[C#] Exception deserializing null for optional enum specified via reference property

Description

Given an object with an inline enum property "type" and a referenced enum "country", data like {"country": null} will throw an exception on deserialization, while data like {} and {"type": null} will work as expected.

Swagger-codegen version

2.2.2-SNAPSHOT

Swagger declaration file content or url

{
    "swagger": "2.0",
    "info": {
        "version": "1.0.0",
        "title": "Swagger Test"
    },
    "paths": {},
    "definitions": {
        "Address": {
            "type": "object",
            "properties": {
                "type": {
                    "type": "string",
                    "enum": [
                        "inline1",
                        "inline2"
                    ]
                },
                "country": {
                    "$ref": "#/definitions/Country"
                }
            }
        },
        "Country": {
            "type": "string",
            "enum": [
                "Ref1",
                "Ref2"
            ]
        }
    }
}

Command line used for generation

http://editor.swagger.io/#/
and
java -jar ~/swagger-codegen/modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i test.json -l csharp -o test/

Steps to reproduce

Generate the code. Write a sample program like:

RestResponse rsp;
Address addr;

rsp = new RestResponse();
rsp.Content = "{}";
addr = (Address)Configuration.Default.ApiClient.Deserialize(rsp, typeof(Address));
Console.WriteLine(addr.Country);

rsp = new RestResponse();
rsp.Content = "{\"type\": null}";
addr = (Address) Configuration.Default.ApiClient.Deserialize(rsp, typeof(Address));
Console.WriteLine(addr.Type);

rsp = new RestResponse();
rsp.Content = "{\"country\": null}";
addr = (Address)Configuration.Default.ApiClient.Deserialize(rsp, typeof(Address));
Console.WriteLine(addr.Country);

The first two will work, and the last will throw an exception.

Related issues

https://github.com/swagger-api/swagger-codegen/pull/4145 fixes the compilation issue but still doesn't work if a null is actually passed in the message.

Suggest a Fix

It seems to me that the isEnum flag is not getting set for RefProperties in DefaultCodegen.fromProperty. Could also be the case that C# needs to check for some other property in modelGeneric.mustache to know whether to force a ? reference.

Start with DefaultCodegen.fromProperty and the C# modelGeneric.mustache template, comparing how inline and referenced enums are represented. Generate the sample C# client and run the three deserialization cases from the issue. Done means a null referenced enum deserializes without an exception while the existing empty-object and inline-enum cases continue to work.