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.NET Forum / Languages / C# / September 2007Generics with multiple constrains
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. 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".  SignatureDavid 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: > [quoted text clipped - 11 lines] > > Is it possible to create such a generics class? Tks. 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 [quoted text clipped - 20 lines] > > > > Is it possible to create such a generics class? Tks. 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#. SignatureDavid 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 [quoted text clipped - 26 lines] > > > > > > Is it possible to create such a generics class? Tks. 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). Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > So you would want to specify a constraint list where just one of the items > is [quoted text clipped - 37 lines] >> > > >> > > Is it possible to create such a generics class? Tks. Right - nearly anything can be done with creative coding, but I was just looking at this very narrowly in terms of generic constraint lists. SignatureDavid 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 > 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 [quoted text clipped - 49 lines] > >> > > > >> > > Is it possible to create such a generics class? Tks. 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. > Right - nearly anything can be done with creative coding, but I was just > looking at this very narrowly in terms of generic constraint lists. [quoted text clipped - 52 lines] > > >> > > > > >> > > Is it possible to create such a generics class? Tks. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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