GraphQL–DotNet–Nullability

In GraphQL all types are nullable by default. In the first version of GraphQL non-null fields were even not supported at all(!).

There is a good reason for this; evolvability of your API. Non-nullable fields make it hard to change your contract. Switching from a non-nullable to a nullable field will always be a breaking change. This explains why GraphQL makes every type nullable by default, and you have to explicitly mark a field as non-null by adding the bang character (!) to your schema definition.

When you are using GraphQL.NET, the principles above still apply but the library follows some different rules to mark a field as nullable or not. For structs (integers, booleans, …) GraphQL.NET will mark them as not-null by default. You even have to explicitly mark them as nullable(see this post: https://bartwullems.blogspot.com/2018/11/graphqldotnetnullable-types-nullable.html). For all other types the rules above are followed and every type is nullable by default.

If you don’t want this, you have to wrap your GraphQL.NET type in a NonNullGraphType wrapper.

Here is a example for a single type:

	public ProductType()
	{
	Field(m => m.Id, type : typeof(NonNullGraphType<IdGraphType>));
	Field(d => d.Description); //--> Nullable
	Field(d => d.Category, type:typeof(NonNullGraphType<CategoryType>));
	}

view raw ProductType.cs hosted with ❤ by GitHub

This will generate the following schema:

	type ProductType {
	id: ID!
	description: String
	category: CategoryType!
	}

view raw ProductTypeSchema.gql hosted with ❤ by GitHub

And also an example for a list:

	public ProductType()
	{
	Field(m => m.Id, type : typeof(NonNullGraphType<IdGraphType>));
	Field(d => d.Description); //--> Nullable
	Field<NonNullGraphType<ListGraphType<SupplierType>>>("suppliers");
	}

view raw ProductTypeWithList.cs hosted with ❤ by GitHub

This will generate the following schema:

	type ProductType {
	id: ID!
	description: String
	suppliers: [SupplierType]!
	}

view raw ProductTypeWithList.gql hosted with ❤ by GitHub

More information:

• https://medium.com/@calebmer/when-to-use-graphql-non-null-fields-4059337f6fc8
• https://blog.apollographql.com/using-nullability-in-graphql-2254f84c4ed7