Embedded Assembly Loading with support for Symbols and Portable Class Libraries in C#

REPOST: Of an old blog post from 2014.

Jeffrey Richter has previously written about how to deploy a single executable file for an application by embedding dependencies as resources in the main application assembly in Jeffrey Richter: Excerpt #2 from CLR via C#, Third Edition.

However, this solution has a few issues. It does not handle Portable Class Libraries (PCLs) and does not show how to support loading symbols from embedded pdb-files either. The code presented below handles both.

As with the solution Jeffrey Richter details, one simply adds a handler to the current domains AssemblyResolve event, which is called whenever an assembly could not be resolved directly. However, this also occurs when an embedded portable class library (such as Autofac) has defined a dependency towards any BCL assembly (e.g. System.Core 2.0.5.0), in this case you have to check if the assembly is retargetable and then load it directly via the usual CLR mechanism so the appropriate version is loaded.

For a better debug experience and better exception stack traces it is recommended to include pdb-files as well. pdb-files are handled by simply loading these if they have been embedded, have the same name as the dll-file and then using the Assembly.Load overload that also loads raw symbol data from a byte array.

To use the code do the following:

  • Call SetupEmbeddedAssemblyResolve as the first thing in your application
  • Add dependencies incl. pdb-files, if needed, to your project via Add as Link and change the build action to Embedded Resource and Copy to Output Directory to Do not copy
  • Change the assembly references properties under References and set Copy Local to false

UPDATE: See Automatically Embed Copy Local Assemblies with Symbols in MSBuild for how to automatically embed assemblies instead of doing this manually.

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private static void SetupEmbeddedAssemblyResolve()
{
    AppDomain.CurrentDomain.AssemblyResolve += (sender, args) =>
    {
        var name = args.Name;
        var asmName = new AssemblyName(name);

        // Any retargetable assembly should be resolved directly using normal load e.g. System.Core issue 
        if (name.EndsWith("Retargetable=Yes"))
        {
            return Assembly.Load(asmName);
        }

        var executingAssembly = Assembly.GetExecutingAssembly();
        var resourceNames = executingAssembly.GetManifestResourceNames();

        var resourceToFind = asmName.Name + ".dll";
        var resourceName = resourceNames.SingleOrDefault(n => n.Contains(resourceToFind));

        if (string.IsNullOrWhiteSpace(resourceName)) { return null; }

        var symbolsToFind = asmName.Name + ".pdb";
        var symbolsName = resourceNames.SingleOrDefault(n => n.Contains(symbolsToFind));

        var assemblyData = LoadResourceBytes(executingAssembly, resourceName);

        if (string.IsNullOrWhiteSpace(symbolsName))
        { 
            Trace.WriteLine(string.Format("Loading '{0}' as embedded resource '{1}'", resourceToFind, resourceName));

            return Assembly.Load(assemblyData);
        }
        else
        {
            var symbolsData = LoadResourceBytes(executingAssembly, symbolsName);

            Trace.WriteLine(string.Format("Loading '{0}' as embedded resource '{1}' with symbols '{2}'", resourceToFind, resourceName, symbolsName));

            return Assembly.Load(assemblyData, symbolsData);
        }
    };
}

private static byte\[\] LoadResourceBytes(Assembly executingAssembly, string resourceName)
{
    using (var stream = executingAssembly.GetManifestResourceStream(resourceName))
    {
        var data = new byte\[stream.Length\];

        stream.Read(data, 0, data.Length);

        return data;
    }
}

Based on Jeffrey Richter: Excerpt #2 from CLR via C#, Third Edition and FileNotFoundException when trying to load Autofac as an embedded assembly.

2020.10.01