What CLR do when compare T with null, and T is a struct?

Question

private static void SaveOrRemove<T>(string key, T value)
{
    if (value == null)
    {
        Console.WriteLine("Remove: " + key);
    }

    //...
}

If I call passing 0 to value: SaveOrRemove("MyKey", 0), the condition value == null is false, then CLR dont make a value == default(T). What really happens?

Answer 1

The JIT compiler basically removes any comparisons with null when T is a non-nullable value type, assuming them all to be false. (Nullable value types will compare with the null value for that type, which is probably what you expect.)

If you want it to compare to the default value, you could use:

if (EqualityComparer<T>.Default.Equals(value, default(T))
{
    ...
}

Answer 2

If you want default(T) you have to say it, not null which has its own meaning. If you want the ability to actually pass in a null in place of a value type, you should use Nullable<T> instead.

So, your code would become:

private static void SaveOrRemove<T>(string key, Nullable<T> value)
{
    if (!value.HasValue()) // is null
    {
        Console.WriteLine("Remove: " + key);
    }
    else
    {
        T val = value.Value;
        // ...
    }
}

Note that Nullable<T> is only useful with value types (structs, "builtins" other than string); for reference types you can't use it anyway.

MSDN link

Answer 3

Your question is answered in section 7.9.6 of the C# specification:

If an operand of a type parameter type T is compared to null, and the runtime type of T is a value type, the result of the comparison is false.