C# Error CS0706 – Invalid constraint type. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

C# Error

CS0706 – Invalid constraint type. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

Reason for the Error & Solution

Invalid constraint type. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

This error occurs when an invalid construct is used in a constraint clause. To avoid this error, use an interface or non-sealed class instead of the construct that caused the error.

Example

The following sample generates CS0706.

// CS0706.cs  
// compile with: /target:library  
class A {}  
class C<T> where T : int[] {}  // CS0706  
class D<T> where T : A {}  // OK