List & Non-Null

In addition to the six fundamental graph types, GraphQL contains two meta graph types: LIST and NON_NULL.

• NON_NULL : Indicates that the Graph Type its describing must not be a null value, be that as an input argument or returned from a field
• LIST: Indicates that GraphQL should expect a collection of objects instead of just a single item.

These meta types aren't anything concrete like a scalar or an enum. Instead they "wrap" another graph type (such as int or Donut). They are used to describe the usage of a graph type in a field or input argument:

For example, we would say:

• "A field that returns a Float number."
• "A field that must return a Person."
• "An input argument that must be a Date."

We can even describe complex scenarios:

• "A field that might return a collection of persons but when returned, each person must be a valid reference."
• "An input argument that must be a list that contains lists of integers." (e.g. [[1, 2], [5, 15]])

Type Expressions

Together these "wrappers" make up a field's Type Expression. GraphQL ASP.NET will automatically infer a type expression for every field and every input argument when generating your schema.

The following assumptions about your data are made when creating type expressions:

✅ Reference types can be null
✅ Value types cannot be null
✅ Nullable value types (e.g. int?) can be null
✅ When a reference type implements IEnumerable<TType> it will be expressed as a "list of TType"

Type Expressions are commonly shown in the GraphQL schema syntax for field definitions. Here are a few examples of a .NET type and its equivalent type expression in schema syntax.

.NET Type	Type Expression
int	Int!
float?	Float
IEnumerable<Person>	[Person]
Person[]	[Person]
List<bool>	[Boolean!]
IReadOnlyList<long>	[Long!]
IReadOnlyList<long?>	[Long]
IEnumerable<List<ICollection<Donut>>>	[[[Donut]]]

The ! indicates NON_NULL and [] for a LIST.

Overriding Type Expressions

You may need to override the default behavior from time to time. For instance, a string, which is a reference type, is nullable by default but you may want to enforce non-nullability at the query level and declare that null is not valid for a given argument. Or, perhaps, an object implements IEnumerable but you don't want graphql to treat it as a list.

You can override the default type expression of any field or argument by defining a custom type expression when needed.