405 method not allowed Web API

You are POSTing from the client:

await client.PostAsJsonAsync("api/products", product);

not PUTing.

Your Web API method accepts only PUT requests.

So:

await client.PutAsJsonAsync("api/products", product);

My problem turned out to be Attribute Routing in WebAPI. I created a custom route, and it treated it like a GET instead of WebAPI discovering it was a POST

    [Route("")]
    [HttpPost] //I added this attribute explicitly, and it worked
    public void Post(ProductModel data)
    {
        ...
    }

I knew it had to be something silly (that consumes your entire day)

I tried many thing to get DELETE method work (I was getting 405 method not allowed web api) , and finally I added [Route("api/scan/{id}")] to my controller and was work fine. hope this post help some one.

     // DELETE api/Scan/5
    [Route("api/scan/{id}")]
    [ResponseType(typeof(Scan))]
    public IHttpActionResult DeleteScan(int id)
    {
        Scan scan = db.Scans.Find(id);
        if (scan == null)
        {
            return NotFound();
        }

        db.Scans.Remove(scan);
        db.SaveChanges();

        return Ok(scan);
    }

I had the same exception. My problem was that I had used:

using System.Web.Mvc; // Wrong namespace for HttpGet attribute !!!!!!!!!
[HttpGet]
public string Blah()
{
    return "blah";
}

SHOULD BE

using System.Web.Http; // Correct namespace for HttpGet attribute !!!!!!!!!
[HttpGet]
public string Blah()
{
    return "blah";
}