Re: A small problem comparing types

From: Jon Skeet [C# MVP] (skeet_at_pobox.com)
Date: 10/12/04 

Date: Tue, 12 Oct 2004 11:12:54 +0100

Simon Harvey <sh856531@microsofts_free_email_service.com> wrote:
> I hope someone can help me with the following:
>
> I have a number of usercontrols that I've made which provides certain audit
> functions for any data inserted into it. Each audit control is a descendant
> of BaseAuditControl. Descedants include AuditableTextControl and
> AuditableBoolControl.
>
> The problem I'm having is determining which control I'm actually dealing
> with at any one time. I often need to treat them generically for most of a
> method and then need to implemement some logic that is specific to the
> particlar type of audit control.
>
> An example:
>
> To update the main ui element of my audit control I make a method in my base
> control. I need to be able to get the type of the current instance that I'm
> dealing with, via the 'this' keyword, and then compare it to the various
> types of AuditControl that I have made. Something like this:
> if((this.GetType()) == (typeof(AuditableTextControl)){
> //Do one thing
> return false;
> }
>
> if((this.GetType()) == (typeof(AuditableBoolControl)){
> //Do another
> return false;
> }

That's usually not a good way of working - you should use polymorphism
instead, usually.
 
> The problem I'm getting is that the Types returned from each of the two
> expressions are different.
>
> this.GetType() returns something like ASP.AuditableTextControl_ascx
> typeof(AuditableTextControl) returns
> <mynamespace>.controls.AuditableTextControl

Right - that's because the control itself is generated from the ascx
file, and (I believe - I haven't created any user controls myself) only
derives from your actual user control class.

> I don't understand whats going on behind the scenes, but it seems to me that
> when you get the type of an actual object instance, it is different from
> trying to get the type using just the class definition.
>
> If anyone can help me figure out how to get a successful comparison between
> them. I've tried to make a fake instance of an AuditableTextControl and then
> tried to get the type of that, but that doesnt work either. Seems a bit
> messy as well. I'm sure that there is a really easy way to day this

Using the more normal form of type comparison - the "is" operator - may
well work, if the ..._ascx type really does inherit from
AuditableTextControl.

if (this is AuditableTextControl)
{
...
}

Once again though, I'd suggest using polymorphism - define a virtual
(or abstract) method in the base control, and override it in each
derived class. 

-- 
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

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