Working with AppDomain.AssemblyResolve event

You can define a dictionary of the assemblies from your directory, like this:

private readonly IDictionary<string,Assembly> additional =
    new Dictionary<string,Assembly>();

Load this dictionary with the assemblies from your known directory, like this:

foreach ( var assemblyName ... corresponding to DLL names in your directory... ) {
    var assembly = Assembly.Load(assemblyName);
    additional.Add(assembly.FullName, assembly);
}

Provide an implementation for the hook...

private Assembly ResolveAssembly(Object sender, ResolveEventArgs e) {
    Assembly res;
    additional.TryGetValue(e.Name, out res);
    return res;
}

...and hook it up to the event:

AppDomain.CurrentDomain.ReflectionOnlyAssemblyResolve += ResolveAssembly;
AppDomain.CurrentDomain.AssemblyResolve += ResolveAssembly;

This should do the trick.