.Net Core: Filter records based on parameter value using Linq

Hi, 

I have a simple controller which will returning a list based on parameter

The schema is from service will give result list data then it will  IQueryable  based parameter 

my problem is why is data result not filtered by param

thank you

public async Task<IActionResult> AssignRole(string SearchString)
{
    List<ViewUser> user = new List<ViewUser>();
    user = await _iuser.GetUser();
    IQueryable<ViewUser> data = user.AsQueryable();
    try
    {
        switch (SearchString)
        {
            case  null:
                data.ToList();
                break;
            default:
                data.Where(m => m.user_name == SearchString).ToList();
                break;
        }         
    }
    catch (Exception ex)
    {
        
    }
    return View(data);
}