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
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