>My favourite of
>

>>    protected override void OnTextChanged(EventArgs e)
>>    {
>>        if (this.Parent is IMyInterface)
>>        {
>>            ((IMyInterface)this.Parent).MyMethod(true);
>>        }
>>        base.OnTextChanged(e);
>>    }
>

>
>is
>

>>    protected override void OnTextChanged(EventArgs e)
>>    {
>          IMyInterface o;
>>        if ( ( o = this.Parent as IMyInterface) != null )
>>        {
>>           o.MyMethod(true);
>>        }
>>        base.OnTextChanged(e);
>>    }
>

Hmm. I defintely win on readability.
You might win on efficiency. Have you compared both in ILDASM ?