Will Dispose() be called in a using statement with a null object?

Is it safe to use the using statement on a (potentially) null object?
Consider the following example:

class Test {
    IDisposable GetObject(string name) {
        // returns null if not found
    }

    void DoSomething() {
        using (IDisposable x = GetObject("invalid name")) {
            if (x != null) {
                 // etc...
            }
        }
    }
}

Is it guaranteed that Dispose will be called only if the object is not null, and I will not get a NullReferenceException?

Yes, Dispose() is only called on non-null objects:

http://msdn.microsoft.com/en-us/library/yh598w02.aspx

The expansion for using checks that the object is not null before calling Dispose on it, so yes, it's safe.

In your case you would get something like:

IDisposable x = GetObject("invalid name");
try
{
    // etc...
}
finally
{
    if(x != null)
    {
        x.Dispose();
    }
}