Why is resharper making the following recommendation?

Question

My code is...

    public static void AssertNotNull<T>(string name, T val)
    {
        if (val == null)
            throw new ArgumentNullException(String.Format("{0} must not be null", name));
    }

Resharper is recommending...

    public static void AssertNotNull<T>(string name, T val)
    {
        if (Equals(val, default(T)))
            throw new ArgumentNullException(String.Format("{0} must not be null", name));
    }

Answer 1

Because it doesn't know if T is a value type or reference type, so it makes the code work with both.

Answer 2

I second Berado's answer, but would add that you can prevent this by adding the constraint below:

public static void AssertNotNull<T>(string name, T val) where T : class

Answer 3

Those two methods are not equivalent. The first one allows AssertNotNull( "foo", 0 ) while the second throws. I think Resharper is being overzealous in this case.

Answer 4

This obviously isn't what you want in this instance, but it's just trying to be helpful, making sure that you don't introduce a bug by forgetting that reference types can be used for T. Like @Michael Meadows said, you probably want to add the class constraint to T.

Answer 5

I suppose because T could be a non-reference type.

Answer 6

if you know that T will always be a class then add a where clause to say so then your origional code will be OK.

public static void AssertNotNull<T>(string name, T val)
    where T : class
    {
        if (val == null)
            throw new ArgumentNullException(String.Format("{0} must not be null", name));
    }