.net core中Grpc使用报错：The remote certificate is invalid according to the validation procedure.

因为Grpc采用HTTP/2作为通信协议，默认采用LTS/SSL加密方式传输，比如使用.net core启动一个服务端（被调用方）时：　　

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.ConfigureKestrel(options =>
                {
                    options.ListenAnyIP(5000, listenOptions =>
                    {
                        listenOptions.Protocols = HttpProtocols.Http2;
                        listenOptions.UseHttps("xxxxx.pfx", "password");
                    });
                });
                webBuilder.UseStartup<Startup>();
            });

　　其中使用UseHttps方法添加证书和秘钥。

　　但是，有时候，比如开发阶段，我们可能没有证书，或者是一个自己制作的临时测试证书，那么在客户端（调用方）调用是可能就会出现下面的异常：　　

　　Call failed with gRPC error status. Status code: 'Internal', Message: 'Error starting gRPC call. HttpRequestException: The SSL connection could not be established, see inner exception. AuthenticationException: The remote certificate is invalid according to the validation procedure.'.
　　fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1]
   An unhandled exception has occurred while executing the request.
　　Grpc.Core.RpcException: Status(StatusCode="Internal", Detail="Error starting gRPC call. HttpRequestException: The SSL connection could not be established, see inner exception. AuthenticationException: The remote certificate is invalid according to the validation procedure.", DebugException="System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception.
　　---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.
   at System.Net.Security.SslStream.StartSendAuthResetSignal(ProtocolToken message, AsyncProtocolRequest asyncRequest, ExceptionDispatchInfo exception)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.ProcessReceivedBlob(Byte[] buffer, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.StartReadFrame(Byte[] buffer, Int32 readBytes, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslStream.PartialFrameCallback(AsyncProtocolRequest asyncRequest)
    --- End of stack trace from previous location where exception was thrown ---
   at System.Net.Security.SslStream.ThrowIfExceptional()
   at System.Net.Security.SslStream.InternalEndProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.Security.SslStream.EndProcessAuthentication(IAsyncResult result)
   at System.Net.Security.SslStream.EndAuthenticateAsClient(IAsyncResult asyncResult)
　　..........

　　然而我们可能没有办法得到有效的证书，这时，我们有两个办法：

　　1、使用http协议

　　想想，我们为什么要使用Grpc？因为高性能，高效率，简单易用吧，但是https相比http就是多个加密的过程，这可能会有一定的性能损失（一般可忽略）。

　　而一般的，我们在微服务架构中使用Grpc比较多，而微服务一般部署在我们自己的一个子网下，这也就没必要使用https了吧？

　首先我们知道，Grpc是基于HTTP/2作为通信协议的，而HTTP/2默认是基于LTS/SSL加密技术的，或者说默认需要https协议支持（https=http+lts/ssl），而HTTP/2又支持明文传输，即对http也是支持，但是一般需要我们自己去设置。

　　当我们使用Grpc时，又不去改变这个默认行为，那可能就会导致上面的报错。

　　在.net core开发中，Grpc要支持http，我们需要显示的指定不需要TLS支持，官方给出的做法是添加如下配置（比如客户端（调用方）在ConfigureServices添加）：　　

    public void ConfigureServices(IServiceCollection services)
    {
        //显式的指定HTTP/2不需要TLS支持
        AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true); 
        AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2Support", true);

        services.AddGrpcClient<Greeter.GreeterClient>(nameof(Greeter.GreeterClient), options =>
        {
            options.Address = new Uri("http://localhost:5000");
        });

        ...
    }

　　2、调用时不对证书进行验证

　　如果是控制台程序，我们可以这么做：　　

    public static void Main(string[] args)
    {
        var channel = GrpcChannel.ForAddress("https://localhost:5000", new GrpcChannelOptions()
        {
            HttpClient = null,
            HttpHandler = new HttpClientHandler
            {
                //方法一
                ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator
                //方法二
                //ServerCertificateCustomValidationCallback = (a, b, c, d) => true
            }
        });

        var client = new Greeter.GreeterClient(channel);
        var result = client.SayHello(new HelloRequest() { Name = "Grpc" });
    }

　　其中 HttpClientHandler 的 ServerCertificateCustomValidationCallback 是对证书的自定义验证，上面给出了两种方式验证。

　　如果是.net core的webmvc或者webapi程序，因为.net core 3.x开始已经支持了Grpc的引入，所以我只需要在ConfigureServices中注入Grpc的客户端是进行设置：　　

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddGrpcClient<Greeter.GreeterClient>(nameof(Greeter.GreeterClient), options =>
        {
            options.Address = new Uri("https://localhost:5000");
        }).ConfigurePrimaryHttpMessageHandler(() =>
        {
            return new HttpClientHandler
            {
                //方法一
                ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator
                //方法二
                //ServerCertificateCustomValidationCallback = (a, b, c, d) => true
            };
        });

        ...
    }

　　因为.net core3.x中Grpc的使用是基于它的HttpClient机制，比如 AddGrpcClient 方法返回的就是一个 IHttpClientBuilder 接口对象，上面的配置我们还可以这么写：　　

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddGrpcClient<Greeter.GreeterClient>(nameof(Greeter.GreeterClient));
        services.AddHttpClient(nameof(Greeter.GreeterClient), httpClient =>
        {
            httpClient.BaseAddress = new Uri("https://localhost:5000");
        }).ConfigurePrimaryHttpMessageHandler(() =>
        {
            return new HttpClientHandler
            {
                //方法一
                ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator
                //方法二
                //ServerCertificateCustomValidationCallback = (a, b, c, d) => true
            };
        });

        ...
    }