[REQ][csharp-netcore] nullable value types support

[lucamazzanti]

I want to open that feature request to recap some features\PR I see in the project.

I would understand the behavior with required and nullable options and nullable value types in the models codegen.
Moving from Swagger I have a breaking change I want to understand.
In Swagger we have that all the value type fields of a model are nullable, here not.
I see in the migration guideline a few informations about nullables.

example

The specification:

Pet:
    type: object
    required:
      - id
      - name
    properties:
      id:
        type: integer
      name:
        type: string
      age:
        type: integer
      owner:
        type: string      

Here the Swagger version:

public class Pet
{
    public long? Id {get; set; }
    public string Name {get; set; }
    public long? Age {get; set; }
    public string Owner {get; set; }
}

In this version everything is nullable to better specify a default value when data is missing in serialization\deserialization. It's useful, but having required, validated fields managed always as nullable is not so much good.

Here the OpenApi version:

public class Pet
{
    public long Id {get; set; }
    public string Name {get; set; }
    public long Age {get; set; }
    public string Owner {get; set; }
}

In this version it's difficult to manage not required fields, when the default value is available or not? You cannot know it.

Here my expected version:

public class Pet
{
    public long Id {get; set; }
    public string Name {get; set; }
    public long? Age {get; set; }
    public string Owner {get; set; }
}

Here the not required value type Age is proposed as nullable, the required Id not.

Must I change the specification generating supporting definitions such x-nullable?
I understand the difference between required and nullable, they have different meanings, but in C# an optional value type seems right to be managed as nullable.

Here past issues involved:

• Fixed valuetype parameters and discriminator deserialization: fix made after the csharp-netcore branching
  Here open issues involved:
• x-csharp-value-type doesn't help to generate nullable type
• Not possible to generate nullable enum in an API model
• Add option for nullable reference types

Thank you all.

People involved: @Blackclaws @devhl-labs @DavidJeppesen-Seges

[lucamazzanti]

I will understand how to generate x-nullable: true and nullable: true in the server part.

[lucamazzanti]

My issue was to have a NetFramework WebApi project with Swagger, and the OpenApi client generator reading the server documentation, missing of x-nullable informations.

So I added server side a Swagger SchemaFilter to produce for not required value types the missing informations.

public class NullableSchemaFilter : ISchemaFilter
{
  public void Apply(Schema schema, SchemaRegistry schemaRegistry, Type type)
  {
    if (schema.properties == null) return;
    
    var optionalValueTypeProperties = schema.properties
    .Where(i =>
  	  (i.Value.type == "string" && i.Value.format == "date") ||
  	  (i.Value.type == "string" && i.Value.format == "date-time") ||
  	  (i.Value.type == "string" && i.Value.format == "uuid") ||
  	  i.Value.type == "number" ||
  	  i.Value.type == "integer" ||
  	  i.Value.type == "boolean")
    .Where(i => !schema.required?.Contains(i.Key) ?? false);
    
    foreach (var property in optionalValueTypeProperties)
    {
      property.Value.vendorExtensions.Add("nullable", true);
      property.Value.vendorExtensions.Add("x-nullable", true);
    }
  }
}

Now the optional value types are decorated with x-nullable and the vanilla OpenApi codegen manage them as nullable.