防御性编程的原则一 - 始终检查NULL!

public override bool Equals(object obj)
{
    // this statement CAN FAIL if your "obj" is *NOT* of type "Parameter"
    var par = obj as Parameter;

    // so therefore you MUST be ready to deal with a NULL value here!
    if (par != null && ParameterName == par.ParameterName)  
    {
        return true;
    }
    else
    {
        return false;
    }

}