How do I check to see if a user exists already in the controller?
<p>Here is my register method. I changed the register method into a Admin tool. The admin tool works properly without the if statement. However, it does not like the way I am looking for existing users.</p>

<pre><code>    // POST: /Account/Register
[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task&lt;ActionResult&gt; Register(RegisterViewModel model)
{    
    if (ModelState.IsValid)
    {
        if(_context.Users.Contains(model.UserName))//Errors out here
        {
            var user = new ApplicationUser { UserName = model.UserName};
            user.UserName = model.UserName;
            var result = await UserManager.CreateAsync(user, model.Password);
            if (result.Succeeded)
            {
                await this.UserManager.AddToRoleAsync(user.Id, model.Name);
                return RedirectToAction("Index","User");
            }
        AddErrors(result);
        }
    }
    return View(model);
}
</code></pre>