Check if object is null using ReferenceEquals object static method.

public bool IsNull(object obj)
{
    //ReferenceEquals - >Determines whether the specified System.Object instances are the same instance.
    return object.ReferenceEquals(obj, null);
}

string st = null;
Response.Write(IsNull(st) + "<br/>"); //True             
st = "1";
Response.Write(IsNull(st) + "<br/>"); //False