides in System.Web.ModelBinding namespace which is newly introduced with ASP.NET 4.5. For any value provider to work with model binder it requires two components one is implementation of value provider which reads data from request and forward it to model binder and other one is value provider source attribute which expose the actual value provider instance. Have a look at below code snippet here QueryStringAttribute is value provider source attribute which expose object of QueryStringValueProvider so model binder can use it to fetch data.

public IQueryable<Blog> SelectMethod([QueryString]int? id)

Creating Value Provider Source Attribute

To create custom value provider attribute we can derive Attribute class and implement IValueProviderSource interface as displayed in below code snippet.

public class CustomValueProviderAttribute : Attribute, IValueProviderSource

{

    public IValueProvider GetValueProvider(ModelBindingExecutionContext modelBindingExecutionContext)

    {

        return new CustomValueProvider(modelBindingExecutionContext);

    }

}

Here we can have access of ModelBindingExecutionContext and we can pass same to value provider if it is required. Through ModelBindingExecutionContext we can also have access of HttpContextBase and ModelStateDictionary.

Creating Value Provider

Same way we can create Custom Value Provider by implementing IValueProvider interface. Below code snippet shows pseudo code for the same.

public class CustomValueProvider : IValueProvider

{

    ModelBindingExecutionContext _modelBindingExecutionContext;

    public CustomValueProvider(ModelBindingExecutionContext modelBindingExecutionContext)

    {

        this._modelBindingExecutionContext = modelBindingExecutionContext;

    }

    public bool ContainsPrefix(string prefix)

    {

        // validate if requested key is exist or not

    }

    public ValueProviderResult GetValue(string key)

    {

        // return ValueProviderResult object we 

        // can use ModelBindingExecutionContext

        // to access request data

    }

}

Once we are ready we can use created value provider as

SelectMethod([CustomValueProvider]int? id, [CustomValueProvider]string name)

As noted earlier, there are few inbuilt classes in ASP.NET 4.5 model binding framework which give more focused control over custom business logic. Here we will also examine one of its which is SimpleValueProvider. Here we will examine how we can focus on core logic and leaving other responsibility on core framework.

public class CustomValueProvider : SimpleValueProvider

{

    public CustomValueProvider(ModelBindingExecutionContext modelBindingExecutionContext)

        : base(modelBindingExecutionContext)

    {

    }

    protected override object FetchValue(string key)

    {

        // here we can access this.ModelBindingExecutionContext

        // and can look into request data. Once we fetch requested

        // data we just need to return actual value for e.g.            

        return "dotnetExpertGuide.com";

        // NOTE: WE ARE NOT RETURNING ValueProviderResult INSTANCE

    }

}

Earlier with IValueProvider, we had to check if requested key exist or not and if it is then instantiating ValueProviderResult and return it. While with SimpleValueProvider we only need to return actual value of requested key or null incase if it does not exist rest will be taken care by SimpleValueProvider class. Another such framework class is NameValueCollectionValueProvider which act as a base class to create value provider from name value collection. Here I am not demonstrating it. I am leaving it for reader :).

SimpleValueProvider and ASP.NET MVC

Can’t we have/introduce SimpleValueProvider class for MVC in upcoming version?

ModelStateDictionary

Once model binding is done for parameter it is added to ModelStateDictionary dictionary along with its value. For e.g.

SelectMethod([CustomValueProvider]int? id, [CustomValueProvider]string name)

In above code once model binding is done for parameter id, it is added to ModelStateDictionary and it is accessible in rest of the parameter model binding i.e. parameter name here.

Hope this would be helpful. Stay tuned for more post.

You can follow me on twitter for latest link and update on ASP.NET & MVC.