DotNetGraphQLQueryGen

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DotNetGraphQLQueryGen

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Generate dotnet interfaces/classes from a GraphQL schema to perform type-safe querying from dotnet

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# dotnet GraphQL Query generator

Given a GraphQL schema file (or a GraphQL endpoint where we can do a schema introspection), this tool will generate interfaces and classes to enable strongly typed querying from C# to a GraphQL API.

Example GraphQL schema
```
schema {
    query: Query
    mutation: Mutation
}

scalar Date

type Query {
	actors: [Person]
	directors: [Person]
	movie(id: Int!): Movie
	movies: [Movie]
}

type Movie {
	id: Int
	name: String
	genre: Int
	released: Date
	actors: [Person]
	writers: [Person]
	director: Person
	rating: Float
}

type Person {
	id: Int
	dob: Date
	actorIn: [Movie]
	writerOf: [Movie]
	directorOf: [Movie]
	died: Date
	age: Int
	name: String
}

type Mutation {
    addPerson(dob: Date!, name: String!): Person
}
```

## Query Generator Command

To generate from a graphql schema file:\
&nbsp;&nbsp;&nbsp; `dotnet run -- schema.graphql -m Date=DateTime`

To generate from a live GraphQL server (*note:* introspection may not be supported or enabled on your server):\
&nbsp;&nbsp;&nbsp; `dotnet run -- http://myapi.app/gql -m Date=DateTime -h "Authorization=Bearer eyAfd..."`

For all command line options, use --help or -?

Both commands will generate the files [GraphQLClient.cs](#graphqlclient.cs) and [GeneratedTypes.cs](#generatedtypes.cs).

### GeneratedTypes.cs
Based on the example schema.graphql the `GeneratedTypes.cs` file will have the following content:

```c#
public interface RootQuery
{
    [GqlFieldName("actors")]
    List<Person> Actors();

[GqlFieldName("actors")]
    List<TReturn> Actors<TReturn>(Expression<Func<Person, TReturn>> selection);

[GqlFieldName("directors")]
    List<Person> Directors();

[GqlFieldName("directors")]
    List<TReturn> Directors<TReturn>(Expression<Func<Person, TReturn>> selection);

[GqlFieldName("movie")]
    Movie Movie();

[GqlFieldName("movie")]
    TReturn Movie<TReturn>(int id, Expression<Func<Movie, TReturn>> selection);

[GqlFieldName("movies")]
    List<Movie> Movies();

[GqlFieldName("movies")]
    List<TReturn> Movies<TReturn>(Expression<Func<Movie, TReturn>> selection);
}

public interface Movie
{
    [GqlFieldName("id")]
    int Id { get; }
    [GqlFieldName("name")]
    string Name { get; }
    [GqlFieldName("genre")]
    int Genre { get; }
    [GqlFieldName("released")]
    DateTime Released { get; }
    [GqlFieldName("actors")]
    List<Person> Actors();
    [GqlFieldName("actors")]
    List<TReturn> Actors<TReturn>(Expression<Func<Person, TReturn>> selection);
    [GqlFieldName("writers")]
    List<Person> Writers();
    [GqlFieldName("writers")]
    List<TReturn> Writers<TReturn>(Expression<Func<Person, TReturn>> selection);
    [GqlFieldName("director")]
    Person Director();
    [GqlFieldName("director")]
    TReturn Director<TReturn>(Expression<Func<Person, TReturn>> selection);
    [GqlFieldName("rating")]
    double Rating { get; }
}

public interface Person
{
    [GqlFieldName("id")]
    int Id { get; }
    [GqlFieldName("dob")]
    DateTime Dob { get; }
    [GqlFieldName("actorIn")]
    List<Movie> ActorIn();
    [GqlFieldName("actorIn")]
    List<TReturn> ActorIn<TReturn>(Expression<Func<Movie, TReturn>> selection);
    [GqlFieldName("writerOf")]
    List<Movie> WriterOf();
    [GqlFieldName("writerOf")]
    List<TReturn> WriterOf<TReturn>(Expression<Func<Movie, TReturn>> selection);
    [GqlFieldName("directorOf")]
    List<Movie> DirectorOf();
    [GqlFieldName("directorOf")]
    List<TReturn> DirectorOf<TReturn>(Expression<Func<Movie, TReturn>> selection);
    [GqlFieldName("died")]
    DateTime Died { get; }
    [GqlFieldName("age")]
    int Age { get; }
    [GqlFieldName("name")]
    string Name { get; }
}

public interface Mutation
{
    [GqlFieldName("addPerson")]
    TReturn AddPerson<TReturn>(DateTime dob, string name, Expression<Func<Person, TReturn>> selection);
}
```

### GraphQLClient.cs

The generated `GraphQLClient` class instance acts as a session to send GraphQL requests.

```c#
var httpClient = new HttpClient { BaseAddress = new Uri("http://myapi.app/gql") };
var client = new GraphQLClient(httpClient);
```

For custom headers like Authorization, configure them in the HttpClient. These will be used during the requests.
```c#
var httpClient = new HttpClient { BaseAddress = new Uri("http://myapi.app/") };
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "eyAf3s...");

var client = new GraphQLClient(new Uri("gql", UriKind.Relative), httpClient);
```

To use the GraphQLClient, copy the following files into your project. These will all be in the output folder:
1. GraphQLClient.cs - a simple client you can modify or extend
2. GeneratedResultTypes.cs - generated types from the schema
2. GeneratedQueryTypes.cs - extensions to allow the linq style querying
3. BaseGraphQLClient.cs - a base client that GraphQLClient inherits from
4. GqlFieldNameAttribute.cs - required by the generated code

Ideally move this to a dependency style library and tool in the future.

## Request Methods

`GraphQLClient` exposes 2 methods:

* [Query](#query)\
  `async Task<GqlResult<TQuery>> QueryAsync<TQuery>(Expression<Func<RootQuery, TQuery>> query)`
* [Mutation](#mutation)\
  `async Task<GqlResult<TQuery>> MutateAsync<TQuery>(Expression<Func<Mutation, TQuery>> query)`

### Query

```c#
// queries
var result = await client.QueryAsync(q => new {
    Movie = q.Movie(2, m => new {
        m.Id,
        m.Name,
        ReleaseDate = m.Released,
        Director = m.Director(d => new {
            d.Name
        })
    }),
    Actors = q.Actors(a => new {
        a.Name,
        a.Dob
    })
});
```

This looks similar to the GraphQL. And now `result.Data` is a strongly type object with the expected types.

If a field in the schema is an object (can have a selection performed on it) it will be exposed in the generated code as a method where you pass in any field arguments first and then the selection.

The GraphQL created by the above will look like this:
```
{
    query {
        Movie: movie(id: 2) {
            Id: id
            Name: name
            ReleaseDate: released
            Director: director {
                Name: name
            }
        }
        Actors: actors {
            Name: name
            Dob: dob
        }
    }
}
```

### Mutations

Mutations look very similar to the above query, just as they do in GraphQL. The above schema had one mutation that can be called like so

```c#
// mutations
var mutationResult = await client.MutateAsync(m => new {
    NewPerson = m.AddPerson(new DateTime(1801, 7, 3), "John Johny", p => new { p.Id })
});
```

`m` is the `Mutation` type interface and lets you call any of the mutations generated from the schema. Like a query, you create a new anonymous object and call as many mutations as you wish. Each mutation method has all their arguments and the last one is a selection query on the mutation's return type.

The above has `mutationResult` strongly typed. E.g. `mutationResult.Data.NewPerson` will only have an `Id` field.

## Input types

Any input types are generated as actual `class`es as the are used as arguments. The `-m Date=DateTime` tells the tool that the `scalar` type `Date` should be the `DateTime` type in C#. You can use this to support any other custom `scalar` types. E.g. `-m Date=DateTime;Point=PointF`, just comma separate a `GqlScalar=DotnetType` list.

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