Per default all fields on an object type can be either null or the specified type.

type User {
  name: String
}

In the above example name can either be null or a String.

Being nullable does not make sense for every field though. Maybe we have some database constraint which enforces the name to never be null. GraphQL allows us to be specific about this, by marking a field as non-null.

type User {
  name: String!
}

The exclamation mark (!) denotes that the field can never be null. This is also enforced by the execution engine. If we were to return a null value in the name resolver, the execution engine would throw an error. This prevents unexpected null values from causing issues in the consuming applications.

Implicit nullability

Hot Chocolate automatically infers the nullability of the schema type from the nullability of the used CLR type.

Value types are non-null per default, unless they have been marked as nullable.

Reference types are always nullable, unless we have enabled nullable reference types. With nullable reference types enabled all fields are non-null per default.

We strongly encourage the use of nullable reference types.

Explicit nullability

We can also be explicit about the nullability of our fields.

public class Query
{
    [GraphQLNonNullType]
    public Book GetBook()
    {
        return new Book { Title  = "C# in depth", Author = "Jon Skeet" };
    }
}

public class Book
{
    [GraphQLNonNullType]
    public string Title { get; set; }

    public string Author { get; set; }
}

The inner type of a list can be made non-null like the following.

public class Book
{
    [GraphQLType(typeof(ListType<NonNullType<StringType>>))]
    public List<string> Genres { get; set; }
}