WebApi routing for asp.net WebForms returns 404

I think others were very close. Try this:

[RoutePrefix("api")]  // or maybe "api/", can't recall OTTOMH...
public class MyController : ApiController
{
    [Route("MyController")]
    [HttpPost]
    public HttpResponseMessage Post([FromBody]string value)

and then request /api/MyController

If this doesn't work, use RouteDebugger to analyze your routes, and why it rejects a match. Amend your question with what you see in RouteDebugger so I can trace down what is not matching.

Also, you may need to call MapHttpAttributeRoutes in your Register function - but not sure about that.

Edit

Now that I'm looking at it again, I think I see more issues with it.

First, let's start with the template:

Here is what you have (from your question):

config.Routes.MapHttpRoute(
                    name: "DefaultApi",
                    routeTemplate: "api/{controller}",
                    defaults: new { id = System.Web.Http.RouteParameter.Optional });

The odd part here is that your template does not have {id} segment, but is defined as optional. Sounds like it is missing from the template, and the route should be changed to:

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = System.Web.Http.RouteParameter.Optional });

Note that you also removed default action - I'm not sure if MVC automatically uses convention method of locating a method called Post, but I suppose it does.

Second problem is that your method signature is (again from your question):

public HttpResponseMessage Post([FromBody]string value)

It defines Post that takes in a parameter named value, whereas your route defines parameter named id. Hence there is another mismatch. You can rename the variable or decorate (see below). Also, {id} is marked optional, but I believe (and here I don't remember exactly OTTOMH) you need to give default value to to value for those cases where {id} is not supplied, and so combining together:

public HttpResponseMessage Post([FromBody(Name="id")]string value = null)

This should fix it, although there may be more issues... but let's start with these.

Change url: "/api/MyController" to url: "/api/My"

Have you considered using Route attributes instead? I find them to be easier than route configs.

public class MyController : ApiController
{
    [Authorize]
    [Route("MyController")]
    [HttpPost]
    public HttpResponseMessage Post([FromBody]string value)
    {....}
}

perhaps you could additionally try configuring your routes inside of a dedicated RouteConfig class that mvc4 also supports. My route config setups look like this:

/// <summary>
/// The route config class
/// </summary>
public class RouteConfig
{
    /// <summary>
    /// Registers the routes.
    /// </summary>
    /// <param name="routes">The routes.</param>
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );
    }
}

Use [RoutePrefix] on your Controller Class

[RoutePrefix("api/My")]
public class MyController : ApiController
{
    [Route("MyController")]
    [HttpPost]
    public HttpResponseMessage Post([FromBody]string value)
    {....}
}

Then you should be able to navigate to api/My/MyController

Change [Route("MyController")] to [Route("api/MyController")].

It will then work as you expect it to.

Your RouteAttribute overrides config settings, so you have two options:

1) remove RouteAttribute and use URL: /api/my

2) keep RouteAttribute and use URL /MyController (in this case config settings ignored)

From comments i see, that you want to navigate to: api/MyController. So third option not tested, but I think it might work:

3) [Route("api/MyController")]

My challenges were slightly different. My setup is asp.net webforms project and using VS2013. I wanted to introduce Web Api. The curve ball was my web forms implementation is in VB.net.

I wanted to write my APIs in C# so I had to enable codeSubDirectories option via web.config.

<codeSubDirectories>
    <add directoryName="VBCode"/>
    <add directoryName="CSCode"/>        
</codeSubDirectories>

I created two folders (VBCode and CSCode) under App_Code folder respectively.

I then moved all my vb.net classes to VBCode sub directory.

Created UserController.cs under CSCode and wrote regular web api code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;

namespace WebAPIDemo
{

public class UserController : ApiController
{
    static IUserRepository _repo = new UserRepository();
    // GET api/<controller>

    public IEnumerable<Users> GetAllUsers()
    {
        return _repo.GetAllUsers();            
    }

    // GET api/<controller>/5
    public IHttpActionResult GetUserById(int id)
    {

        return Json(_repo.GetUserById(id));
    }

Modified the global.asax

<%@ Application Language="VB" %>
<%@ Import Namespace="System.Web.Routing" %>
<%@ Import Namespace="System.Web.Http" %>

<script RunAt="server">

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs on application startup
        RouteTable.Routes.MapHttpRoute(
             "defaultApi",
            "api/{controller}/{action}/{id}",
            defaults:=New With {.id =     System.Web.Http.RouteParameter.Optional})

    End Sub

The api call worked with and without this decorator [RoutePrefix("api")].

Took me several hours to figure out this implementation.

Call 1:
http://localhost:60203/api/user/getuserbyid/2

Result:
{"Id":2,"FirstName":"Alan","LastName":"Wake","Company":"XYZ Corp","Email":"[email protected]","PhoneNo":"64649879"}

Call 2:
http://localhost:60203/api/user/getallusers

Result:
<ArrayOfUsers xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/WebAPIDemo">
<Users>
<Company>ABC</Company>
<Email>[email protected]</Email>
<FirstName>Madhur</FirstName>
<Id>1</Id>
<LastName>Kapoor</LastName>
<PhoneNo>65431546</PhoneNo>
</Users>
<Users>
<Company>XYZ Corp</Company>
<Email>[email protected]</Email>
<FirstName>Alan</FirstName>
<Id>2</Id>
<LastName>Wake</LastName>
<PhoneNo>64649879</PhoneNo>
</Users>
</ArrayOfUsers>