Unit Testing: Moq doesn’t support property indexers

I hit a road block today writing some tests which were dependent upon another class’s property indexer. When trying to do the following using Moq:

   1: [TestMethod]

   2: public void Test1()

   3: {

   4:     Mock<HttpContextBase> context = new Mock<HttpContextBase>();

   5:  

   6:     Object actual = null;

   7:  

   8:     context.ExpectSet(c => c.Session["SomeKey"]).Callback(o => actual = o);

   9:  

  10:     context.Object.Session["SomeKey"] = "some value";

  11:     

  12:     Assert.IsNotNull(actual, "Value was not set");

  13: }

Line 8 fails with this error:

System.ArgumentException: Expression is not a property access: c => c.Session.get_Item("SomeKey").

It appears the current stable build of Moq (version 2.6.1014.1) doesn’t support property indexers. The good news is that a few days ago support was added for property indexers. However, if you’re like me and you’re not working with the latest and greatest, then there’s a workaround. Just create your own mock implementation of the class with the indexed property. In my case the class in question was HttpSessionStateBase. Here’s my mock implementation:

/// <summary>

/// Mock implementation of HttpSessionStateBase

/// </summary>

/// <remarks>

/// Current stable build of Moq (2.6.1014.1) doesn't support property indexers 

/// at this time, so this is an implementation of HttpSessionStateBase 

/// which allows us to verify that the session cache is being updated.

/// </remarks>

internal class MockSession : HttpSessionStateBase

{

    Dictionary<string, object> session = new Dictionary<string, object>();

    public override void Add(string name, object value)

    {

        session.Add(name, value);

    }

    public override object this[string name]

    {

        get

        {

            if (!session.ContainsKey(name))

                return null;

            return session[name];

        }

        set

        {

            session[name] = value;

        }

    }

}

Then you can do the following:

// setup

MockSession session = new MockSession();

Mock<HttpContextBase> context = new Mock<HttpContextBase>();

context.ExpectGet(c => c.Session).Returns(session);

// exercise

context.Session["SomeKey"] = "some value";

// assert

Assert.IsNotNull(context.Session["SomeKey"]);

Happy Moqing!