public interface IUrlRewriter
{
    Task<Uri> RewriteUri(HttpContext context);
}

public class PrefixRewriter : IUrlRewriter
  {
    private readonly PathString _prefix; //前缀值
    private readonly string _newHost; //转发的地址

    public PrefixRewriter(PathString prefix, string newHost)
    {
      _prefix = prefix;
      _newHost = newHost;
    }

    public Task<Uri> RewriteUri(HttpContext context)
    {
      if (context.Request.Path.StartsWithSegments(_prefix))//判断访问是否含有前缀
      {
        var newUri = context.Request.Path.Value.Remove(0, _prefix.Value.Length) + context.Request.QueryString;
        var targetUri = new Uri(_newHost + newUri);
        return Task.FromResult(targetUri);
      }

      return Task.FromResult((Uri)null);
    }
  }

public class ProxyHttpClient
  {
    public HttpClient Client { get; private set; }

    public ProxyHttpClient(HttpClient httpClient)
    {
      Client = httpClient;
    }
  }

public class ProxyMiddleware
  {
    // private ProxyHttpClient _proxyHttpClient;
    private const string CDN_HEADER_NAME = "Cache-Control";
    private static readonly string[] NotForwardedHttpHeaders = new[] { "Connection", "Host" };
    private readonly RequestDelegate _next;
    private readonly ILogger<ProxyMiddleware> _logger;

    public ProxyMiddleware(
        RequestDelegate next,
        ILogger<ProxyMiddleware> logger
        
        )
    {
      _next = next;
      _logger = logger;
      //_proxyHttpClient = proxyHttpClient;
    }

    /// <summary>
    /// 通过中间件,拦截访问,检测前缀,并转发
    /// </summary>
    /// <param name="context"></param>
    /// <param name="urlRewriter"></param>
    /// <returns></returns>
    public async Task Invoke(HttpContext context, IUrlRewriter urlRewriter, ProxyHttpClient proxyHttpClient)
    {
      var targetUri = await urlRewriter.RewriteUri(context);

      if (targetUri != null)
      {
        var requestMessage = GenerateProxifiedRequest(context, targetUri);
        await SendAsync(context, requestMessage, proxyHttpClient);

        return;
      }

      await _next(context);
    }

    private async Task SendAsync(HttpContext context, HttpRequestMessage requestMessage, ProxyHttpClient proxyHttpClient)
    {
    

      using (var responseMessage = await proxyHttpClient.Client.SendAsync(requestMessage, HttpCompletionOption.ResponseHeadersRead, context.RequestAborted))
      {
        context.Response.StatusCode = (int)responseMessage.StatusCode;

        foreach (var header in responseMessage.Headers)
        {
          context.Response.Headers[header.Key] = header.Value.ToArray();
        }

        foreach (var header in responseMessage.Content.Headers)
        {
          context.Response.Headers[header.Key] = header.Value.ToArray();
        }

        context.Response.Headers.Remove("transfer-encoding");

        if (!context.Response.Headers.ContainsKey(CDN_HEADER_NAME))
        {
          context.Response.Headers.Add(CDN_HEADER_NAME, "no-cache, no-store");
        }

        await responseMessage.Content.CopyToAsync(context.Response.Body);
      }
    }

    private static HttpRequestMessage GenerateProxifiedRequest(HttpContext context, Uri targetUri)
    {
      var requestMessage = new HttpRequestMessage();
      CopyRequestContentAndHeaders(context, requestMessage);
      requestMessage.RequestUri = targetUri;
      requestMessage.Headers.Host = targetUri.Host;
      requestMessage.Method = GetMethod(context.Request.Method);
      return requestMessage;
    }

    private static void CopyRequestContentAndHeaders(HttpContext context, HttpRequestMessage requestMessage)
    {
      var requestMethod = context.Request.Method;
      if (!HttpMethods.IsGet(requestMethod) &&
        !HttpMethods.IsHead(requestMethod) &&
        !HttpMethods.IsDelete(requestMethod) &&
        !HttpMethods.IsTrace(requestMethod))
      {
        var streamContent = new StreamContent(context.Request.Body);
        requestMessage.Content = streamContent;
      }

      foreach (var header in context.Request.Headers)
      {
        if (!NotForwardedHttpHeaders.Contains(header.Key))
        {
          if (header.Key != "User-Agent")
          {
            if (!requestMessage.Headers.TryAddWithoutValidation(header.Key, header.Value.ToArray()) && requestMessage.Content != null)
            {
              requestMessage.Content" " + context.TraceIdentifier) : string.Empty;

            if (!requestMessage.Headers.TryAddWithoutValidation(header.Key, userAgent) && requestMessage.Content != null)
            {
              requestMessage.Content"color: #ff0000">四.注入和启用我们的中间件和ProxyHttpClient

我们在Startup的ConfigureServices中添加如下代码,注入我们的HttpClient与IUrlRewriter,如下:


services.AddHttpClient<ProxyHttpClient>()
      .ConfigurePrimaryHttpMessageHandler(x => new HttpClientHandler()
      {
        AllowAutoRedirect = false,
        MaxConnectionsPerServer = int.MaxValue,
        UseCookies = false,
      }); //注入我们定义的HttpClient
 services.AddSingleton<IUrlRewriter>(new PrefixRewriter("/webapp", "http://localhost:63445"));//这里填写前缀与需要转发的地址


然后在Startup的Configure中,启动我们的中间件,如下:


app.UseMiddleware<ProxyMiddleware>();


五.测试中间件效果
我们编写前端代码如下:


created: function () {
        this.mockTableData1();
        axios.get("/webapp/api/values/get", "123").then(res => { alert(res.data[0]) });
        axios.post("/webapp/api/values/post",{value: 'david'}).then(res => { alert(res.data.message) });

      }


在另外的WebApi项目,编写接口如下:


[HttpGet]
    public ActionResult<IEnumerable<string Get()
    {
      return new string[] { "value1", accstring.ToString() };
    }

    [HttpPost]
    public AjaxResult Post(dynamic value)
    {
      string aaa = JsonConvert.SerializeObject(value);
      return Success("OK");
    }


效果如下,可以看到我们的视图正确的获取到了返回值:

写在最后
这里我们通过中间件的形式实现了接口的代理转发,在具体的使用过程中肯定还会有一些小问题,而且这里我们只实现了Http的转发.ws的则没有.
如果要使用的话,其实国外有一个开源的项目:https://github.com/ProxyKit, 已经有900多个star了.应该还不错.