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POWhy am I getting this exception when emitting classes that reference each other via value-type generics?
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  1. POWhy am I getting this exception when emitting classes that reference each other via value-type generics?
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    copied!<p>This code snippet is a simplified extract of my class-generation code, which creates two classes that reference each other as arguments in a generic type:</p> <pre><code>namespace Sandbox { using System; using System.Reflection; using System.Reflection.Emit; internal class Program { private static void Main(string[] args) { var assembly = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("Test"), AssemblyBuilderAccess.Run); var module = assembly.DefineDynamicModule("Test"); var typeOne = module.DefineType("TypeOne", TypeAttributes.Public); var typeTwo = module.DefineType("TypeTwo", TypeAttributes.Public); typeOne.DefineField("Two", typeof(TestGeneric&lt;&gt;).MakeGenericType(typeTwo), FieldAttributes.Public); typeTwo.DefineField("One", typeof(TestGeneric&lt;&gt;).MakeGenericType(typeOne), FieldAttributes.Public); typeOne.CreateType(); typeTwo.CreateType(); Console.WriteLine("Done"); Console.ReadLine(); } } public struct TestGeneric&lt;T&gt; { } } </code></pre> <p>Which should produce MSIL equivalent to the following:</p> <pre><code>public class TypeOne { public Program.TestGeneric&lt;TypeTwo&gt; Two; } public class TypeTwo { public Program.TestGeneric&lt;TypeOne&gt; One; } </code></pre> <p>But instead throws this exception on the line <code>typeOne.CreateType()</code>:</p> <pre><code>System.TypeLoadException was unhandled Message=Could not load type 'TypeTwo' from assembly 'Test, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'. Source=mscorlib TypeName=TypeTwo StackTrace: at System.Reflection.Emit.TypeBuilder.TermCreateClass(RuntimeModule module, Int32 tk, ObjectHandleOnStack type) at System.Reflection.Emit.TypeBuilder.CreateTypeNoLock() at System.Reflection.Emit.TypeBuilder.CreateType() at Sandbox.Program.Main(String[] args) in C:\Users\aca1\Code\Sandbox\Program.cs:line 20 </code></pre> <p>Interesting things to note:</p> <ul> <li>The circular reference isn't required to cause the exception; if I don't define field <code>One</code> on <code>TypeTwo</code>, creating <code>TypeOne</code> before <code>TypeTwo</code> still fails, but creating <code>TypeTwo</code> before <code>TypeOne</code> succeeds. Therefore, the exception is specifically caused by using a type that has not yet been created as an argument in a generic field type; however, because I need to use a circular reference, I cannot avoid this situation by creating the types in a specific order.</li> <li>Yes, I <em>do</em> need to use a circular reference.</li> <li>Removing the wrapper <code>TestGeneric&lt;&gt;</code> type and declaring the fields as <code>TypeOne</code> &amp; <code>TypeTwo</code> directly does not produce this error; thus I <em>can</em> use dynamic types that have been defined but not created.</li> <li>Changing <code>TestGeneric&lt;&gt;</code> from a <code>struct</code> to a <code>class</code> does not produce this error; so this pattern <em>does</em> work with most generics, just not generic value types.</li> <li>I can't change the declaration of <code>TestGeneric&lt;&gt;</code> in my case as it is declared in another assembly - specifically, <code>System.Data.Linq.EntityRef&lt;&gt;</code> declared in System.Data.Linq.dll.</li> <li>My circular reference is caused by representing two tables with foreign key references to each other; hence the need for that specific generic type and this specific pattern.</li> <li>Changing the circular reference to a self-reference <strong>edit</strong> succeeds. This failed originally because I had <code>TestGeneric&lt;&gt;</code> as a nested type in Program, so it inherited the <code>internal</code> visibility. I've fixed this now in the code sample above, and it does in fact work.</li> <li>Compiling the generated code manually (as C# code) also works, so it's not an obscure compiler issue.</li> </ul> <p>Any ideas on a) why this occuring, b) how I can fix this and/or c) how I can work around it?</p> <p>Thanks.</p>
 

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