如何使用中间件拦截botbuilder sdk v4中的消息？

using Microsoft.Bot.Builder;
using Microsoft.Bot.Schema;
using System.Threading;
using System.Threading.Tasks;

public class CustomMiddleware : IMiddleware
{
    public async Task OnTurnAsync(ITurnContext turnContext, NextDelegate next, CancellationToken cancellationToken = default(CancellationToken))
    {
        // 拦截消息
        if (turnContext.Activity.Type == ActivityTypes.Message)
        {
            // 处理消息
            await turnContext.SendActivityAsync("这是自定义中间件拦截的消息");
        }

        // 调用下一个中间件或处理程序
        await next(cancellationToken);
    }
}

// 在Bot类中注册中间件
public class Bot : IBot
{
    public async Task OnTurnAsync(ITurnContext turnContext, CancellationToken cancellationToken = default(CancellationToken))
    {
        // 创建中间件实例
        var middleware = new CustomMiddleware();

        // 注册中间件
        turnContext.Adapter.Use(middleware);

        // 处理消息
        await turnContext.SendActivityAsync("这是Bot类中的消息处理");
    }
}