Generics with multiple constrains

G

Guest

I am trying to create a generics class with multiple constrains, as follows:

public class KeyHandler<T> where T : TextBoxBase, ComboBox

When I try that, the compiler would complain:

The class type constraint 'System.Windows.Forms.ComboBox' must come before
any other constraints

If I switch place of ComboBox with TextBoxBase, it would gripe:

The class type constraint 'System.Windows.Forms.TextBoxBase' must come
before any other constraints

Is it possible to create such a generics class? Tks.
 
G

Guest

You can specify multiple constraints, but in your example there must be only
one base class (since C# doesn't allow multiple inheritance) and the
following constraints could indicate interfaces.
e.g.,
public class KeyHandler<T> where T : TextBoxBase, ISomeInterface

For your example, the compiler should actually be providing a more
intelligent message, such as "T can only derive from one base class".
--
David Anton
http://www.tangiblesoftwaresolutions.com
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G

Guest

Thanks for your response. In my case, I want it specifically for TextBox and
ComboBox. Their base class Control, however, does not have the common
methods/properties that they share, such as Select and SelectText.

I guess generics may not be applied to my specific example.
 
G

Guest

So you would want to specify a constraint list where just one of the items is
true. The constraint list items must all be true, so you could never do this
in C#.
--
David Anton
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N

Nicholas Paldino [.NET/C# MVP]

Well, I wouldn't say that you can't do it in C#. If you are willing to
trade compile-time checking for run-time checking (which is not the best
solution, admittedly), then you can have no constraint and then perform the
check to make sure that the type parameter is a ComboBox or TextBox in the
static constructor. If it isn't, then you can throw an exception.

Also, you could define a common interface which has these properties,
and then implement these interfaces on wrappers that would take instances of
either TextBox or ComboBox and then use the interface in the class you are
writing (you won't need generics, since you are only ever dealing with the
interface).
 
G

Guest

Right - nearly anything can be done with creative coding, but I was just
looking at this very narrowly in terms of generic constraint lists.
--
David Anton
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Nicholas Paldino said:
Well, I wouldn't say that you can't do it in C#. If you are willing to
trade compile-time checking for run-time checking (which is not the best
solution, admittedly), then you can have no constraint and then perform the
check to make sure that the type parameter is a ComboBox or TextBox in the
static constructor. If it isn't, then you can throw an exception.

Also, you could define a common interface which has these properties,
and then implement these interfaces on wrappers that would take instances of
either TextBox or ComboBox and then use the interface in the class you are
writing (you won't need generics, since you are only ever dealing with the
interface).

--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)

David Anton said:
So you would want to specify a constraint list where just one of the items
is
true. The constraint list items must all be true, so you could never do
this
in C#.
--
David Anton
http://www.tangiblesoftwaresolutions.com
Convert between VB, C#, C++, and Java
Instant C#
Instant VB
Instant C++
C++ to C# Converter
C++ to VB Converter
C++ to Java Converter
Click to expand...
Click to expand...
 
G

Guest

It seems that I can't use generics in my example, even though I want to,
since this is my first foray into generics stuff.

I took Nic's suggestions and developed a wrapper class that implements an
interface that features the common properties/methods of the two Windows
Forms controls. It works nicely.

Thanks for your inputs. Have a great day.

David Anton said:
Right - nearly anything can be done with creative coding, but I was just
looking at this very narrowly in terms of generic constraint lists.
--
David Anton
http://www.tangiblesoftwaresolutions.com
Convert between VB, C#, C++, and Java
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Nicholas Paldino said:
Well, I wouldn't say that you can't do it in C#. If you are willing to
trade compile-time checking for run-time checking (which is not the best
solution, admittedly), then you can have no constraint and then perform the
check to make sure that the type parameter is a ComboBox or TextBox in the
static constructor. If it isn't, then you can throw an exception.

Also, you could define a common interface which has these properties,
and then implement these interfaces on wrappers that would take instances of
either TextBox or ComboBox and then use the interface in the class you are
writing (you won't need generics, since you are only ever dealing with the
interface).

--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)

David Anton said:
So you would want to specify a constraint list where just one of the items
is
true. The constraint list items must all be true, so you could never do
this
in C#.
--
David Anton
http://www.tangiblesoftwaresolutions.com
Convert between VB, C#, C++, and Java
Instant C#
Instant VB
Instant C++
C++ to C# Converter
C++ to VB Converter
C++ to Java Converter


:

Thanks for your response. In my case, I want it specifically for TextBox
and
ComboBox. Their base class Control, however, does not have the common
methods/properties that they share, such as Select and SelectText.

I guess generics may not be applied to my specific example.

:

You can specify multiple constraints, but in your example there must be
only
one base class (since C# doesn't allow multiple inheritance) and the
following constraints could indicate interfaces.
e.g.,
public class KeyHandler<T> where T : TextBoxBase, ISomeInterface

For your example, the compiler should actually be providing a more
intelligent message, such as "T can only derive from one base class".
--
David Anton
http://www.tangiblesoftwaresolutions.com
Convert between VB, C#, C++, and Java
Instant C#
Instant VB
Instant C++
C++ to C# Converter
C++ to VB Converter
C++ to Java Converter


:

I am trying to create a generics class with multiple constrains, as
follows:

public class KeyHandler<T> where T : TextBoxBase, ComboBox

When I try that, the compiler would complain:

The class type constraint 'System.Windows.Forms.ComboBox' must come
before
any other constraints

If I switch place of ComboBox with TextBoxBase, it would gripe:

The class type constraint 'System.Windows.Forms.TextBoxBase' must
come
before any other constraints

Is it possible to create such a generics class? Tks.
Click to expand...
Click to expand...
Click to expand...
 

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