For what do we need the type conversion API on Hot Chocolate?

Let us have a look at a simple example to answer this question and also to show how this is solved with Hot Chocolate.

Assume we have a mongo database entity representation in c# that looks like the following:

public class Message
{
    public ObjectId Id { get; set; }
    public DateTimeOffset Created { get; set; }
    public string Text { get; set; }
}

We want the Id property to be of the IdType in the GraphQL schema. The Hot Chocolate query execution engine does not know how ObjectId is serialized or deserialized.

Moreover, IdType uses System.String as .NET representation of its values.

In order to be able to use ObjectId through out our code, we have to explain to the query execution engine how to serialize ObjectId to System.String and also how to deserialize it.

This can be done in simple cases with two lines of code:

TypeConversion.Default.Register<string, ObjectId>(from => ObjectId.Parse(from));
TypeConversion.Default.Register<ObjectId, string>(from => from.ToString());

Dependency Injection Support

You can also add your type converters to the dependency injection. Using dependency injection for the type converters lets you more easily write tests that verify behaviour of your API in various scenarious.

The first thing you have to ensure is that your schema has access to the service provider, which can be done like the following:

service.AddGraphQL(sp => SchemaBuilder.New()
  .AddServices(sp)
  ...
  .Create()

After this is done converters can be registered like the following:

services.AddTypeConverter<string, ObjectId>(from => ObjectId.Parse(from));

Moreover, you are able to put your conversion code into a class like the follwing:

public class StringObjectIdConverter
    : TypeConverter<string, ObjectId>
{
    public ObjectId Convert(string from) => ObjectId.Parse(from);
}

This makes sense if you have more complex code to write to specify your conversion.

The class can also be registered with the dependency injection like the following:

services.AddTypeConverter<StringObjectIdConverter>();