GraphQL requires knowledge of the types it returns at build time. When wrapping a REST API, the most reliable approach is to generate a typed .NET client from an OpenAPI specification and inject it into your resolvers.

Generating a Client from OpenAPI

If your REST endpoint exposes an OpenAPI specification (Swagger), you can generate a fully typed .NET client for it.

Step 1: Get the OpenAPI Specification

Download the swagger.json from your REST endpoint:

curl -o swagger.json http://localhost:5000/swagger/v1/swagger.json

Step 2: Generate the Client

Use the NSwag CLI tool to generate a C# client:

dotnet new tool-manifest
dotnet tool install NSwag.ConsoleCore
dotnet nswag swagger2csclient /input:swagger.json /classname:TodoService /namespace:TodoReader /output:TodoService.cs

This generates a TodoService.cs file with a typed client for your REST API. The generated client requires Newtonsoft.Json:

dotnet add package Newtonsoft.Json

Exposing the REST API

Register the generated client in your DI container and inject it into your resolvers.

// Types/TodoQueries.cs
[QueryType]
public static partial class TodoQueries
{
    public static async Task<ICollection<TodoItem>> GetTodosAsync(
        TodoService service,
        CancellationToken ct)
        => await service.GetAllAsync(ct);

    public static async Task<TodoItem> GetTodoByIdAsync(
        long id,
        TodoService service,
        CancellationToken ct)
        => await service.GetByIdAsync(id, ct);
}

// Program.cs
builder.Services.AddHttpClient<TodoService>();

builder
    .AddGraphQL()
    .AddTypes();

You can now open Nitro on your GraphQL server at /graphql and query your REST data:

{
  todoById(id: 1) {
    id
    isComplete
    name
  }
  todos {
    id
    isComplete
    name
  }
}

Using DataLoaders with REST

When multiple GraphQL fields resolve data from the same REST endpoint, use a DataLoader to batch and deduplicate calls. This prevents sending redundant HTTP requests for the same resource.

// DataLoaders/TodoByIdDataLoader.cs
public class TodoByIdDataLoader : BatchDataLoader<long, TodoItem>
{
    private readonly TodoService _service;

    public TodoByIdDataLoader(
        TodoService service,
        IBatchScheduler batchScheduler,
        DataLoaderOptions? options = null)
        : base(batchScheduler, options)
    {
        _service = service;
    }

    protected override async Task<IReadOnlyDictionary<long, TodoItem>> LoadBatchAsync(
        IReadOnlyList<long> keys,
        CancellationToken ct)
    {
        var todos = await _service.GetByIdsAsync(keys, ct);
        return todos.ToDictionary(t => t.Id);
    }
}

Next Steps

• Need to batch REST calls? See DataLoader.
• Need to fetch from a database instead? See Fetching from Databases.
• Need to understand resolvers? See Resolvers.