请从System.Web程序使用的属性的 HTTP 您的WebAPI行动命名空间：

  [System.Web.Http.AcceptVerbs（GET，POST）]
    [System.Web.Http.HttpGet]
    公共字符串验证（用户名字符串，字符串密码）
    {...}
 

为什么它不工作的原因是因为你使用的是从 MVC 空间 System.Web.Mvc 属性。在 System.Web.Http 命名空间中的类对于的WebAPI

My route is correctly configured, and my methods have the decorated tag. I still get "The requested resource does not support HTTP method 'GET'" message?

[System.Web.Mvc.AcceptVerbs("GET", "POST")]
[System.Web.Mvc.HttpGet]
public string Auth(string username, string password)
{
  // Décoder les paramètres reçue.
  string decodedUsername = username.DecodeFromBase64();
  string decodedPassword = password.DecodeFromBase64();

  return "value";
}

Here are my routes:

config.Routes.MapHttpRoute(
    name: "AuthentificateRoute",
    routeTemplate: "api/game/authentificate;{username};{password}",
    defaults: new { controller = "Game",
                    action = "Auth", 
                    username = RouteParameter.Optional, 
                    password = RouteParameter.Optional },
    constraints: new { httpMethod = new HttpMethodConstraint(HttpMethod.Get) }
);

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

Please use the attributes from the System.Web.Http namespace on your WebAPI actions:

    [System.Web.Http.AcceptVerbs("GET", "POST")]
    [System.Web.Http.HttpGet]
    public string Auth(string username, string password)
    {...}

The reason why it doesn't work is because you were using the attributes that are from the MVC namespace System.Web.Mvc. The classes in the System.Web.Http namespace are for WebAPI.