c# - 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 value. After all, anonymous object creation is what it is all about.

What I'm struggling with is when I care about one or more of the constructor parameters. Take ExampleComponent below:

public class ExampleComponent
{
    public ExampleComponent(IService service, string someValue)
    {
    }
}

I want to write a test where I supply a specific value for someValue but leave IService to be created automatically by AutoFixture.AutoMoq.

I know how to use Freeze on my IFixture to keep hold of a known value that will be injected into a component but I can't quite see how to supply a known value of my own.

Here is what I would ideally like to do:

[TestMethod]
public void Create_ExampleComponent_With_Known_SomeValue()
{
    // create a fixture that supports automocking
    IFixture fixture = new Fixture().Customize(new AutoMoqCustomization());

    // supply a known value for someValue (this method doesn't exist)
    string knownValue = fixture.Freeze<string>("My known value");

    // create an ExampleComponent with my known value injected 
    // but without bothering about the IService parameter
    ExampleComponent component = this.fixture.Create<ExampleComponent>();

    // exercise component knowning it has my known value injected
    ...
}

I know I could do this by calling the constructor directly but this would no longer be anonymous object creation. Is there a way to use AutoFixture.AutoMock like this or do I need to incorporate a DI container into my tests to be able to do what I want?

---

EDIT:

I probably should have been less absract in my original question so here is my specific scenario.

I have an ICache interface which has generic TryRead<T> and Write<T> methods:

public interface ICache
{
    bool TryRead<T>(string key, out T value);

    void Write<T>(string key, T value);

    // other methods not shown...  
}

I'm implementing a CookieCache where ITypeConverter handles converting objects to and from strings and lifespan is used to set the expiry date of a cookie.

public class CookieCache : ICache
{
    public CookieCache(ITypeConverter converter, TimeSpan lifespan)
    {
        // usual storing of parameters
    }

    public bool TryRead<T>(string key, out T result)
    {
        // read the cookie value as string and convert it to the target type
    }

    public void Write<T>(string key, T value)
    {
        // write the value to a cookie, converted to a string

        // set the expiry date of the cookie using the lifespan
    }

    // other methods not shown...
}

So when writing a test for the expiry date of a cookie, I care about the lifespan but not so much about the converter.

Answer 1

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; }
}