Compiler Error CS0012

The type 'type' is defined in an assembly that is not referenced. You must add a reference to assembly 'assembly'.

The definition for a referenced type was not found. This could occur if a required .DLL file is not included in the compilation. For more information, see Add Reference Dialog Box and /reference (Import Metadata) (C# Compiler Options).

The following sequence of compilations will result in CS0012:

// cs0012a.cs
// compile with: /target:library
public class A {}

Then:

// cs0012b.cs
// compile with: /target:library /reference:cs0012a.dll
public class B
{
   public static A f()
   {
      return new A();
   }
}

Then:

// cs0012c.cs
// compile with: /reference:cs0012b.dll
class C
{
   public static void Main()
   {
      object o = B.f();   // CS0012
   }
}

You could resolve this CS0012 by compiling with /reference:cs0012b.dll;cs0012a.dll, or in Visual Studio by using the Add Reference Dialog Box to add a reference to cs0012a.dll in addition to cs0012b.dll.

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