library-api

Custom Operation Names with Swashbuckle 5.0

This is a post about Swashbuckle –  a .NET library that seamlessly adds Swagger support to WebAPI projects.  If you aren’t familiar with Swashbuckle then stop reading right now and go look into it – it’s awesome.

library-api

Swashbuckle has recently released version 5.0 which includes (among other things) a ridiculous array of ways to customise your generated swagger spec.

One such customisation point allows you to change the operationId (and other properties) manually against each operation once the auto-generator has done it’s thing.

Why Bother?

Good question.  For me, I decided to bother for one very specific reason: swagger-js.  This library can auto-generate a nice accessor object based on any valid swagger specification with almost no effort, whilst doing lots of useful things like handling authorization and parsing responses.

swagger-js uses the operationId property for method names and the default ones coming out of Swashbuckle weren’t really clear or consistent enough.

Injecting an Operation Filter

The means for customising operations lies with the IOperationFilter interface exposed by Swashbuckle.

public interface IOperationFilter
{
  void Apply(Operation operation, 
    SchemaRegistry schemaRegistry, 
    ApiDescription apiDescription);
}

When implemented and plugged-in (see below), the Apply method will be called for each operation located by Swashbuckle and allows you to mess around with its properties.  We have a very specific task in mind so we can create a SwaggerOperationNameFilter class for our purpose:

public class SwaggerOperationNameFilter : IOperationFilter
{
  public void Apply(Operation operation, SchemaRegistry schemaRegistry, ApiDescription apiDescription)
  {
    operation.operationId = "???";
  }
}

When you installed the Swashbuckle nuget package it will have created a SwaggerConfig file in your App_Start folder.  In this file you will likely have a long and well-commented explanation of all available configuration points, but to keep things simple we can insert the reference to our filter at the end:

GlobalConfiguration.Configuration
  .EnableSwagger(c =>
  {
    //...
    c.OperationFilter<SwaggerOperationNameFilter>();
  });

Getting the Name

At this point you have a lot of flexibility in how you generate the name for the operation.  The parameters passed in to the Apply method give you access to a lot of contextual information but in my case I wanted to manually specify the name of each operation using a custom attribute.

The custom attribute itself contains a single OperationId property…

[AttributeUsage(AttributeTargets.Method)]
public sealed class SwaggerOperationAttribute : Attribute
{
  public SwaggerOperationAttribute(string operationId)
  {
    this.OperationId = operationId;
  }

  public string OperationId { get; private set; }
}

…and can be dropped onto any action method as required…

[SwaggerOperation("myCustomName")]
public async Task<HttpResponseMessage> MyAction()
{
  //…
}

Once the attributes are in place we can pull the name from our filter using the ActionDescriptor

operation.operationId = apiDescription.ActionDescriptor
  .GetCustomAttributes<SwaggerOperationAttribute>()
  .Select(a => a.OperationId)
  .FirstOrDefault();

Voila!

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