Using an in-memory assembly as metadata reference in another in-memory assembly in .NET

Question

I'm using Roslyn to compile strings of C# code. I'd like to use the output of one of the compilations as metadata reference for another, but when I try to invoke a method that forces the loading of the referenced assembly, I get an ImageNotFoundException, complaining that the referenced assembly (MyReference.dll in the sample case) could not be found. This is the minified version of my problem:

internal class Program
{
    public static MemoryStream CompileCSharp(
        string assemblyName,
        string code,
        params MetadataReference[] additionalReferences)
    {
        var sourceText = SourceText.From(code, Encoding.UTF8);
        var tree = SyntaxFactory.ParseSyntaxTree(sourceText);

        var defaultReferences = Basic.Reference.Assemblies.Net70.ReferenceInfos.All
            .Select(r => MetadataReference.CreateFromStream(new MemoryStream(r.ImageBytes)));

        var compilation = CSharpCompilation.Create(
            assemblyName: assemblyName,
            syntaxTrees: new[] { tree },
            references: defaultReferences.Concat(additionalReferences),
            options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

        var stream = new MemoryStream();
        var emitResult = compilation.Emit(stream);
        Debug.Assert(emitResult.Success);

        stream.Position = 0;
        return stream;
    }

    static void Main(string[] args)
    {
        var referenceBytes = CompileCSharp(
            "MyReference.dll",
            """
                public class IdentityProvider
                {
                    public T Identity<T>(T x) => x;
                }
                """);
        var reference = MetadataReference.CreateFromStream(referenceBytes);

        var mainAssembly = CompileCSharp(
            "Main.dll",
            """
                using System;

                public class Program
                {
                    public static void Main()
                    {
                        var provider = new IdentityProvider();
                        Console.WriteLine(provider.Identity(123));
                    }
                }
                """,
            reference);

        var asm = Assembly.Load(mainAssembly.ToArray());
        asm.GetType("Program")!.GetMethod("Main")!.Invoke(null, null);
    }
}

The references I use:

<PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.6.0-1.final" /> (Roslyn itself)
<PackageReference Include="Basic.Reference.Assemblies.Net70" Version="1.4.1" /> (to not have to deal with metadata management of .NET Core)

Without referencing anything, a hello world program compiles fine, making me believe that there is some runtime quirk I do not know about, like requiring the file to be on disk or something. However, I could not verify this.

Peter Lenkefi · Accepted Answer · 2023-05-20 15:03:30Z

1

As it turns out, the runtime simply does not know where to locate the assembly. While the other metadata references are in well-known places, MyReference.dll does not exist anywhere other than in-memory. I suppose this is because the runtime does not really care about where a metadata reference was read from, but I'm really not sure.

The solution is simple, one can just create an AssemblyLoadContext and manually point at the correct bytes:

var loadContext = new AssemblyLoadContext("testLoadContext");
loadContext.Resolving += (loader, name) =>
{
    if (name.Name!.Contains("MyReference.dll"))
    {
        return Assembly.Load(referenceBytes.ToArray());
    }
    return null;
};

var asm = loadContext.LoadFromStream(mainAssembly);
asm.GetType("Program")!.GetMethod("Main")!.Invoke(null, null);

answered May 20, 2023 at 15:03

Peter Lenkefi

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