Override ActionFilter attribute is no working in ASP.NET Web APi 2

I'm using ASP.NET WEB API 2 to migrate an existing web service.

Below is set of filter that I use

public class ValidateSession : ActionFilterAttribute
{

    public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        //var requestMessage = actionContext.Request.Content.ToString();
        if (!actionContext.Request.Headers.Contains("source"))
        {
            actionContext.Request.Headers.Add("source", "1");
        }
        if (!actionContext.Request.Headers.Contains("appstore_session_id"))
        {
            actionContext.Response = actionContext.Request.CreateErrorResponse(System.Net.HttpStatusCode.BadRequest, "Session id is not included in the header");
        }
    }


}

Below is the code for override the previous ActionFilter attribute

public class OverrideSessionValidation : ActionFilterAttribute, IOverrideFilter
{
    public Type FiltersToOverride
    {
        get { return typeof(ValidateSession); }
    }

    public bool AllowMultiple
    {
        get { return true; }
    }


}

The code for controller is also give below

[ValidateSession]
public class SampleController : ApiController
{
    public string GetSessionValues()
    {
        return "from session vals";
    }
    [OverrideSessionValidation]
    public string GetDefaultVals()
    {
        return "from DefVals";
    }
}

It can be seen that I have placed the validate session at the class level and for one method I want to override the same. Hence for the second method I used overrideSessionValidation. Though the

FiltersToOverride of OverrideSessionValidation is called, I see that onActionExcuting for ValidateSession is also called. I expect that the class filter onActionExecuting should not be called as I have Override for the same.

Please let me know what is the error so that I can solve this problem

Thanks and regards Venkatesh