Asas gRPC & Protobuf

// greet.proto
syntax = "proto3";

option csharp_namespace = "GrpcService";

package greet;

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply);
}

message HelloRequest {
  string name = 1;
}

message HelloReply {
  string message = 1;
}

// .csproj snippet
<ItemGroup>
  <Protobuf Include="Protos\greet.proto" GrpcServices="Server" />
</ItemGroup>

// Program.cs
builder.Services.AddGrpc();

var app = builder.Build();
app.MapGrpcService<GreeterService>();
app.Run();

using Grpc.Core;
using GrpcService;

public class GreeterService : Greeter.GreeterBase
{
    private readonly ILogger<GreeterService> _logger;
    public GreeterService(ILogger<GreeterService> logger) => _logger = logger;

    public override Task<HelloReply> SayHello(
        HelloRequest request,
        ServerCallContext context)
    {
        _logger.LogInformation("Saying hello to {Name}", request.Name);
        return Task.FromResult(new HelloReply
        {
            Message = $"Hello, {request.Name}!"
        });
    }
}

// Client project: add Grpc.Net.Client package
using var channel = GrpcChannel.ForAddress("https://localhost:7042");
var client = new Greeter.GreeterClient(channel);

var reply = await client.SayHelloAsync(
    new HelloRequest { Name = "Alice" });

Console.WriteLine(reply.Message); // Hello, Alice!

message Product {
  int32  id          = 1;
  string name        = 2;
  double price       = 3;
  int32  stock       = 4;
  bool   is_active   = 5;
  repeated string tags = 6; // array
}

// Supported scalar types:
// int32, int64, uint32, uint64, float, double
// bool, string, bytes, enum

enum OrderStatus {
  ORDER_STATUS_UNSPECIFIED = 0;
  ORDER_STATUS_PENDING     = 1;
  ORDER_STATUS_SHIPPED     = 2;
  ORDER_STATUS_CANCELLED   = 3;
}

message Order {
  int32       id       = 1;
  OrderStatus status   = 2;
  repeated OrderLine lines = 3;
}

message OrderLine {
  int32  product_id = 1;
  int32  quantity   = 2;
  double price      = 3;
}

// gRPC requires HTTP/2
// For local dev with HTTP (not HTTPS), enable HTTP/2 cleartext:
builder.WebHost.ConfigureKestrel(opt =>
    opt.ListenLocalhost(5000, o => o.Protocols = HttpProtocols.Http2));

// Client for cleartext (dev only)
AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true);
using var channel = GrpcChannel.ForAddress("http://localhost:5000");

public override Task<ProductReply> GetProduct(
    ProductRequest request,
    ServerCallContext context)
{
    var product = _repo.FindById(request.Id);
    if (product is null)
        throw new RpcException(
            new Status(StatusCode.NotFound, $"Product {request.Id} not found"));

    return Task.FromResult(MapToReply(product));
}

// dotnet add package Grpc.AspNetCore.Server.Reflection

builder.Services.AddGrpcReflection();

if (app.Environment.IsDevelopment())
    app.MapGrpcReflectionService();

// Now use grpcurl:
// grpcurl -plaintext localhost:5000 list
// grpcurl -plaintext localhost:5000 greet.Greeter/SayHello

// product.proto
service ProductService {
  rpc GetProduct (GetProductRequest) returns (ProductResponse);
  rpc ListProducts (ListProductsRequest) returns (ListProductsResponse);
}

// Order service calls it:
public class OrderService
{
    private readonly ProductService.ProductServiceClient _products;
    public OrderService(ProductService.ProductServiceClient p) => _products = p;

    public async Task<Order> PlaceOrderAsync(int productId, int qty)
    {
        var product = await _products.GetProductAsync(
            new GetProductRequest { Id = productId });
        return new Order { ProductName = product.Name, Quantity = qty };
    }
}