Dynamically Loading an ASP.NET Core Assembly

I am trying to dynamically load a .NET Core DLL containing an ASP.NET Core's Startup class. I then want to instantiate the Startup class and hand it over to the TestHost API, so I can start the site in memory. I've written the following code:

var directoryPath = @"C:\Dlls";
var assemblyFilePath = Path.Combine(directoryPath, "Foo.dll");
var assemblyName = AssemblyLoadContext.GetAssemblyName(assemblyFilePath);
var assembly = new AssemblyLoader(directoryPath).LoadFromAssemblyName(assemblyName);
var startupType = assembly.ExportedTypes
    .FirstOrDefault(x => string.Equals(x.Name, "Startup"));
var webHostBuilder = new WebHostBuilder()
    .UseStartup(startupType)
    .UseUrls(new string[] { "http://localhost" });
using (var testServer = new TestServer(webHostBuilder))
{
    var response = testServer.CreateClient().GetAsync("/");
}

public class AssemblyLoader : AssemblyLoadContext
{
    private readonly string directoryPath;

    public AssemblyLoader(string directoryPath) =>
        this.directoryPath = directoryPath;

    protected override Assembly Load(AssemblyName assemblyName)
    {
        var dependencyContext = DependencyContext.Default;
        var compilationLibraries = dependencyContext
            .CompileLibraries
            .Where(x => x.Name.Contains(assemblyName.Name))
            .ToList();
        if (compilationLibraries.Count > 0)
        {
            return Assembly.Load(new AssemblyName(compilationLibraries.First().Name));
        }
        else
        {
            var file = new FileInfo($"{this.directoryPath}{Path.DirectorySeparatorChar}{assemblyName.Name}.dll");
            if (File.Exists(file.FullName))
            {
                var asemblyLoader = new AssemblyLoader(file.DirectoryName);
                return asemblyLoader.LoadFromAssemblyPath(file.FullName);
            }
        }

        return Assembly.Load(assemblyName);
    }
}

However, this throws a TypeLoadException when assembly.ExportedTypes is called. The full stack trace:

System.TypeLoadException occurred HResult=0x80131522
Message=Method 'ConfigureServices' in type 'Foo.Startup' from assembly 'Foo, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation. Source= StackTrace: at System.Reflection.RuntimeAssembly.GetExportedTypes(RuntimeAssembly assembly, ObjectHandleOnStack retTypes) at System.Reflection.RuntimeAssembly.GetExportedTypes()

I think it's having trouble loading the ASP.NET Core and other third party DLL's like Newtonsoft.Json.