[Required] attribute not applied to generated C# int properties

[ChristophHornung]

Describe the bug

Generating the code for

{
  "swagger": "2.0",
  "info": {
    "title": "Swagger specification",
    "version": "1.0.0"
  },
  "paths": {
    "/Product": {
      "get": {
        "operationId": "Product",
        "responses": {
          "200": {
            "description": "",
            "schema": {
              "$ref": "#/definitions/Product"
            }
          }
        }
      }
    }
  },
  "definitions": {
    "Product": {
      "type": "object",
      "required": [
        "Id",
        "StringId"
      ],
      "properties": {
        "Id": {
          "type": "integer",
          "format": "int32"
        },
        "StringId": {
          "type": "string",
          "minLength": 1
        }
      }
    }
  }
}

via (e.g.)

var settings = new CSharpClientGeneratorSettings();
settings.CSharpGeneratorSettings.JsonLibrary = NJsonSchema.CodeGeneration.CSharp.CSharpJsonLibrary.SystemTextJson;
settings.CSharpGeneratorSettings.JsonSerializerSettingsTransformationMethod = "TestJsonSerializerSettingsTransformationMethod";

var generator = new CSharpClientGenerator(document, settings);

Generates a C# class of

[System.CodeDom.Compiler.GeneratedCode("NJsonSchema", "14.2.0.0 (NJsonSchema v11.1.0.0 (Newtonsoft.Json v13.0.0.0))")]
public partial class Product
    {

        [System.Text.Json.Serialization.JsonPropertyName("Id")]
        public int Id { get; set; }

        [System.Text.Json.Serialization.JsonPropertyName("StringId")]
        [System.ComponentModel.DataAnnotations.Required]
        public string StringId { get; set; }

    }

I would have expected for the Id property to retain a Required attribute as well. Or am I missing something here?

Version used

14.2.0

[roeldewit]

The Required attribute checks if the property is not null, a integer can't be null so the Required attribute is redundant. If you want the integer value to be in a specific range you can use the Range attribute for validation or you could make the integer nullable if that's what you're looking for.

[ChristophHornung]

@roeldewit The Required attribute is not synonymous with not null as they are two different concepts. It is true that in C# the concept of "required" is weaker than in other languages but that is exactly what the attribute is for.
But especially in the context of OpenAPI this distinction matters.

That is why

public class Product
{
	[Required]
	public int Id { get; set; }
}

with the the JsonSchemaGenerator will generate

"definitions": {
    "Product": {
      "type": "object",
      "required": [
        "Id"
      ],
      "properties": {
        "Id": {
          "type": "integer",
          "format": "int32"
        }
      }
    }
  }

while

public class Product
{
	public int Id { get; set; }
}

will generate

"Product": {
      "type": "object",
      "properties": {
        "Id": {
          "type": "integer",
          "format": "int32"
        }
      }
    }

So why after a simple roundtrip with NSwag from C# class to openapi.json back to C# class is this information lost?

E.g. when simply using the generated class in an API that also generates an OpenAPI suddenly the definitions would look different.