The new dynamic type in C# 4 was mainly added to offer simpler creation of objects using reflection and the ability to treat objects as they are no matter where they were created: COM interop, DLR, etc. It basically defers static typing until runtime. The object itself is statically typed, however the compiler bypasses compile-time static type checking until runtime.
One place where this kind of late bound typing is really useful is unit testing ASP.NET MVC action methods that return JSON via JsonResult. For example, given the following controller action method:
And the following unit test you’ll quickly discover that while you can see the values as a watch in Visual Studio or while debugging, you cannot actual run an assert on any of the members (Assert.AreEqual(1, data.FeedId) will throw an exception:
It should be relatively obvious as to why this will not work, notably anonymous types are internal to the assembly that they were created in and results.Data returns a System.Object. Using the dynamic keyword, one might assume that the we should be able to run asserts on the properties of the JSON result, but unfortunately, it’s not that simple:
The exception gives some subtle hints as to what the problem is:
'MyFantasyFootBallGuruTests.Controllers.AlertsControllerTests.GetCurrentPick2' failed: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException : 'object' does not contain a definition for 'FeedId' at CallSite.Target(Closure , CallSite , Object ) at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0) Controllers\AlertsControllerTests.cs(47,0): at MyFantasyFootBallGuruTests.Controllers.AlertsControllerTests.GetCurrentPick2()
Well it’s somewhat of a hint…the object does not contain a definition for ‘FeedId’ is caused by the fact that while we can see the values with the debugger, the reality is that the ‘object’ is really an anonymous type, which exists in the assembly containing the controller. Since anonymous types by definition are always internal to the assembly they are defined in, the runtime throws an exception. Fortunately, the fix is for this is easy.
When you create a new project (assembly) in Visual Studio, a “Properties” folder with a class called “AssemblyInfo.cs” is automatically created. This class contains attributes that define the metadata of the assembly: the name, description, title, whether or not the assembly is visible to COM components, the version, and other information related to the assembly. There is also another very important attribute to those wishing to test anonymous types in assemblies other than the assembly that the anonymous type is defined in: InternalsVisibleToAttribute.
The InternalsVisibleToAttribute makes types that are normally only visible in the defined assembly to other assemblies specified in the attribute. For instance:
Adding this attribute to the assembly that contains the controller classes, makes any anonymous types visible to the unit test assembly, MyFantasyFootballGuruTests. After adding this attribute, the unit tests will not only compile, but allow the properties of the anonymous type to be tested: