C# Language Generics Type constraints (classes and interfaces)
Example
Type constraints are able to force a type parameter to implement a certain interface or class.
interface IType;
interface IAnotherType;
// T must be a subtype of IType
interface IGeneric<T>
where T : IType
{
}
// T must be a subtype of IType
class Generic<T>
where T : IType
{
}
class NonGeneric
{
// T must be a subtype of IType
public void DoSomething<T>(T arg)
where T : IType
{
}
}
// Valid definitions and expressions:
class Type : IType { }
class Sub : IGeneric<Type> { }
class Sub : Generic<Type> { }
new NonGeneric().DoSomething(new Type());
// Invalid definitions and expressions:
class AnotherType : IAnotherType { }
class Sub : IGeneric<AnotherType> { }
class Sub : Generic<AnotherType> { }
new NonGeneric().DoSomething(new AnotherType());
Syntax for multiple constraints:
class Generic<T, T1>
where T : IType
where T1 : Base, new()
{
}
Type constraints works in the same way as inheritance, in that it is possible to specify multiple interfaces as constraints on the generic type, but only one class:
class A { /* ... */ }
class B { /* ... */ }
interface I1 { }
interface I2 { }
class Generic<T>
where T : A, I1, I2
{
}
class Generic2<T>
where T : A, B //Compilation error
{
}
Another rule is that the class must be added as the first constraint and then the interfaces:
class Generic<T>
where T : A, I1
{
}
class Generic2<T>
where T : I1, A //Compilation error
{
}
All declared constraints must be satisfied simultaneously for a particular generic instantiation to work. There is no way to specify two or more alternative sets of constraints.
