Compiler Error CS0449

The 'class', 'struct', 'unmanaged', 'notnull', and 'default' constraints cannot be combined or duplicated, and must be specified first in the constraints list.

The constraints on the type parameter of a generic type or method must occur in a specific order: class or struct must be first, if present, then any interface constraints, and finally any constructor constraints. This error is caused by the class or struct constraint not appearing first. To resolve this error, reorder the constraint clauses.

Example

The following sample generates CS0449.

// CS0449.cs  
// compile with: /target:library  
public interface I {}  // Made public to avoid CS0703  

public class C4
{  
   public void F1<T>() where T : class, struct, I {}   // CS0449  
   public void F2<T>() where T : I, struct {}   // CS0449  
   public void F3<T>() where T : I, class {}   // CS0449  

   // OK  
   public void F4<T>() where T : class {}  
   public void F5<T>() where T : struct {}  
   public void F6<T>() where T : I {}  
}