Unit testing ModelBinders in ASP.Net Web Api projects

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My Colleague Matt Davies has done a blog post a couple of years ago about Unit testing MVC3/MVC4 Model Binders. This is a nice post but as you know Web Api Side of ASP.Net is usually using a different set of object model therefore the setting up the BindingContext would be different than what we have in MVC. I also searched and did not find a good blog post about how to unit test ModelBinders in ASP.NEt Web Api project, so I decided to put together this post.

Let’s assume we have following implementation of model binder which bind a sort expression like FirstName desc or LastName to a simple class structure which has FieldName and Ascending as boolean to indicate sorting direction. Following is the code for ModelBinder:

We want to write a few unit test to make sure logic of this ModelBinder is working and in each different test case we want to provide different value and check the result. In order to be able to call .BindModel we need two parameters HttpActionContext and ModelBindingContext. HttpActionContext can be completely mocked. We are using NSubstitute as mocking framework. ModelBindingContext has a property called ValueProvider which obviously provide value of the model. So we create a ModelBindingContext and set the ValueProvider to a mocked version and keep the reference to be able to change value for each test.

Following is the complete code for setting up the model binder and testing that.

In each unit test we set up the _valueProvider mock to return expected sortExpression and then call the method and assert on expected values.

Simple!

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