NullReferenceException when trying to resolve ModuleReference members

The testcode i used is copied from https://docs.microsoft.com/en-us/dotnet/framework/app-domains/build-multifile-assembly

Scenario:

I compiled the tutorial from the link above. This yields the following files: myAssembly.exe Client.netmodule and Stringer.netmodule I’m trying to resolve as many Reference members into their matching Definitions (e.g. TypeReference --> TypeDefinition)

Error Description:

i’m running the following code:

 var appA = AssemblyDefinition.ReadAssembly("myAssembly.exe");

// Get all types from module 'Client.netmodule'
foreach (var t in modules_A.Skip(1).First().Types)
{
    if (t.Name == "<Module>")
        continue;

    // IL_0013: callvirt System.Void myStringer.Stringer::StringerMethod()
    var callInst = t.Methods.First(m => m.Name.Equals("Main")).Body.Instructions[7];

    if (callInst.Operand is not MethodReference methodReference)
        continue;

    // Type Name: myStringer.Stringer
    // TypeRef Scope: Stringer.netmodule ScopeType:ModuleReference
    var methodTypeRef = methodReference.DeclaringType;

    var methodTypeDef = methodTypeRef.Resolve();
}

The line var methodTypeDef = methodTypeRef.Resolve(); fails with an NRE becuase:

• methodTypeRef 's module is ‘Client.netmodule’
• The ModuleDefinition of ‘Client.netmodule’ has no Assembly (null). According to ECMA this is valid.
• The MetadataResolver implementation is as follows: var modules = type.Module.Assembly.Modules; As you see it assumes, a ModuleDefinition always has an Assembly assigned to it.

Solutions

I see a couple of solutions here.

1. Check whether the module has an assembly a. return null in that case, indicating that it was impossible to find a proper TypeDefinition b. Have some heuristic-based logic which tries to find a ModuleDefinition which matches the ModuleReferenc’s name. Either lookup the assembly resolver’s lookup directories for such a module or try on some cached ModuleDefinitions. Return null when no module was found or no module contained to TypeDefinition of the searched TypeReference. Or return the TypeDefinition if found. I think it should be noted however that this approach might return a wrong TypeDefinition if the wrong module matched.
2. Same as for 1. But instead of returning null, throw a proper exception.

There are probably more and smarter solutions but I think that’s up to you to decide @jbevain

[jbevain]

Thank you, I’ll have a look!

(Using netmodules is not a good idea they’re just a source of pain on earth)

[jbevain]

Fixed by #730