Type constraints sadness

argot — Sat, 21 Nov 2009 11:17:58 GMT

Although delegate type and Enum type are class types, we can not inherit from them as they are sealed classes after compile,

public delegate void DelegateTestType();

//Compile time error

class myTest : DelegateTestType

{

...

}

we can not make it as a type constraint either on class or method:

//Compile time error

class myTest<T> where T : DelegateTestType

{

}

From the C# 2.0 specification we can read (20.7, Constraints):

A class-type constraint must satisfy the following rules:

· The type must be a class type.

· The type must not be sealed.

· The type must not be one of the following types: System.Array, System.Delegate, System.Enum, or System.ValueType.

· The type must not be object. Because all types derive from object, such a constraint would have no effect if it were permitted.

· At most one constraint for a given type parameter can be a class type.

In c# world we have to use some indirect ways, like type casting, wrapper class or examine the type is Delegate type, it is still a problem in C# 4.0, this is a bit sad as C++ can do it more than happily!

General .NET

Type constraints sadness