Easier Mock Construction in .NET

We’re currently using Moq as our mocking library on an ASP.NET MVC application. Moq has been working out pretty well, but building mock objects is a little cumbersome. We’re using StructureMap heavily, which leads to frequent updates to the constructors which StructureMap relies on to compose our object graph. Moq requires you to match the arguments of your mock constructor to the parameters of the mocked class’s constructor. Every time we mock an instance of a class we have to figure out how many arguments that class’s constructor takes and pass a default value in for each. To make matters worse, when we add arguments to a constructor, we have to move from failing test to failing test, adding null arguments to the mock constructor to fix runtime errors in our NUnit SetUp.

To address these issues, we threw together a function to dynamically determine the arity of a mocked class’s constructor (which we assume only has one) and invoke the mock constructor appropriately. Since updating our tests to use the function below, we haven’t had to update a single SetUp in response to a constructor signature change.

protected Mock<T> NewMock<T>() where T : class
{
    var type = typeof(T);
    var constructors = type.GetConstructors();
    if (constructors.Length == 0) constructors = type.GetConstructors(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
    if (constructors.Length == 0) return new Mock<T>();

    var constructor = constructors[0];
    var parameters = constructor.GetParameters();

    var objects = parameters
        .Select(p => p.ParameterType.IsValueType ? Activator.CreateInstance(p.ParameterType) : null)
        .ToArray();
    var constructorInfo = typeof(Mock<T>).GetConstructor(new Type[] { typeof(object[]) });
    return (Mock<T>)constructorInfo.Invoke(new object[] { objects });
}