为什么我的代码会抛出Invalid Cast异常?(C#)

错误信息：System.InvalidCastException：无法将"ClassLibrary1.Plugin"类型的对象强制转换为"PluginInterface.IPlugin"类型。

我想做的是让我的程序访问一个程序集，并运行它可能包含的任何内容。这将加载.dll

private void AddPlugin(string FileName)
{
Assembly pluginAssembly = Assembly.LoadFrom(FileName);
foreach (Type pluginType in pluginAssembly.GetTypes())
{
if (pluginType.IsPublic)
{
if (!pluginType.IsAbstract)
{
Type typeInterface = pluginType.GetInterface("PluginInterface… true);
if (typeInterface != null)
{
Types.AvailablePlugin newPlugin = new Types.AvailablePlugin();
newPlugin.AssemblyPath = FileName;
newPlugin.Instance = (IPlugin)Activator.CreateInstance(plugin…
// Above line throws exception.
newPlugin.Instance.Initialize();
this.colAvailablePlugins.Add(newPlugin);
newPlugin = null;
}
typeInterface = null;
}
}
}
pluginAssembly = null;
}

我的程序和程序集都有这两个接口：

using System;
namespace PluginInterface
{
public interface IPlugin
{
IPluginHost Host { get; set; }
string Name { get; }
string Description { get; }
string Author { get; }
string Version { get; }
System.Windows.Forms.Form MainInterface { get; }
void Initialize();
void Dispose();
void ReceivedMessage(PlayerIOClient.Message m);
void Disconnected();
}
public interface IPluginHost
{
void Say(string message);
void Send(PlayerIOClient.Message m);
void Send(string message_Type, params object[] paramss);
}
}

我要添加的类/程序集：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;
using PluginInterface;
namespace ClassLibrary1
{
public class Plugin : IPlugin // <-- See how we inherited the IPlugin interface?
{
public Plugin()
{
}
string myName = "Title";
string myDescription = "Descrip";
string myAuthor = "Me";
string myVersion = "0.9.5";

IPluginHost myHost = null;
Form1 myMainInterface = new Form1();

public string Description
{
get { return myDescription; }
}
public string Author
{
get { return myAuthor; }
}
public IPluginHost Host
{
get { return myHost; }
set { myHost = value; }
}
public string Name
{
get { return myName; }
}
public System.Windows.Forms.Form MainInterface
{
get { return myMainInterface; }
}
public string Version
{
get { return myVersion; }
}
public void Initialize()
{
//This is the first Function called by the host...
//Put anything needed to start with here first
MainInterface.Show();
}
public void ReceivedMessage(PlayerIOClient.Message m)
{
}
public void Disconnected()
{
}
public void Dispose()
{
MainInterface.Dispose();
}
}
}

我们非常感谢所有的帮助。