[HttpPost]
    [Route("api/post/")]
    public IHttpActionResult MyPost([ModelBinder(typeof(MyModelBinder))]IModel viewModel)
    {
        doSomething(viewModel);
        return Ok("ok");
    }

public class MyModelBinder : IModelBinder
{
    public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
    {
        var content = actionContext.Request.Content;
        string json = content.ReadAsStringAsync().Result;

        try
        {
            var DTO = JsonConvert.DeserializeObject<A>(json);
            IModel viewModel = new A()
            {
                // mapping here
            };
            bindingContext.Model = viewModel;
            return true;
        }
        catch(Exception e)
        {

        }
        try
        {
            var DTO = JsonConvert.DeserializeObject<B>(json);
            IModel viewModel = new B()
            {
                // mapping here
            };
            bindingContext.Model = viewModel;
            return true;
        }
        catch (Exception e)
        {

        }
        return false;
    }
}