Give custom validation in controller action method in MVC

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I am doing validation in controller part action method and I got stuck at a point that to give validation for name by checking the condition that the name should not appear already in the table. I have tried many conditions but always getting some error. can anyone please help me to find solution for my problem ???

controller

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Create([Bind(Include = "Id,Name")] Student student)
        {
            TempData["name"] = student.Name;                     
            var show = db.Students.Select(e => e.Name).ToList();
            if (show == TempData["name"])
            {

                ModelState.AddModelError("Name", "Name already exists");

            }
            if (ModelState.IsValid)
                {
                    db.Students.Add(student);
                    await db.SaveChangesAsync();
                    return RedirectToAction("Index");
                }
            return View(student);
        }

This is my controller for checking , I am retrieving all the names from table in var show , then I need to check whether the result in show is there or not and if the new name is not in show I need to enter it into the table else need to show the error message as ModeState.AddModelError(). Can anyone please help me to find a solution for my problem ???

What I have tried:

had checked in many ways but always getting error that the new name given is already exist.

Posted 23-Apr-17 1:57am

ammu11

Updated 23-Apr-17 2:12am

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Afzaal Ahmad Zeeshan · Accepted Answer · 2017-04-23T02:12:00

The way I see your code, I would make a few changes before even rerunning the application,

public async Task<ActionResult> Create([Bind(Include = "Id,Name")] Student student)
{
    // No need for a temp variable, I can get the Name from parameter.           
    var show = db.Students.Select(e => e.Name).ToList();

    // "show" is a List<T> variable; List<string>
    // Thus, check if the list contains this name
    if (show.Contains(student.Name))
    {
        ModelState.AddModelError("Name", "Name already exists");
    }

This would be a proper way to do this, why? Because, previously you were trying to check a list against a temporary variable which would typically return object type data. I am unsure as to why that resolved to true.

There is another approach to this kind of behavior,

C#

var show = db.Students.Where(e => e.Name == student.Name).ToList(); 

if(show != null && show.Count != 0) {
    // There exists a user
} else {
    // Add the user
}

Perhaps, this approach would solve your problem here.