how to specify the T Generic type for a class at runtime or from parameter [modified]

Feras Mazen Taleb

I made class called LinqFilter

public class LinqFilter
{
.......
}

and I made a user control

public class lablab : usercontrol
{
linqFilter<[here i want to not put exact class but i want to still make it generic] > koko;

}

and i want to specifiy the [T] at runtime can I Do that OR can I Get the T from Parameter come from outside the usercontrol I tryid this code but i get an error

public class lablab<z> : usercontrol
{
linqFilter<z> koko;
}

thanks a lot

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modified on Monday, February 9, 2009 12:25 AM

Dave Kreskowiak · modified on Monday, February 9, 2009 12:25 AM

You can't. Generics are (IIRC) really just syntaxic sugar that tells the compiler to write code for each Type you specify in the Generic. Say you pass two different types of objects to a Generic method. The compiler automatically writes two methods, using your code as a template, to handle both cases of those objects passed to it. Types used by Generics are determined at compile-time, not run-time.

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Dave Kreskowiak Microsoft MVP Visual Developer - Visual Basic
2006, 2007, 2008

supercat9

Generic reference types are more than syntactic sugar; the compiler only has to produce one JIT-compiled instance of a method that uses a generic reference type, but the method can produce objects which are 'ready to go' in the desired type without the expense of a typecast or other run-time validation. I know of no way such a result could be achieved without generics. There are limits to what generics can do, but they seem to be a very useful feature with significant semantic meaning. Extension methods, by contrast, seem more like syntactic sugar.

Feras Mazen Taleb

thanks a lot for your replay and explanation but could you tell me how can I use the second choice I mean how to make the user control accept T as Template something like this :

public class kokoControl : UserControl
{
.....
}

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Dave Kreskowiak

What are you ultimately trying to do with this??

A guide to posting questions on CodeProject[^]
Dave Kreskowiak Microsoft MVP Visual Developer - Visual Basic
2006, 2007, 2008

Feras Mazen Taleb

ok I'm sorry my replay was not complete . i will tell you the full story :) I made generic class that can make DynamicLinq For any class this class is

public class LinqFilter<T>
{
some of :
public Fun<T,bool> GetFilter()
{
//here i will return
Expression Filter ;
Filter = Expression.And(.....)
..............................
var predicate = Expression.Lambda. .... .
return (Func<T,bool> )predicate.Compile();
}
}

the previous class work well I want to use it inside a user control called FilterControl but I want to make the control also as a Generic Class just to pass the T inside the control

public Class FilterControl<T> : UserControl
{
LinqFilter<T> F;
................
}

but I got an error when tried to do that how can I pass the T from outside the control to the [F] Variable of linqfilter .:confused: thanks

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Curtis Schlak

So I fired up Snippet Compiler[^], added System.Core.dll as a reference, and wrote this little snippet of code:

public class Base
{
public Func<t,> GetFilter()
{
return delegate(T t) { return true; };
}
}

public class BaseCotnrol : UserControl
{
Base foo;
}

It compiled just fine. Could you post the compiler error?

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Feras Mazen Taleb

first , thanks a lot for your replay :) I thought that I will not get the answer :^) !! I will try that but could you explain more what should this code do ?

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Curtis Schlak

My code doesn't really do anything. &l;smile /> I just wrote some code that looked like your code to make sure that it compiles. It compiled. Unfortunately, that just means that I don't understand the nature of your question. Can you post something more indicative of your plight?

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

Feras Mazen Taleb

I will explain I made this class

public class LinqFilter<T>
{
public Fun<T,bool> GetFilter()
{
// in this function I made an Expression Tree after
// that i make lambda expression that return delegate
// this is not important for my problem
// but I said that just to not write empty function .
var predicate = Expression.Lambda. .... .
return (Func<T,bool> )predicate.Compile();
}
}

there is a user control called FilterControl I need to use object of previous class [LinqFilter] inside this control but I want to get The [T] Type from out side the user control the following code make compiler error when i put <T> in User Control definition

public Class FilterControl<T> : UserControl
{
LinqFilter<T> F;
................
}

I hope to not give me joking code again thanks

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Curtis Schlak

When you write that you "want to get the [T] Type from out side the user control", what do you mean by that? Do you mean that you want to bind T at runtime rather than at compile time? Something like the following pseudocode?

t <- assign some resolved Type
create an instance of FilterControl dynamically and bind t to T
put the instance of FilterControl`1 on your form

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

Feras Mazen Taleb

What I need is : When I put the FilterControl in the window , I want to specify the T Type I mean in Compile Time in this way the control will be able to make filtering on objects of any T class with support from LinqFilter class

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Curtis Schlak

Ah ha! Now I understand. The Visual Studio designers do not support controls with unbound generic parameters. You can't create your FilterControl<T> : UserControl and then use the forms designer to drag it onto a panel or form. You might be able to adapt a TypeDescriptor for the user control for your forms, though I've never seen this done. There's an article on Urban Potato that does this for controls with abstract base classes: Using Visual Studio Whidbey to Design Abstract Forms[^].

Feras Mazen Taleb

thanks a lot I will read this article ( in sha'a allah) and I hope to find what I want Thanks very match :)

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Curtis Schlak

If you find or develop something interesting, then don't forget to come back and post a solution so that we all can learn! :-D

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

Feras Mazen Taleb

Okay my friend :)

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