AutoFixture.AutoMoq supply a known value for one constructor parameter

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<TTarget,TValueType> : 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<TTargetType, TValueType>(
        this IFixture fixture, 
        string paramName,
        TValueType value)
    {
        fixture.Customizations.Add(
           new ConstructorArgumentRelay<TTargetType, TValueType>(paramName, value)
        );
        return fixture;
    }
}

Now we create two similar classes to test with.

    public class TestClass<T>
    {
        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<T>
    {
        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<TestClass<string>, string>("value1", wanted)
               .ConstructorArgumentFor<TestClass<string>, string>("value2", wanted2);

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

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

So I'm sure people could work out the generalized implementation of Mark's suggestion but I thought I'd post it for comments.

I've created a generic ParameterNameSpecimenBuilder based on Mark's LifeSpanArg:

public class ParameterNameSpecimenBuilder<T> : ISpecimenBuilder
{
    private readonly string name;
    private readonly T value;

    public ParameterNameSpecimenBuilder(string name, T value)
    {
        // we don't want a null name but we might want a null value
        if (string.IsNullOrWhiteSpace(name))
        {
            throw new ArgumentNullException("name");
        }

        this.name = name;
        this.value = value;
    }

    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as ParameterInfo;
        if (pi == null)
        {
            return new NoSpecimen(request);
        }

        if (pi.ParameterType != typeof(T) ||
            !string.Equals(
                pi.Name, 
                this.name, 
                StringComparison.CurrentCultureIgnoreCase))
        {
            return new NoSpecimen(request);
        }

        return this.value;
    }
}

I've then defined a generic FreezeByName extension method on IFixture which sets the customization:

public static class FreezeByNameExtension
{
    public static void FreezeByName<T>(this IFixture fixture, string name, T value)
    {
        fixture.Customizations.Add(new ParameterNameSpecimenBuilder<T>(name, value));
    }
}

The following test will now pass:

[TestMethod]
public void FreezeByName_Sets_Value1_And_Value2_Independently()
{
    //// Arrange
    IFixture arrangeFixture = new Fixture();

    string myValue1 = arrangeFixture.Create<string>();
    string myValue2 = arrangeFixture.Create<string>();

    IFixture sutFixture = new Fixture();
    sutFixture.FreezeByName("value1", myValue1);
    sutFixture.FreezeByName("value2", myValue2);

    //// Act
    TestClass<string> result = sutFixture.Create<TestClass<string>>();

    //// Assert
    Assert.AreEqual(myValue1, result.Value1);
    Assert.AreEqual(myValue2, result.Value2);
}

public class TestClass<T>
{
    public TestClass(T value1, T value2)
    {
        this.Value1 = value1;
        this.Value2 = value2;
    }

    public T Value1 { get; private set; }

    public T Value2 { get; private set; }
}

You could do something like this. Imagine that you want to assign a particular value to the TimeSpan argument called lifespan.

public class LifespanArg : ISpecimenBuilder
{
    private readonly TimeSpan lifespan;

    public LifespanArg(TimeSpan lifespan)
    {
        this.lifespan = lifespan;
    }

    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as ParameterInfo;
        if (pi == null)
            return new NoSpecimen(request);

        if (pi.ParameterType != typeof(TimeSpan) ||
            pi.Name != "lifespan")   
            return new NoSpecimen(request);

        return this.lifespan;
    }
}

Imperatively, it could be used like this:

var fixture = new Fixture();
fixture.Customizations.Add(new LifespanArg(mySpecialLifespanValue));

var sut = fixture.Create<CookieCache>();

This approach can be generalized to some degree, but in the end, we're limited by the lack of a strongly typed way to extract a ParameterInfo from a particular constructor or method argument.

You have to replace:

string knownValue = fixture.Freeze<string>("My known value");

with:

fixture.Inject("My known value");

You can read more about Inject here.

---

Actually the Freeze extension method does:

var value = fixture.Create<T>();
fixture.Inject(value);
return value;

Which means that the overload you used in the test actually called Create<T> with a seed: My known value resulting in "My known value4d41f94f-1fc9-4115-9f29-e50bc2b4ba5e".