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Compiler Error C3391

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'type_arg' : invalid type argument for generic parameter 'param' of generic 'generic_type', must be a non-nullable value type

A generic type was instantiated incorrectly. Check the type definition. For more information, see Nullable and Generics.

Example

The following sample uses C# to create a component that contains a generic type that has certain constraints that are not supported when authoring generic types in C++/CLI. For more information, see Constraints on Type Parameters.

// C3391.cs
// Compile by using: csc /target:library C3391.cs
// a C# program
public class GR<N>
where N : struct {}

When the C3391.dll component is available, the following sample generates C3391.

// C3391_b.cpp
// Compile by using: cl /clr C3391_b.cpp
#using <C3391.dll>
using namespace System;
value class VClass {};

int main() {
   GR< Nullable<VClass> >^ a =
      gcnew GR< Nullable<VClass> >();   // C3391 can't be Nullable
   GR<VClass>^ aa = gcnew GR<VClass>(); // OK
}