Compiler Error CS0731

The type forwarder for type 'type' in assembly 'assembly' causes a cycle

This error can only occur with improperly formed imported metadata. It cannot occur with only C# source.

Example

The following sample generates CS0731. The example consists of three files:

1. Circular.IL

2. Circular2.IL

3. CS0731.cs

First compile the .IL files as libraries, and then compile the .cs code referencing the two files.

// Circular.il
// compile with: /DLL /out=Circular.dll
.assembly extern circular2
{
  .ver 0:0:0:0
}
.assembly extern circular3
{
  .ver 0:0:0:0
}
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )   // .z\V.4..
  .ver 2:0:0:0
}
.assembly Circular
{
  .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
  .hash algorithm 0x00008004
  .ver 0:0:0:0
}
.class extern forwarder Circular.Referenced.TypeForwarder
{
  .assembly extern circular2
}
.module Circular.dll
// MVID: {880C2329-C915-42A0-83E9-9D10C3E6DBD0}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       // WINDOWS_CUI
.corflags 0x00000001    //  ILONLY
// Image base: 0x04E40000
// ======== CLASS MEMBERS DECLARATION =========
.class public abstract auto ansi sealed beforefieldinit User
       extends [mscorlib]System.Object
{
  .method public hidebysig static class [circular2]Circular.Referenced.TypeForwarder 
          F() cil managed
  {
    .maxstack  1
    newobj     instance void [circular2]Circular.Referenced.TypeForwarder::.ctor()
    ret
  }
} 

// Circular2.il
// compile with: /DLL /out=Circular2.dll
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )   // .z\V.4..
  .ver 2:0:0:0
}
.assembly extern Circular
{
  .ver 0:0:0:0
}
.assembly circular2
{
  .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
  .hash algorithm 0x00008004
  .ver 0:0:0:0
}
.class extern forwarder Circular.Referenced.TypeForwarder
{
  .assembly extern Circular
}
.module circular2.dll
// MVID: {8B3BE5C8-DBE1-49C4-BC72-DF35F0387C21}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       //   WINDOWS_CUI
.corflags 0x00000001    //  ILONLY
// Image base: 0x04E40000

// CS0731.cs
// compile with: /reference:circular.dll /reference:circular2.dll
// CS0731 expected
class A {
   public static void Main() {
      User.F();
   }
}