# Compiler Error CS0701

'identifier' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

This error occurs if a sealed type is used as a constraint. To resolve this error, use only non-sealed types as constraints.

## Example

The following sample generates CS0701.

// CS0701.cs
// compile with: /target:library
class C<T> where T : System.String {}   // CS0701
class D<T> where T : System.Attribute {}   // OK