一、“标准”的gRPC定义、承载和调用

可能有些读者朋友们对ASP.NET Core gRPC还不是太熟悉，所以我们先来演示一下如何在一个ASP.NET Core应用中如何定义和承载一个简单的gRPC服务，并使用自动生成的客户端代码进行调用。我们新建一个空的解决方案，并在其中添加如下所示的三个项目。

我们在类库项目Proto中定义了如下所示Greeter服务，并利用其中定义的四个操作分别模拟四种消息交换模式。HelloRequest 和HelloReply 是它们涉及的两个ProtoBuf消息。

syntax = "proto3";import "google/protobuf/empty.proto";service Greeter {  rpc SayHelloUnary (HelloRequest) returns ( HelloReply);  rpc SayHelloServerStreaming (google.protobuf.Empty) returns (stream HelloReply);  rpc SayHelloClientStreaming (stream HelloRequest) returns (HelloReply);  rpc SayHelloDuplexStreaming (stream HelloRequest) returns (stream HelloReply);}message HelloRequest {  string name = 1;}message HelloReply {  string message = 1;}

ASP.NET Core项目中定义了如下的GreeterServce服务实现了定义的四个操作，基类GreeterBase是针对上面这个.proto文件生成的类型。

public class GreeterService: GreeterBase{    public override Task<HelloReply> SayHelloUnary(HelloRequest request, ServerCallContext context)    => Task.FromResult(new HelloReply { Message = $"Hello, {request.Name}" });    public override async Task<HelloReply> SayHelloClientStreaming(IAsyncStreamReader<HelloRequest> reader, ServerCallContext context)    {        var list = new List<string>();        while (await reader.MoveNext(CancellationToken.None))        {            list.Add(reader.Current.Name);        }        return new HelloReply { Message = $"Hello, {string.Join(",", list)}" };    }    public  override async Task SayHelloServerStreaming(Empty request, IServerStreamWriter<HelloReply> responseStream, ServerCallContext context)    {        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Foo!" });        await Task.Delay(1000);        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Bar!" });        await Task.Delay(1000);        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Baz!" });    }    public override async Task SayHelloDuplexStreaming(IAsyncStreamReader<HelloRequest> reader, IServerStreamWriter<HelloReply> writer, ServerCallContext context)    {        while (await reader.MoveNext())        {            await writer.WriteAsync(new HelloReply { Message = $"Hello {reader.Current.Name}" });        }    }}

具体的服务承载代码如下。我们采用Minimal API的形式，通过调用IServiceCollection接口的AddGrpc扩展方法注册相关服务，并调用MapGrpcService<TService>将定义在GreeterServce中的四个方法映射我对应的路由终结点。

var builder = WebApplication.CreateBuilder(args);builder.Services.AddGrpc();builder.WebHost.ConfigureKestrel(kestrel => kestrel.ConfigureEndpointDefaults(options => options.Protocols = HttpProtocols.Http2));var app = builder.Build();app.MapGrpcService<GreeterService>();app.Run();

在控制台项目Client中，我们利用生成出来的客户端类型GreeterClient分别一对应的服务交换模式调用了四个gRPC方法。

var channel = GrpcChannel.ForAddress(";);var client = new GreeterClient(channel);Console.WriteLine("Unary");await UnaryCallAsync();Console.WriteLine("\nServer Streaming");await ServerStreamingCallAsync();Console.WriteLine("\nClient Streaming");await ClientStreamingCallAsync();Console.WriteLine("\nDuplex Streaming");await DuplexStreamingCallAsync();Console.ReadLine();async Task UnaryCallAsync(){    var request = new HelloRequest { Name = "foobar" };    var reply = await client.SayHelloUnaryAsync(request);    Console.WriteLine(reply.Message);}async Task ServerStreamingCallAsync(){    var streamingCall = client.SayHelloServerStreaming(new Empty());    var reader = streamingCall.ResponseStream;    while (await reader.MoveNext(CancellationToken.None))    {        Console.WriteLine(reader.Current.Message);    }}async Task ClientStreamingCallAsync(){    var streamingCall = client.SayHelloClientStreaming();    var writer = streamingCall.RequestStream;    await writer.WriteAsync(new HelloRequest { Name = "Foo" });    await Task.Delay(1000);    await writer.WriteAsync(new HelloRequest { Name = "Bar" });    await Task.Delay(1000);    await writer.WriteAsync(new HelloRequest { Name = "Baz" });    await writer.CompleteAsync();    var reply = await streamingCall.ResponseAsync;    Console.WriteLine(reply.Message);}async Task DuplexStreamingCallAsync(){    var streamingCall = client.SayHelloDuplexStreaming();    var writer = streamingCall.RequestStream;    var reader = streamingCall.ResponseStream;    _ = Task.Run(async () =>    {        await writer.WriteAsync(new HelloRequest { Name = "Foo" });        await Task.Delay(1000);        await writer.WriteAsync(new HelloRequest { Name = "Bar" });        await Task.Delay(1000);        await writer.WriteAsync(new HelloRequest { Name = "Baz" });        await writer.CompleteAsync();    });    await foreach (var reply in reader.ReadAllAsync())    {        Console.WriteLine(reply.Message);    }}

如下所示的是客户端控制台上的输出结果。

二、将gRPC方法抽象成委托

通过上面的演示我们也知道，承载的gRPC类型最终会将其实现的方法注册成路由终结点，这一点其实和MVC是一样的。但是gRPC的方法和定义在Controller类型中的Action方法不同之处在于，前者的签名其实是固定的。如果我们将请求和响应消息类型使用Request和Reply来表示，四种消息交换模式的方法签名就可以写成如下的形式。

Task<Reply> Unary(Request request, ServerCallContext context);Task<Reply> ClientStreaming(IAsyncStreamReader<Request> reader, ServerCallContext context);Task ServerStreaming(Empty request, IServerStreamWriter<Reply> responseStream, ServerCallContext context);Task DuplexStreaming(IAsyncStreamReader<Request> reader, IServerStreamWriter<Reply> writer, ServerCallContext context);

“流式”方法中用来读取请求和写入响应的IAsyncStreamReader<T>和IServerStreamWriter<T>定义如下。

public interface IAsyncStreamReader<out T>{    T Current { get; }    Task<bool> MoveNext(CancellationToken cancellationToken = default);}public interface IAsyncStreamWriter<in T>{    Task WriteAsync(T message, CancellationToken cancellationToken = default);}public interface IServerStreamWriter<in T> : IAsyncStreamWriter<T>{}public interface IClientStreamWriter<in T> : IAsyncStreamWriter<T>{    Task CompleteAsync();}

表示服务端调用上下文的ServerCallContext 类型具有丰富的成员，但是它的本质就是对HttpContext上下文的封装，所以我们对它进行了简化。如下面的代码片段所示，我们给予这个上下文类型两个属性成员，一个是表示请求上下文的HttpContext，另一个则是用来设置响应状态StatusCode，后者对应的枚举定义了完整的gRPC状态码。

public class ServerCallContext{    public StatusCode StatusCode { get; set; } = StatusCode.OK;    public HttpContext HttpContext { get; }    public ServerCallContext(HttpContext httpContext)=> HttpContext = httpContext;}public enum StatusCode{    OK = 0,    Cancelled = 1,    Unknown = 2,    InvalidArgument = 3,    DeadlineExceeded = 4,    NotFound = 5,    AlreadyExists = 6,    PermissionDenied = 7,    Unauthenticated = 0x10,    ResourceExhausted = 8,    FailedPrecondition = 9,    Aborted = 10,    OutOfRange = 11,    Unimplemented = 12,    Internal = 13,    Unavailable = 14,    DataLoss = 0xF}

既然方法签名固定，意味着我们可以将四种gRPC方法定义成如下四个对应的委托，泛型参数TService、TRequest和TResponse分别表示服务、请求和响应类型。

public delegate Task<TResponse> UnaryMethod<TService, TRequest, TResponse>(TService service, TRequest request, ServerCallContext context)    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>;public delegate Task<TResponse> ClientStreamingMethod<TService, TRequest, TResponse>(TService service, IAsyncStreamReader<TRequest> reader, ServerCallContext context)    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>;public delegate Task ServerStreamingMethod<TService, TRequest, TResponse>(TService service, TRequest request, IServerStreamWriter<TResponse> writer, ServerCallContext context)    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>;public delegate Task DuplexStreamingMethod<TService, TRequest, TResponse>(TService service, IAsyncStreamReader<TRequest> reader, IServerStreamWriter<TResponse> writer, ServerCallContext context)    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>;

我们知道路由的本质就是创建一组路由模式（Pattern）和对应处理器之间的映射关系。路由模式很简单，对应的路由模板为“{ServiceName}/{MethodName}”，并且采用Post请求方法。对应的处理器最终体现为一个RequestDelegate。那么只要我们能够将上述四种委托类型都转换成RequestDelegate委托，一切都迎刃而解了。

三、将委托转换成RequestDelegate

为了将四种委托类型转化成RequestDelegate，我们将后者实现为一个ServiceCallHandler类型，并为其定义了如下两个基类。ServerCallHandlerBase的HandleCallAsync方法正好与RequestDelegate委托的签名一致，所以这个方法最终会用来处理gRPC请求。不同的消息交换模式采用不同的请求处理方式，只需实现抽象方法HandleCallAsyncCore就可以了。HandleCallAsync方法在调用此抽象方法之前将响应的ContentType设置成gRPC标准的响应类型“application/grpc”。在此之后将状态码设置为“grpc-status”首部，它将在HTTP2的DATA帧发送完毕后，以HEADERS帧发送到客户端。这两项操作都是gRPC协议的一部分。

public abstract class ServerCallHandlerBase{    public async Task HandleCallAsync(HttpContext httpContext)    {        try        {            var serverCallContext = new ServerCallContext(httpContext);            var response = httpContext.Response;            response.ContentType = "application/grpc";            await HandleCallAsyncCore(serverCallContext);            SetStatus(serverCallContext.StatusCode);        }        catch        {            SetStatus(StatusCode.Unknown);        }        void SetStatus(StatusCode statusCode)        {            httpContext.Response.AppendTrailer("grpc-status", ((int)statusCode).ToString());        }    }    protected abstract Task HandleCallAsyncCore(ServerCallContext serverCallContext);}public abstract class ServerCallHandler<TService, TRequest, TResponse> : ServerCallHandlerBase    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>{    protected ServerCallHandler(MessageParser<TRequest> requestParser)=> RequestParser = requestParser;    public MessageParser<TRequest> RequestParser { get; }}

ServerCallHandler<TService, TRequest, TResponse>派生自ServerCallHandlerBase，并利用三个泛型参数TService、TRequest、TResponse来表示服务、请求和响应类型，RequestParser用来提供发序列化请求消息的MessageParser<TRequest>对象。针对四种消息交换模式的ServiceCallHandler类型均继承这个泛型基类。

UnaryCallHandler

基于Unary消息交换模式的ServerCallHandler的具体类型为UnaryCallHandler<TService, TRequest, TResponse>，它由上述的UnaryMethod<TService, TRequest, TResponse>委托构建而成。在重写的HandleCallAsyncCore方法中，我们利用HttpContext提供的IServiceProvider对象将服务实例创建出来后，从请求主体中将请求消息读取出来，然后交给指定的委托对象进行处理并得到响应消息，该响应消息最终用来对当前请求予以回复。

internal class UnaryCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>{    private readonly UnaryMethod<TService, TRequest, TResponse> _handler;    public UnaryCallHandler(UnaryMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser):base(requestParser)    => _handler = handler;        protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)    {        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);        var httpContext = serverCallContext.HttpContext;        var request = await httpContext.Request.BodyReader.ReadSingleMessageAsync<TRequest>(RequestParser);        var reply = await _handler(service, request!, serverCallContext);        await httpContext.Response.BodyWriter.WriteMessageAsync(reply);    }}

请求消息是通过如下这个ReadSingleMessageAsync<TMessage>方法读取出来的。按照gRPC协议，通过网络传输的请求和响应消息都会在前面追加5个字节，第一个字节表示消息是否经过加密，后面四个字节是一个以大端序表示的整数，表示消息的长度。对于其他消息交换模式，也是调用Buffers的TryReadMessage<TRequest>方法从缓冲区中读取请求消息。

public static async Task<TMessage> ReadSingleMessageAsync<TMessage>(this PipeReader reader, MessageParser<TMessage> parser) where TMessage:IMessage<TMessage>{    while (true)    {        var result = await reader.ReadAsync();        var buffer = result.Buffer;        if (Buffers.TryReadMessage(parser, ref buffer, out var message))        {            return message!;        }        reader.AdvanceTo(buffer.Start, buffer.End);        if (result.IsCompleted)        {            break;        }    }    throw new IOException("Fails to read message.");}internal static class Buffers{    public static readonly int HeaderLength = 5;    public static bool TryReadMessage<TRequest>(MessageParser<TRequest> parser, ref ReadOnlySequence<byte> buffer, out TRequest? message) where TRequest: IMessage<TRequest>    {        if (buffer.Length < HeaderLength)        {            message = default;            return false;        }        Span<byte> lengthBytes = stackalloc byte[4];        buffer.Slice(1, 4).CopyTo(lengthBytes);        var length = BinaryPrimitives.ReadInt32BigEndian(lengthBytes);        if (buffer.Length < length + HeaderLength)        {            message = default;            return false;        }        message = parser.ParseFrom(buffer.Slice(HeaderLength, length));        buffer = buffer.Slice(length + HeaderLength);        return true;    }}

如下这个WriteMessageAsync扩展方法负责输出响应消息。

public static ValueTask<FlushResult> WriteMessageAsync(this PipeWriter writer, IMessage message){    var length = message.CalculateSize();    var span = writer.GetSpan(5 + length);    span[0] = 0;    BinaryPrimitives.WriteInt32BigEndian(span.Slice(1, 4), length);    message.WriteTo(span.Slice(5, length));    writer.Advance(5 + length);    return writer.FlushAsync();}

ClientStreamingCallHandler<TService, TRequest, TResponse>代表Client Streaming模式下的ServerCallHandler，它由对应的ClientStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要一个用来以“流”的方式读取请求的IAsyncStreamReader<TRequest>对象，它们都将作为参数传递给指定的委托，后者执行后会返回最终的响应消息。此消息同样通过上面这个WriteMessageAsync扩展方法予以回复。

internal class ClientStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>{    private readonly ClientStreamingMethod<TService, TRequest, TResponse> _handler;    public ClientStreamingCallHandler(ClientStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser)        :base(requestParser)    {        _handler = handler;    }    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)    {        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);        var reader = serverCallContext.HttpContext.Request.BodyReader;        var writer = serverCallContext.HttpContext.Response.BodyWriter;        var streamReader = new HttpContextStreamReader<TRequest>(serverCallContext.HttpContext, RequestParser);        var response = await _handler(service, streamReader, serverCallContext);        await writer.WriteMessageAsync(response);    }}

IAsyncStreamReader<T>接口的实现类型为如下这个HttpContextStreamReader<T>。在了解了请求消息在网络中的结构之后，对于实现在该类型中针对请求的读取操作，应该不难理解。

public class HttpContextStreamReader<T> : IAsyncStreamReader<T> where T : IMessage<T>{    private readonly PipeReader _reader;    private readonly MessageParser<T> _parser;    private ReadOnlySequence<byte> _buffer;    public HttpContextStreamReader(HttpContext httpContext, MessageParser<T> parser)    {        _reader = httpContext.Request.BodyReader;        _parser = parser;    }    public T Current { get; private set; } = default!;    public async Task<bool> MoveNext(CancellationToken cancellationToken)    {        var completed = false;        if (_buffer.IsEmpty)        {            var result = await _reader.ReadAsync(cancellationToken);            _buffer = result.Buffer;            completed = result.IsCompleted;        }        if (Buffers.TryReadMessage(_parser, ref _buffer, out var mssage))        {            Current = mssage!;            _reader.AdvanceTo(_buffer.Start, _buffer.End);            return true;        }        _reader.AdvanceTo(_buffer.Start, _buffer.End);        _buffer = default;        return !completed && await MoveNext(cancellationToken);    }}

ServerStreamingCallHandler

ServerStreamingCallHandler<TService, TRequest, TResponse>代表Server Streaming模式下的ServerCallHandler，它由对应的ServerStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要一个用来以“流”的方式写入响应的IAsyncStreamWriter<TResponse>对象，它们都将作为参数传递给指定的委托。

internal class ServerStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>{    private readonly ServerStreamingMethod<TService, TRequest, TResponse> _handler;    public ServerStreamingCallHandler(ServerStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser):base(requestParser)        => _handler = handler;    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)    {        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);        var httpContext = serverCallContext.HttpContext;        var streamWriter = new HttpContextStreamWriter<TResponse>(httpContext);        var request = await httpContext.Request.BodyReader.ReadSingleMessageAsync(RequestParser);        await _handler(service, request, streamWriter, serverCallContext);    }}

IAsyncStreamWriter<T>接口的实现类型为如下这个HttpContextStreamWriter<T>，它直接调用上面定义的WriteMessageAsync扩展方法将指定的消息写入响应主体的输出流。

public class HttpContextStreamWriter<T> : IServerStreamWriter<T> where T : IMessage<T>{    private readonly PipeWriter _writer;    public HttpContextStreamWriter(HttpContext httpContext) => _writer = httpContext.Response.BodyWriter;    public Task WriteAsync(T message, CancellationToken cancellationToken = default)    {        cancellationToken.ThrowIfCancellationRequested();        return _writer.WriteMessageAsync(message).AsTask();    }}

DuplexStreamingCallHandler<TService, TRequest, TResponse>代表Duplex Streaming模式下的ServerCallHandler，它由对应的DuplexStreamingMethod<TService, TRequest, TResponse>委托创建而成。在重写的HandleCallAsyncCore方法中，除了服务实例，它还需要分别创建以“流”的方式读/写请求/响应的IAsyncStreamReader<TRequest>和IAsyncStreamWriter<TResponse>对象，对应的类型分别为上面定义的HttpContextStreamReader<TRequest>和HttpContextStreamWriter<TResponse>。

internal class DuplexStreamingCallHandler<TService, TRequest, TResponse> : ServerCallHandler<TService, TRequest, TResponse>    where TService : class    where TRequest : IMessage<TRequest>    where TResponse : IMessage<TResponse>{    private readonly DuplexStreamingMethod<TService, TRequest, TResponse> _handler;    public DuplexStreamingCallHandler(DuplexStreamingMethod<TService, TRequest, TResponse> handler, MessageParser<TRequest> requestParser) :base(requestParser)        => _handler = handler;    protected override async Task HandleCallAsyncCore(ServerCallContext serverCallContext)    {        using var scope = serverCallContext.HttpContext.RequestServices.CreateScope();        var service = ActivatorUtilities.CreateInstance<TService>(scope.ServiceProvider);        var reader = serverCallContext.HttpContext.Request.BodyReader;        var writer = serverCallContext.HttpContext.Response.BodyWriter;        var streamReader = new HttpContextStreamReader<TRequest>(serverCallContext.HttpContext, RequestParser);        var streamWriter = new HttpContextStreamWriter<TResponse>(serverCallContext.HttpContext);        await _handler(service, streamReader, streamWriter, serverCallContext);    }}

目前我们将针对四种消息交换模式的gRPC方法抽象成对应的泛型委托，并且可以利用它们创建ServerCallHandler，后者可以提供作为路由终结点处理器的RequestDelegate委托。枚举和对应ServerCallHandler之间的映射关系如下所示：

UnaryMethod<TService, TRequest, TResponse>：UnaryCallHandler<TService, TRequest, TResponse>ClientStreamingMethod<TService, TRequest, TResponse>：ClientStreamingCallHandler<TService, TRequest, TResponse>ServerStreamingMethod<TService, TRequest, TResponse>：ServerStreamingCallHandler<TService, TRequest, TResponse>DuplexStreamingMethod<TService, TRequest, TResponse>：DuplexStreamingCallHandler<TService, TRequest, TResponse>

现在我们将整个路由注册的流程串起来，为此我们定义了如下这个IServiceBinder<TService>接口，它提供了两种方式将定义在服务类型TService中的gRPC方法注册成对应的路由终结点。

public interface IServiceBinder<TService> where TService : class{    IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;    IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>;}

路由终结点由路由模式和处理器两个元素组成，路由模式主要体现在由gRPC服务和操作名称组成的路由模板，我们默认使用服务类型的名称和方法名称（提出Async后缀）。为了能够对这两个名称进行定制，我们定义了如下两个特性GrpcServiceAttribute和GrpcMethodAttribute，它们可以分别标注在服务类型和操作方法上来指定一个任意的名称。

[AttributeUsage(AttributeTargets.Class)]public class GrpcServiceAttribute: Attribute{    public string? ServiceName { get; set; }}[AttributeUsage(AttributeTargets.Method)]public class GrpcMethodAttribute : Attribute{    public string? MethodName { get; set; }}

如下所示的ServiceBinder<TService> 是对IServiceBinder<TService> 接口的实现，它是对一个IEndpointRouteBuilder 对象的封装。对于实现的第一组方法，我们利用提供的方法名称与解析TService类型得到的服务名称合并，进而得到路由终结点的URL模板。这些方法还提供了一个针对gRPC方法签名的Func<TService,Func<…>>委托，我们利用它来将提供用于构建对应ServiceCallHandler的委托。我们最终利用IEndpointRouteBuilder 对象完成针对路由终结点的注册。

public class ServiceBinder<TService> : IServiceBinder<TService> where TService : class{    private readonly IEndpointRouteBuilder _routeBuilder;    public ServiceBinder(IEndpointRouteBuilder routeBuilder) => _routeBuilder = routeBuilder;    public IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        Task<TResponse> GetMethod(TService service, TRequest request, ServerCallContext context) => methodAccessor(service)(request, context);        var callHandler = new UnaryCallHandler<TService, TRequest, TResponse>(GetMethod, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, ServerCallContext, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        Task<TResponse> GetMethod(TService service, IAsyncStreamReader<TRequest> reader, ServerCallContext context) => methodAccessor(service)(reader, context);        var callHandler = new ClientStreamingCallHandler<TService, TRequest, TResponse>(GetMethod, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<TRequest, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        ServerStreamingMethod<TService, TRequest, TResponse> handler = (service, request, writer, context) => methodAccessor(service)(request, writer, context);        var callHandler = new ServerStreamingCallHandler<TService, TRequest, TResponse>(handler, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(string methodName, Func<TService, Func<IAsyncStreamReader<TRequest>, IServerStreamWriter<TResponse>, ServerCallContext, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        DuplexStreamingMethod<TService, TRequest, TResponse> handler = (service, reader, writer, context) => methodAccessor(service)(reader, writer, context);        var callHandler = new DuplexStreamingCallHandler<TService, TRequest, TResponse>(handler, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    private static string GetPath(string methodName)    {        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;        if (methodName.EndsWith("Async"))        {            methodName = methodName.Substring(0, methodName.Length - 5);        }        return $"{serviceName}/{methodName}";    }    public IServiceBinder<TService> AddUnaryMethod<TRequest, TResponse>(Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        var method = CreateDelegate<UnaryMethod<TService, TRequest,TResponse>>(methodAccessor, out var methodName);        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;        var callHandler = new UnaryCallHandler<TService, TRequest, TResponse>(method, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddClientStreamingMethod<TRequest, TResponse>( Expression<Func<TService, Task<TResponse>>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        var method = CreateDelegate<ClientStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;        var callHandler = new ClientStreamingCallHandler<TService, TRequest, TResponse>(method, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddServerStreamingMethod<TRequest, TResponse>(Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        var method = CreateDelegate<ServerStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;        var callHandler = new ServerStreamingCallHandler<TService, TRequest, TResponse>(method, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    public IServiceBinder<TService> AddDuplexStreamingMethod<TRequest, TResponse>(Expression<Func<TService, Task>> methodAccessor, MessageParser<TRequest> parser)        where TRequest : IMessage<TRequest>        where TResponse : IMessage<TResponse>    {        var method = CreateDelegate<DuplexStreamingMethod<TService, TRequest, TResponse>>(methodAccessor, out var methodName);        var serviceName = typeof(TService).GetCustomAttribute<GrpcServiceAttribute>()?.ServiceName ?? typeof(TService).Name;        var callHandler = new DuplexStreamingCallHandler<TService, TRequest, TResponse>(method, parser);        _routeBuilder.MapPost(ServiceBinder<TService>.GetPath(methodName), callHandler.HandleCallAsync);        return this;    }    private TDelegate CreateDelegate<TDelegate>(LambdaExpression expression, out string methodName) where TDelegate : Delegate    {        var method = ((MethodCallExpression)expression.Body).Method;        methodName = method.GetCustomAttribute<GrpcMethodAttribute>()?.MethodName ?? method.Name;        return (TDelegate)Delegate.CreateDelegate(typeof(TDelegate), method);    }}

由于第二组方法提供的针对gRPC方法调用的表达式，所以我们可以得到描述方法的MethodInfo对象，该对象不但解决了委托对象的创建问题，还可以提供方法的名称，所以这组方法无需提供gRPC方法的名称。但是提供的表达式并不能严格匹配方法的签名，所以无法提供编译时的错误检验，所以各有优缺点。

五、为gRPC服务定义一个接口

由于路由终结点的注册是针对服务类型进行的，所以我们决定让服务类型自身来完成所有的路由注册工作。在这里我们使用C# 11中一个叫做“静态接口方法”的特性，为服务类型定义如下这个IGrpcService<TService>接口，服务类型TService定义的所有gRPC方法的路由注册全部在静态方法Bind中完成，该方法将上述的IServiceBinder<TService>作为参数。

public interface  IGrpcService<TService> where TService:class{     static abstract void Bind(IServiceBinder<TService> binder);}

我们定义了如下这个针对IEndpointRouteBuilder 接口的扩展方法完成针对指定服务类型的路由注册。为了与现有的方法区别开来，我特意将其命名为MapGrpcService2。该方法根据指定的IEndpointRouteBuilder 对象将ServiceBinder<TService>对象创建出来，并作为参数调用服务类型的静态Bind方法。到此为止，整个Mini版的gRPC服务端框架就构建完成了，接下来我们看看它能否工作。

public static class EndpointRouteBuilderExtensions{    public static IEndpointRouteBuilder MapGrpcService2<TService>(this IEndpointRouteBuilder routeBuilder) where TService : class, IGrpcService<TService>    {        var binder = new ServiceBinder<TService>(routeBuilder);        TService.Bind(binder);        return routeBuilder;    }}

我们开篇演示了ASP.NET Core gRPC的服务定义、承载和调用。如果我们上面构建的Mini版gRPC框架能够正常工作，意味着客户端代码可以保持不变，我们现在就来试试看。我们在Server项目中将GreeterService服务类型改成如下的形式，它不再继承任何基类，只实现IGrpcService<GreeterService>接口。针对四种消息交换模式的四个方法的实现方法保持不变，在实现的静态Bind方法中，我们采用两种形式完成了针对这四个方法的路由注册。

[GrpcService(ServiceName = "Greeter")]public class GreeterService: IGrpcService<GreeterService>{    public Task<HelloReply> SayHelloUnaryAsync(HelloRequest request, ServerCallContext context)    => Task.FromResult(new HelloReply { Message = $"Hello, {request.Name}" });    public async Task<HelloReply> SayHelloClientStreamingAsync(IAsyncStreamReader<HelloRequest> reader, ServerCallContext context)    {        var list = new List<string>();        while (await reader.MoveNext(CancellationToken.None))        {            list.Add(reader.Current.Name);        }        return new HelloReply { Message = $"Hello, {string.Join(",", list)}" };    }    public  async Task SayHelloServerStreamingAsync(Empty request, IServerStreamWriter<HelloReply> responseStream, ServerCallContext context)    {        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Foo!" });        await Task.Delay(1000);        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Bar!" });        await Task.Delay(1000);        await responseStream.WriteAsync(new HelloReply { Message = "Hello, Baz!" });    }    public async Task SayHelloDuplexStreamingAsync(IAsyncStreamReader<HelloRequest> reader, IServerStreamWriter<HelloReply> writer, ServerCallContext context)    {        while (await reader.MoveNext())        {            await writer.WriteAsync(new HelloReply { Message = $"Hello {reader.Current.Name}" });        }    }        public static void Bind(IServiceBinder<GreeterService> binder)        {            binder               .AddUnaryMethod<HelloRequest, HelloReply>(it =>it.SayHelloUnaryAsync(default!,default!), HelloRequest.Parser)               .AddClientStreamingMethod<HelloRequest, HelloReply>(it => it.SayHelloClientStreamingAsync(default!, default!), HelloRequest.Parser)               .AddServerStreamingMethod<Empty, HelloReply>(nameof(SayHelloServerStreamingAsync), it => it.SayHelloServerStreamingAsync, Empty.Parser)               .AddDuplexStreamingMethod<HelloRequest, HelloReply>(nameof(SayHelloDuplexStreamingAsync), it => it.SayHelloDuplexStreamingAsync, HelloRequest.Parser);        }    }

服务承载程序直接将针对MapGrpcService<GreeterService>方法的调用换成MapGrpcService2<GreeterService>。由于整个框架根本不需要预先注册任何的服务，所以针对AddGrpc扩展方法的调用也可以删除。

using GrpcMini;using Microsoft.AspNetCore.Server.Kestrel.Core;var builder = WebApplication.CreateBuilder(args);builder.WebHost.ConfigureKestrel(kestrel => kestrel.ConfigureEndpointDefaults(options => options.Protocols = HttpProtocols.Http2));var app = builder.Build();app.MapGrpcService2<Server.Greeter>();app.Run();

在此运行我们的程序，客户端依然可以得到相同的输出。