title: Swagger showing no parameters to controller actions that have paramters
body: <p>Swagger <strong>not showing parameters</strong> in UI and JSON, even tho my method has parameters, This particularly happens when I add the <strong>[FromBody]</strong> tag <br/>
  <a href="https://i.stack.imgur.com/cLVDF.png" rel="nofollow noreferrer">swagger UI no parameters</a>
  <a href="https://i.stack.imgur.com/8Xff2.png" rel="nofollow noreferrer">JSON file no parameters</a> <br/>
  The action method:</p>
  
  <pre><code>    [HttpPost("Action")]
      public IActionResult Action([FromBody] string message)
      {
          return Ok(message);
      }
  </code></pre>
  
  <hr>
  
  <p>I used fresh Asp.net core <strong>3.1</strong> and <strong>2.2</strong> web app with API template to test this,
  I configured it exactly like the <a href="https://github.com/domaindrivendev/Swashbuckle.AspNetCore#getting-started" rel="nofollow noreferrer">documnetaiton </a> <br/>
  ConfigureServices:</p>
  
  <pre><code>services.AddMvc();
  
  services.AddSwaggerGen(c =&gt;
  {
          c.SwaggerDoc("v1", new OpenApiInfo { Title = "My API", Version = "v1" });
  }
  </code></pre>
  
  <p>Configure: <br/></p>
  
  <pre><code> app.UseSwagger();
  
     app.UseSwaggerUI(c =&gt;
   {
      c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
   });
  </code></pre>
  
  <hr>
  
  <p>It does <strong>work</strong> when I use <strong>other attributes</strong> like [FromRoute]&amp;[FromHeader] so on... I looked at the <a href="https://petstore.swagger.io/#/pet/updatePetWithForm" rel="nofollow noreferrer">examples</a> and swagger does show parameters from a post method <br/></p>
  
  <hr>
  
  <p>I also tried objects like this :</p>
  
  <pre><code>    public class Message
  {
      public int ID { get; set; }
      public string body { get; set; }
  }
  </code></pre>
  
  <p>With this action :</p>
  
  <pre><code> [HttpPost("Action")]
   public IActionResult Action([FromBody] Message message)
   {
      return Ok(message);
   }
  </code></pre>
  
  <p>same <a href="https://i.stack.imgur.com/wKQiO.png" rel="nofollow noreferrer">result</a> (no parameters) but it does show the schema</p>
  
  <hr>
  
  <p>So what am I doing wrong? how can I document post parameters like the examples </p>