GraphQL C# Library (.NET): Guide

GraphQL Library for C# (.NET): GraphQL.NET

GraphQL is a modern query language for APIs that allows clients to request only the necessary data, reducing redundancy and speeding up applications. For the .NET platform, there is a powerful library GraphQL.NET that enables C# developers to create server-side components fully compatible with the GraphQL specification. In this article, we will thoroughly examine its capabilities, advantages, and provide practical examples.

What is GraphQL.NET?

GraphQL.NET is an open-source library for C# (.NET) that implements the GraphQL specification. It provides tools for defining schemas (types, queries, mutations, and subscriptions), processing incoming requests, validation, and executing logic. The library is actively maintained by the community and used in production environments.

Key features:

Flexible schema definition — types, interfaces, union types, and enums.
Support for Queries, Mutations, and Subscriptions.
Integration with DI containers (e.g., Microsoft.Extensions.DependencyInjection).
Server-side request validation and parsing.
DataLoader support for optimizing N+1 queries.
Testing and debugging tools.

Installation and Setup

Installation is done via NuGet Package Manager. Run the command:

dotnet add package GraphQLdotnet add package GraphQL.SystemTextJson

For ASP.NET Core, you will also need the middleware package:

dotnet add package GraphQL.Server.Transports.AspNetCore

Creating a Simple GraphQL API

1. Defining Data Types

First, let's create a data model and the corresponding GraphQL type:

public class Book{    public int Id { get; set; }    public string Title { get; set; }    public string Author { get; set; }}

public class BookType : ObjectType<Book>{    protected override void Configure(IObjectTypeDescriptor<Book> descriptor)    {        descriptor.Field(b => b.Id).Type<NonNullType<IntType>>();        descriptor.Field(b => b.Title).Type<NonNullType<StringType>>();        descriptor.Field(b => b.Author).Type<StringType>();    }}

2. Defining Queries

Now let's create a Query class that will handle requests:

public class Query{    private readonly List<Book> _books = new()    {        new Book { Id = 1, Title = "1984", Author = "George Orwell" },        new Book { Id = 2, Title = "Brave New World", Author = "Aldous Huxley" }    };

    public Book GetBook(int id)    {        return _books.FirstOrDefault(b => b.Id == id);    }

    public List<Book> GetBooks()    {        return _books;    }}

Then register this class in the schema:

public class BookSchema : Schema{    public BookSchema(IServiceProvider provider) : base(provider)    {        Query = provider.GetRequiredService<Query>();    }}

3. Configuration in Startup.cs

Add services and middleware in ASP.NET Core:

public void ConfigureServices(IServiceCollection services){    services.AddGraphQL(builder => builder        .AddSchema<BookSchema>()        .AddSystemTextJson());}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env){    app.UseGraphQL<BookSchema>();    app.UseGraphQLPlayground(); // For testing}