AutoFixture.AutoMoq supply a known value for one constructor parameter

Question

I\'ve just started to use AutoFixture.AutoMoq in my unit tests and I\'m finding it very helpful for creating objects where I don\'t care about the specific

Answer 1

I fee like @Nick was almost there. When overriding the constructor argument, it needs to be for the given type and have it limited to that type only.

First we create a new ISpecimenBuilder that looks at the "Member.DeclaringType" to keep the correct scope.

public class ConstructorArgumentRelay : ISpecimenBuilder
{
    private readonly string _paramName;
    private readonly TValueType _value;

    public ConstructorArgumentRelay(string ParamName, TValueType value)
    {
        _paramName = ParamName;
        _value = value;
    }

    public object Create(object request, ISpecimenContext context)
    {
        if (context == null)
            throw new ArgumentNullException("context");
        ParameterInfo parameter = request as ParameterInfo;
        if (parameter == null)
            return (object)new NoSpecimen(request);
        if (parameter.Member.DeclaringType != typeof(TTarget) ||
            parameter.Member.MemberType != MemberTypes.Constructor ||
            parameter.ParameterType != typeof(TValueType) ||
            parameter.Name != _paramName)
            return (object)new NoSpecimen(request);
        return _value;
    }
}

Next we create an extension method to allow us to easily wire it up with AutoFixture.

public static class AutoFixtureExtensions
{
    public static IFixture ConstructorArgumentFor(
        this IFixture fixture, 
        string paramName,
        TValueType value)
    {
        fixture.Customizations.Add(
           new ConstructorArgumentRelay(paramName, value)
        );
        return fixture;
    }
}

Now we create two similar classes to test with.

    public class TestClass
    {
        public TestClass(T value1, T value2)
        {
            Value1 = value1;
            Value2 = value2;
        }

        public T Value1 { get; private set; }
        public T Value2 { get; private set; }
    }

    public class SimilarClass
    {
        public SimilarClass(T value1, T value2)
        {
            Value1 = value1;
            Value2 = value2;
        }

        public T Value1 { get; private set; }
        public T Value2 { get; private set; }
    }

Finally we test it with an extension of the original test to see that it will not override similarly named and typed constructor arguments.

[TestFixture]
public class AutoFixtureTests
{
    [Test]
    public void Can_Create_Class_With_Specific_Parameter_Value()
    {
        string wanted = "This is the first string";
        string wanted2 = "This is the second string";
        Fixture fixture = new Fixture();
        fixture.ConstructorArgumentFor, string>("value1", wanted)
               .ConstructorArgumentFor, string>("value2", wanted2);

        TestClass t = fixture.Create>();
        SimilarClass s = fixture.Create>();

        Assert.AreEqual(wanted,t.Value1);
        Assert.AreEqual(wanted2,t.Value2);
        Assert.AreNotEqual(wanted,s.Value1);
        Assert.AreNotEqual(wanted2,s.Value2);
    }        
}