Compiler Error C3218

  • Article

'type' : type not allowed as a constraint

For a type to be a constraint, it must be either a value type or reference to a managed class or interface.

Example

The following sample generates C3218.

// C3218.cpp
// compile with: /clr /c
class A {};
ref class B {};

// Delete the following 3 lines to resolve.
generic <class T>
where T : A   // C3218
ref class C {};

// OK
generic <class T>
where  T : B
ref class D {};