Program.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;
using HostApp;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using PlugInInterface;

namespace PlugIns
{
    class Program
    {
        static void Main()
        {
            string plugInDllPath = Path.Combine(
                Path.GetDirectoryName(typeof(Program).Assembly.Location),
                "MyPlugIn.dll");

            Assembly[] plugInApiAssemblies =
            {
                typeof(IPlugIn).Assembly,
                typeof(JsonReader).Assembly
            };
            var plugInAssemblyNames = new HashSet<string>(
                plugInApiAssemblies.Select(a => a.GetName().Name));

            var ctx = new PlugInLoadContext(plugInDllPath, plugInAssemblyNames);
            Assembly plugInAssembly = ctx.LoadFromAssemblyPath(plugInDllPath);

            var pi = (IPlugIn)plugInAssembly.CreateInstance("MyPlugIn.MyPlugInType");

            pi.Foo(new JObject());
        }
    }
}