Generated JSON deserializer fails at runtime when schema enum contains duplicate keys

[GeorgDangl]

The following is the definition of the System.Net.HttpStatusCode type (shortened) from .nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref\netstandard.dll:

namespace System.Net
{
    //
    // Summary:
    //     Contains the values of status codes defined for HTTP.
    public enum HttpStatusCode
    {
        //
        // Summary:
        //     Equivalent to HTTP status 300. System.Net.HttpStatusCode.Ambiguous indicates
        //     that the requested information has multiple representations. The default action
        //     is to treat this status as a redirect and follow the contents of the Location
        //     header associated with this response.
        Ambiguous = 300,
        //
        // Summary:
        //     Equivalent to HTTP status 300. System.Net.HttpStatusCode.MultipleChoices indicates
        //     that the requested information has multiple representations. The default action
        //     is to treat this status as a redirect and follow the contents of the Location
        //     header associated with this response.
        MultipleChoices = 300,
    }
}

Per this StackOverflow question (which references the C# spec), this is allowed and correct syntax.

Generating the schema

var jsonSchema = (await NJsonSchema.JsonSchema4.FromTypeAsync(typeof(System.Net.HttpStatusCode))).ToJson();

Produces this (shortened) JSON schema:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "title": "HttpStatusCode",
  "type": "integer",
  "description": "",
  "x-enumNames": [
    "MultipleChoices",
    "Ambiguous"
  ],
  "enum": [
    300,
    300
  ]
}

After a bit more processing (I'm using a model containing a HttpStatusCode property in a Web API, which gets then converted via NSwag to a Swagger definition, which then again is used to create a C# client) this is the generated C# client code (shortened):

[System.CodeDom.Compiler.GeneratedCode("NJsonSchema", "9.11.0.0 (Newtonsoft.Json v9.0.0.0)")]
    public enum HttpStatusCode
    {   
        [System.Runtime.Serialization.EnumMember(Value = "Ambiguous")]
        MultipleChoices = 14,
    
        [System.Runtime.Serialization.EnumMember(Value = "Ambiguous")]
        Ambiguous = 15
    }

This just fails at runtime when deserializing with Newtonsoft.Json:

System.InvalidOperationException : Enum name 'Ambiguous' already exists on enum 'HttpStatusCode'.

The workaround for me is to simply use the System.Net.HttpStatusCode enum in my generated client instead of the generated one.

[GeorgDangl]

To mitigate this, the following PostProcess works for me in ASP.NET Core:

services.AddSwaggerDocument(c =>
{
    c.PostProcess = (x) =>
    {
        // Some enum classes use multiple integer values for the same value, e.g.
        // System.Net.HttpStatusCode uses these:
        // RedirectKeepVerb = 307
        // TemporaryRedirect = 307
        // MVC is configured to use the StringEnumConverter, and NJsonSchema errorenously
        // outputs the duplicates. For the example above, the value 'TemporaryRedirect' is
        // serialized twice, 'RedirectKeepVerb' is missing.
        // The following post process action should remove duplicated enums
        // See https://github.com/RSuter/NJsonSchema/issues/800 for more information
        foreach (var enumType in x.Definitions.Select(d => d.Value).Where(d => d.IsEnumeration))
        {
            var distinctValues = enumType.Enumeration.Distinct().ToList();
            enumType.Enumeration.Clear();
            foreach (var distinctValue in distinctValues)
            {
                enumType.Enumeration.Add(distinctValue);
            }
        }
    };
});

I've blogged about it here:
https://blog.dangl.me/archive/remove-duplicate-enum-entries-in-swagger-documents-with-nswag-in-aspnet-core/

[RicoSuter]

I think you also need to remove the corresponding name in EnumerationNames… otherwise the name-value mapping might be wrong in the generated code. I think this problem should be fixed directly in NJS... i.e. filter out duplicates as you do it manually now...