Why can't I inject value null with Ninjects ConstructorArgument?

Question

When using Ninjects ConstructorArgument you can specify the exact value to inject to specific parameters. Why can't this value be null, or how can I make it work? Maybe it's not something you'd like to do, but I want to use it in my unit tests.. Example:

public class Ninja
{
    private readonly IWeapon _weapon;
    public Ninja(IWeapon weapon)
    {
        _weapon = weapon;
    }
}

public void SomeFunction()
{
    var kernel = new StandardKernel();
    var ninja = kernel.Get<Ninja>(new ConstructorArgument("weapon", null));
}

Answer 1

Looking at the source (and the stack trace I got by reproing which you omitted :P)

This is because it's binding to a different overload of the ConstructorArgument ctor than the normal usage (i.e., where you're passing a Value Type or a non-null Reference Type) does.

The workaround is to cast the null to Object:-

var ninja = kernel.Get<Ninja>( new ConstructorArgument( "weapon", (object)null ) );

Ninject 2 source:

public class ConstructorArgument : Parameter
{
    /// <summary>
    /// Initializes a new instance of the <see cref="ConstructorArgument"/> class.
    /// </summary>
    /// <param name="name">The name of the argument to override.</param>
    /// <param name="value">The value to inject into the property.</param>
    public ConstructorArgument(string name, object value) : base(name, value, false) { }

    /// <summary>
    /// Initializes a new instance of the <see cref="ConstructorArgument"/> class.
    /// </summary>
    /// <param name="name">The name of the argument to override.</param>
    /// <param name="valueCallback">The callback to invoke to get the value that should be injected.</param>
    public ConstructorArgument(string name, Func<IContext, object> valueCallback) : base(name, valueCallback, false) { }
}

Repro:

public class ReproAndResolution
{
    public interface IWeapon
    {
    }

    public class Ninja
    {
        private readonly IWeapon _weapon;
        public Ninja( IWeapon weapon )
        {
            _weapon = weapon;
        }
    }

    [Fact]
    public void TestMethod()
    {
        var kernel = new StandardKernel();
        var ninja = kernel.Get<Ninja>( new ConstructorArgument( "weapon", (object)null ) );
    }
}

Lesson? You'd be crazy not to download the latest source and look at it. Great comments, nice clean codebase. Thanks again to @Ian Davis for that tip/prodding!