ASP.NET Core Form Validation Example

 Form Validation in ASP.NET Core MVC is handled using Model Validation + Data Annotations + ModelState.

ASP.NET Core provides 3 Levels of Validation:

1. Model Level Validation (Recommended)
2. Client-Side Validation
3. Custom Validation

I'll explain step-by-step professionally 👇

---

1. Model Validation (Using Data Annotations)

Create Model with Validation Attributes

using System.ComponentModel.DataAnnotations;

public class Employee
{
    public int Id { get; set; }

    [Required(ErrorMessage ="Name is required")]
    public string Name { get; set; }

    [Required]
    [EmailAddress(ErrorMessage ="Invalid Email")]
    public string Email { get; set; }

    [Required]
    [Range(18,60)]
    public int Age { get; set; }

    [Required]
    [StringLength(10, MinimumLength = 6)]
    public string Password { get; set; }
}

---

Most Important Validation Attributes

Attribute	Purpose
Required	Mandatory Field
StringLength	Min & Max length
Range	Numeric Range
EmailAddress	Validate Email
Phone	Validate Phone
Compare	Compare fields
RegularExpression	Custom Pattern
MinLength	Minimum Length
MaxLength	Maximum Length

---

2. Controller Validation

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Create(Employee emp)
{
    if (ModelState.IsValid)
    {
        // Save Data
        return RedirectToAction("Index");
    }

    return View(emp);
}

---

What is ModelState.IsValid

ASP.NET automatically checks:

• Required Fields
• Format Validation
• Range Validation

If all correct → ModelState.IsValid = true

Else → false

---

3. View Validation (Razor Form)

<form asp-action="Create" method="post">

    <div>
        <label>Name</label>
        <input asp-for="Name" />
        <span asp-validation-for="Name"></span>
    </div>

    <div>
        <label>Email</label>
        <input asp-for="Email" />
        <span asp-validation-for="Email"></span>
    </div>

    <button type="submit">Submit</button>

</form>

---

Important Tag Helpers

Tag	Purpose
asp-for	Bind Model Property
asp-validation-for	Show Error
asp-validation-summary	Show All Errors

---

Show All Errors

<div asp-validation-summary="All"></div>

---

4. Enable Client Side Validation

Add in _Layout.cshtml

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

This enables:

• Real-time validation
• No page reload validation

---

Required jQuery Files

Already included in ASP.NET Core:

jquery.validate.min.js
jquery.validate.unobtrusive.min.js

---

Example Full Form

<form asp-action="Create" method="post">

    <div asp-validation-summary="All"></div>

    <input asp-for="Name" />
    <span asp-validation-for="Name"></span>

    <input asp-for="Email" />
    <span asp-validation-for="Email"></span>

    <button type="submit">Submit</button>

</form>

---

Custom Validation Example

[Required]
[RegularExpression(@"^[a-zA-Z]+$", 
 ErrorMessage ="Only Characters Allowed")]
public string Name { get; set; }

---

Compare Validation Example

Password Confirm

[Required]
public string Password { get; set; }

[Compare("Password", ErrorMessage ="Password mismatch")]
public string ConfirmPassword { get; set; }

---

Server Side vs Client Side

Type	Where Runs
Client Validation	Browser
Server Validation	Controller

Always use Both