Using a null IDisposable value with the using statement

Question

The following code produces no errors when executed:

using ((IDisposable)null) {
    Console.WriteLine("A");
}
Console.WriteLine("B");

Is a null value to using 'allowed'? If so, where is it documented?

Most C# code I've seen will create a "dummy/NOP" IDisposable object - is there a particular need for a non-null Disposable object? Was there ever?

If/since null is allowed, it allows placing a null guard inside the using statement as opposed to before or around.

The C# Reference on Using does not mention null values.

Answer 1

Yes, a null value is allowed. §8.13, "The using statement", of the C# 5.0 specification says:

If a null resource is acquired, then no call to Dispose is made, and no exception is thrown.

The specification then provides the following expansion for a using statement of the form

using (ResourceType resource = expression) statement

when resource is a reference type (other than dynamic):

{
    ResourceType resource = expression;
    try {
        statement;
    }
    finally {
        if (resource != null) ((IDisposable)resource).Dispose();
    }
}