How to use a custom model binder with Swashbuckle, Swagger and NSwag?

Question

I have an ASP.NET Core Web API that contains the following endpoint.

[HttpGet]
[Route("models/{ids}")]
[Produces(typeof(IEnumerable<Model>))]
public IActionResult Get
(
    [ModelBinder(typeof(CsvModelBinder<string>))] IEnumerable<string> ids
)
{
    // Get models

    return Ok(models);
}

This endpoint takes a CSV list of Ids (e.g. /models/a,b,c) and returns a JSON array of the corresponding Model objects. CsvModelBinder<string> is a custom implementation of IModelBinder I wrote that splits the CSV list of Ids into an IEnumerable<string> that I can use in my query to go find the objects. This all works great.

What I'm now trying to do is generate a client library using NSwag, but this is proving problematic because Swashbuckle is generating Swagger that describes the ids parameter as an IEnumerable<string>, not a string.

Option A: Is there a way to tell Swashbuckle to describe the parameter as a string instead of as an IEnumerable<string>?

Option B: Is there a way to tell NSwag that this IEnumerable<string> parameter should be marshalled into a CSV when generating the request URL?

Answer 1

I figured it out. I needed to create a custom model use MapType() in Startup.cs

Csv.cs

public class Csv<T> : List<T> where T : IConvertible
{
    public Csv<T> Append(string delimitedValues)
    {
        var splitValues = delimitedValues
            .Split(',', StringSplitOptions.RemoveEmptyEntries)
            .Cast<string>();

        var convertedValues = splitValues
            .Select(str => Convert.ChangeType(str, typeof(T)))
            .Cast<T>();

        this.AddRange(convertedValues);

        return this;
    }

    public override string ToString()
    {
        return this.Aggregate("", (a,s) => $"{a},{s}").Trim(',');
    }
}

Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddSwaggerGen(c =>
    {
        c.IncludeXmlComments(() => new XPathDocument(new FileStream(Path.Combine(PlatformServices.Default.Application.ApplicationBasePath, "MyApi.xml"), FileMode.Open)));
        c.SwaggerDoc("v1", new Info { Title = "My API", Version = "v1"});
        c.MapType<Csv<string>>(() => new Schema { Type = "string", Format = "string" });

    });
}

Answer 2

You can write a custom operation filter (Swashbuckle) or operation processor (NSwag) to transform the given parameter in the spec to a plain string.