GraphQL in .NET Core project

In last few days I worked on integration GraphQL into my .NET Core project. I would like to share with you with all steps that I had to do.


I found only one library which we can use when we want to create GraphQL API in .NET Core project: http://graphql-dotnet.github.io/graphql-dotnet/.

Unfortunately documentation of that project isn’t extended. However there is separate github project where we can find more information: https://github.com/JacekKosciesza/StarWars.

Based on that information I successfully created GraphQL endpoint which serves the same information which produces my REST API but now my clients can execute more queries in one request. And this is real performance boost.

Add nuget package

First we have to add nuget package to our csproj file. We can execute in command line action: dotnet add package GraphQL. After that command in project file we should have a new entry:

<PackageReference Include="GraphQL" Version="0.17.3" />

Controller

We can start our work from controller. GraphQL requires only one HTTP endpoint with POST method. Below there is simple example of controller.

In our controller we have four things which are mandatory for .NET GraphQL:

  • GraphQLQuery — all query roots which we can execute. This class we have to create.
  • IDocumentExecuter — interface from .NET GrapQL. It is responsible for executing queries.
  • ISchema — interfece which we have to implement to tell GraphQL about all our types which can be returned.
  • GraphQLParameter — class which stores query parameters.

GraphQLParameter

The simples one is last class. It stores one property for GraphQL query.

IDocumentExecuter

This is interface from .NET GraphQL. Also there is a default implementation of that interface. We have to do only one thing with it — add to our dependency injection container. For autofac we have to add line:

ISchema

If we want to inject services/repositories to our GraphQL types we have to implement that interface. It’s very simple.

Also we have to add to dependency injection ISchema implementation (and resolveType constructor parameter).

GraphQLQuery

Last thing is class which will be responsible for storing all of our query roots. I decided to move implementation of the queries to separate resolvers. Thanks to this I don’t have one big class which is responsible for different things. My GraphQLQuery looks like on below snippet.

So I’m using reflection to get all classes which implement IResolver interface. Then I’m creating instances thanks to ServiceProvider which exists in .NET Core. Resolvers are described below.

Resolvers

Resolvers are classes which are responsible for retrieve data which should be returned by specific query. All my resolvers are marked by IResolver interface, which has only one method.

Below there is an implementation of one of the resolver.

Above resolver is responsible only for one thing. It registers one query: groups. And that query returns list of groups.

We can see that resolver uses IGroupsServices. That service can retrieve groups from database for example.

Here we have also GroupType class which is really important from GraphQL perspective. It stores information which GraphQL uses to introspection (whole information about API schema is built thanks to this).

All resolvers can be registerd in dependency injection container like that:

Add GraphQL types

The last thing are GraphQL types. Type system is the heart of GraphQL. Below there is an simple example of GraphQL type.

Above type uses GroupDto. That class is returned by IGroupService in resolver (described above) and transformed to GroupType class by .NET GraphQL.

All types should be registered in dependency injection (especially when we inject to the type services which will resolve property data). Below there is a code which I used to register my GraphQL types.

All my types are marked by IGraphType interface (which is empty). Thanks to this I can register easily all of types in DI container.


You can find more source code connected to GraphQL in one of my project on github.

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