When can a generic parameter never be null

Question

In a generic GetHashCode(T foo) method, I check whether foo is null. However I just stumbled upon a strange Resharper warning.

In the following code, can foo never be null?

private class FooComparer<T> : IEqualityComparer<T> where T: Foo
{
    public int GetHashCode(T foo)
    {
        // resharper warning:  "Expression is always false"
        if (Object.ReferenceEquals(null,foo)) return 0; 

        // ... calculate hash
    }
}

However as far as I can tell, the following is perfectly legal:

Foo foo = null;
var fooComparer = new FooComparer<Foo>();
int hash = fooComparer.GetHashCode(foo);

Cardsharp

Yes your code is perfectly legal. – Hedvah

Is Foo a value or a reference type? – Frazee

Foo is a class, not a struct – Cardsharp

Answer 1

Method IEqualityComparer<T>.GetHashCode has contract [NotNull] for its parameter because it has implementations that throw an exception when null is provided as an argument.

If you want to use FooComparer<T>.GetHashCode directly and exception-safe for null as its argument, you can annotate it as follows:

public int GetHashCode([JetBrains.Annotations.CanBeNull] T foo)
{
    // resharper warning:  "Expression is always false"
    if (Object.ReferenceEquals(null,foo)) return 0; 

    // ... calculate hash
}

Nevertheless analysis for [Not-Null]-parameters must be improved. This bug exists for similar code in http://youtrack.jetbrains.com/issue/RSRP-304111

Orphaorphan

Answer 2

MSDN for IEqualityComparer<T>.GetHashCode Method says:

Exceptions:

ArgumentNullException The type of obj is a reference type and obj is null.

This seems to imply that calling GetHashCode<T>(T obj) with a null parameter violates the contract of IEqualityComparer<T>.

I assume Resharper assumes that callers adhere to that contract, and thus never pass in null.

Benignity

Good point! There seems to be some inconsistency in the MDSN documentation, as for example this implementation of IEqulityComparer.GetHashCode uses the same null check as I do. – Cardsharp

I for one implement my GetHashCode just like you do (Expect that I avoid ReferenceEquals, because it causes boxing). Returning a constant seems like the cleanest choice for GetHashCode. – Benignity