Manually add custom route parameter to Swagger docs with Swashbuckle.AspNetCore

Question

So I have encountered an interesting case, where I needed to generate swagger documentation for an REST API whose only documentation was an actual document (no inline XML documentation) and which I do not have direct source code access. So I simple wrote a wrapper controller, and overrid each route as such:

[HttpGet("this/{that}/the/{other}")]
public override IActionResult GetWhatever(String that, String other) => base.GetWhatever(that, other);

and then just documented it with standard summary tags, etc. However, one of the now overridden methods uses a querystring internally, and is not exposed as a parameter with [FromQuery], so it is not able to be auto-documented reflectively (and putting in a tag for it without the actual parameter present does not generate documentation for it)

I need a way to add this parameter documentation manually, but through code somehow (not just by adding it to the swagger.json file). I though I could use SwaggerGen's ISchemaFilter to add a parameter description to the associated route/method, but so far I'm not having much luck.

Does anyone have an example of doing something like this?

Answer 1

So it appears what I was looking for was IOpertationFilter. Coupling that with a custom Attribute, I was able to create what I needed to manually add a custom parameter to the Swagger documentation on the fly. See all associated code below, please note Schema/PartialSchema have many properties, I only set Type as it was all I needed, other cases might require more.

SwaggerParameterAttribute.cs

using System;
using Microsoft.AspNetCore.Mvc.Filters;
using Swashbuckle.AspNetCore.Swagger;

/// <summary>
/// Types of Swagger parameters
/// </summary>
  public enum SwaggerParamType {Body, NonBody};

/// <summary>
/// Attribute to facilitate manually adding a parameter to auto-generated Swagger documentation
/// </summary>
  [AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
  public class SwaggerParameterAttribute : ActionFilterAttribute {

  /// <summary>
  /// Swagger parameter to inject
  /// </summary>
    public IParameter Parameter { get; set; } 

  /// <summary>
  /// Default constructor
  /// </summary>
  /// <param name="ParamType">Type of Swagger parameter (Body/NonBody)</param>
  /// <param name="Name">Name of the parameter</param>
  /// <param name="Type">Primitive type associated with the parameter (int, bool, string, etc.)</param>
  /// <param name="In">Location of the parameter (path, query, etc.)</param>
  /// <param name="Description">Description of the parameter</param>
  /// <param name="Required">Whether the parameter is required or not (true/false)</param>
    public SwaggerParameterAttribute(SwaggerParamType ParamType, String Name, String Type, String In, String Description = "", Boolean Required = false){
  switch (ParamType) {      
    case SwaggerParamType.Body:
      Parameter = new BodyParameter() { Name = Name, In = In, Description = Description, Required = Required, Schema = new Schema() { Type = Type } };
      break;    
    case SwaggerParamType.NonBody:
      Parameter = new NonBodyParameter() { Name = Name, In = In, Description = Description, Required = Required };  
      ((PartialSchema)Parameter).Type = Type;
      break;
    default:
      throw new ArgumentOutOfRangeException("Invalid Swagger parameter type specified.");
  }
}

SwaggerOperationFilter.cs

using System;
using System.Reflection;
using Microsoft.AspNetCore.Mvc.Controllers;
using Swashbuckle.AspNetCore.Swagger;
using Swashbuckle.AspNetCore.SwaggerGen;

using Whatever.NameSpace.Your.Attribute.Is.In;

/// <summary>
/// Custom Swagger Operation Filter
/// </summary>
  public class SwaggerOperationFilter : IOperationFilter {
    public void Apply(Operation operation, OperationFilterContext context) {
    //Check for [SwaggerParameter] add defined parameter to the parameter list
      foreach (Attribute attribute in ((ControllerActionDescriptor)context.ControllerActionDescriptor).MethodInfo.GetCustomAttributes()) {
        if (attribute.GetType() == typeof(SwaggerParameterAttribute)) {
          operation.Parameters.Add(((SwaggerParameterAttribute)attribute).Parameter);
        }
      }
    }
  }

Startup.cs (Just the swagger operation filter part)

using Swashbuckle.AspNetCore.Swagger;

using Whatever.NameSpace.Your.Filter.Is.In;

public void ConfigureServices(IServiceCollection services) {
  services.AddSwaggerGen(options => {
    options.OperationFilter<SwaggerOperationFilter>();
  }
}

SomeController.cs (Example Usage)

using Swashbuckle.AspNetCore.Swagger;
using Swashbuckle.AspNetCore.SwaggerGen;

using Whatever.NameSpace.Your.Attribute.Is.In;

[HttpGet("this/{that}/the/{other}")]
[SwaggerParameter(ParamType: SwaggerParamType.NonBody, Name: "param1", Type: "string", In: "query", Description: "Some description of param1 here")]
[SwaggerParameter(SwaggerParamType.NonBody, "param2", "string", "query", "Some description of param2 here")]
public override IActionResult GetWhatever(String that, String other) => base.GetWhatever(that, other);