Question

What may cause It.IsAny<string>() to return null at every call? Am I incorrect in assuming that it is designed to return a non-null string?

Here’s the usage – where the Login method throws an ArgumentNullException for a null 2nd argument (connection string). I was assuming that It.IsAny<string>() would provide a non-null string, which would bypass the ArgumentNullException.

var mockApiHelper = new Mock<ApiHelper>();
mockApiHelper.Setup(m => m.Connect(It.IsAny<string>(), 
                                   It.IsAny<string>(), 
                                   It.IsAny<string>()));

var repositoryPlugin = new RepositoryPlugin(mockApiHelper.Object);
repositoryPlugin.Login(new CredentialsInfo(), It.IsAny<string>());

Assert.IsTrue(repositoryPlugin.LoggedIn, 
    "LoggedIn property should be true after the user logs in.");

Answer 1

Well, It.IsAny<TValue> just returns the result of calling Match<TValue>.Create – which in turn returns default(TValue). That will be null for any reference type.

It’s not clear whether you’re really calling it on the right object though – shouldn’t you be calling it on the mock rather than on the real code?

All the samples I’ve seen use It.IsAny in the context of a mock.Setup call. Could you give more information about how you’re trying to use it?