This document outlines the concepts needed to write [gRPC](https://grpc.io/docs/guides/) apps in C#. The topics covered here apply to both [C-core](https://grpc.io/blog/grpc-stacks)-based and ASP.NET Core-based gRPC apps.
gRPC uses a contract-first approach to API development. Protocol buffers (protobuf) are used as the Interface Design Language (IDL) by default. The *.proto* file contains:
For more information on the syntax of protobuf files, see the [official documentation (protobuf)](https://developers.google.com/protocol-buffers/docs/proto3).
For example, consider the *greet.proto* file used in [Get started with gRPC service](xref:tutorials/grpc/grpc-start):
* Defines a `Greeter` service.
* The `Greeter` service defines a `SayHello` call.
The tooling package [Grpc.Tools](https://www.nuget.org/packages/Grpc.Tools/) is required to generate the C# assets from *.proto* files. The generated assets (files):
This package is required by both the server and client projects. The `Grpc.AspNetCore` metapackage includes a reference to `Grpc.Tools`. Server projects can add `Grpc.AspNetCore` using the Package Manager in Visual Studio or by adding a `<PackageReference>` to the project file:
Client projects should directly reference `Grpc.Tools` alongside the other packages required to use the gRPC client. The tooling package isn't required at runtime, so the dependency is marked with `PrivateAssets="All"`:
For server-side assets, an abstract service base type is generated. The base type contains the definitions of all the gRPC calls contained in the *.proto* file. Create a concrete service implementation that derives from this base type and implements the logic for the gRPC calls. For the `greet.proto`, the example described previously, an abstract `GreeterBase` type that contains a virtual `SayHello` method is generated. A concrete implementation `GreeterService` overrides the method and implements the logic handling the gRPC call.
For client-side assets, a concrete client type is generated. The gRPC calls in the *.proto* file are translated into methods on the concrete type, which can be called. For the `greet.proto`, the example described previously, a concrete `GreeterClient` type is generated. Call `GreeterClient.SayHelloAsync` to initiate a gRPC call to the server.
By default, server and client assets are generated for each *.proto* file included in the `<Protobuf>` item group. To ensure only the server assets are generated in a server project, the `GrpcServices` attribute is set to `Server`.
