[BUG] Generated SDK for C# does not handle TCP failure

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

It is possible to send a request to an URL that is properly resolved with DNS, but that is not accessible (TCP failure). In this case, the SDK generated with openapi-generator for C# simply returns null, without an exception.

openapi-generator version

I am using openapi-generator-cli-6.5.0.jar downloaded from:
https://repo1.maven.org/maven2/org/openapitools/openapi-generator-cli/6.5.0/

OpenAPI declaration file content or url

https://gist.github.com/MatrixRonny/76a87acaf059685db1fa58d7b5c0855d/

Generation Details

openapi-generator-cli generate -i .\api-docs.json -g csharp-netcore -o "DeploymentServiceSdk" --skip-validate-spec

Steps to reproduce

1. Generate API for specified JSON definition
2. Create the following DNS entry on local machine:
   0.0.0.0 something.does-not-exist
3. Run the following C# code

string noIpUrl = "http://something.does-not-exist"
var apiHandler = new ObfuscatedNameApi(noIpUrl);
var result = apiHandler.ObfuscatedPathPost(new ObfuscatedRequest());
//Actual behavior: result is null and no exception is thrown
//Expected behavior: throw exception

Related issues/PRs

There are issues for specific HTTP status codes. In this case, the internal ISynchronousClient returns a response with HTTP Status 0.

Suggest a fix

In the following generated code:

public static readonly ExceptionFactory DefaultExceptionFactory = (methodName, response) =>
{
    var status = (int)response.StatusCode;
    if (status >= 400)
    {
        return new ApiException(status,
            string.Format("Error calling {0}: {1}", methodName, response.RawContent),
            response.RawContent, response.Headers);
    }
    return null;
};

additional checks need to be made:

if (status == 0)
{
  return new ApiException(0, "Could not connect to API endpoint.");
}
else if (status >= 400)
{
  ...
}

Additional Information

The real problem was encountered within Kubernetees, due to services trying to access other services with external hostname. It was difficult to pinpoint what the actual problem was. The way the generated SDK currently works suggests that the API returns nothing. This is misleading.

Start with the generated C# ExceptionFactory and the internal ISynchronousClient described in the issue, then reproduce the behavior using the linked OpenAPI declaration and the openapi-generator-cli 6.5.0 command. Trace the HTTP status 0 path and add coverage for an unreachable endpoint; done means the generated SDK raises an exception instead of returning null.