Server-Side Validation

1. Description

Server-side validation checks that incoming data meets rules and constraints before processing. In ASP.NET Core, validation is often driven by DataAnnotations on models and verified in controllers using ModelState.IsValid.

2. Why It Is Important

Client-side validation can be bypassed. Server-side validation ensures data integrity, security, and correct business behavior regardless of client capabilities.

3. Real-World Examples

• Validate an order's total is non-negative.
• Ensure required registration fields are provided and email is valid.

4. Syntax & Explanation

Model with annotations (see UserViewModel in earlier file). Controller action performing validation:

[HttpPost]
public IActionResult Register(UserViewModel model)
{
    if (!ModelState.IsValid)
    {
        // Return the view with validation errors displayed
        return View(model);
    }

    // Continue processing (save to DB, etc.)
    return RedirectToAction("Success");
}

If custom validation is needed, you can:

• Implement IValidatableObject on the model and its Validate method.
• Create custom ValidationAttribute classes.

Example custom validation attribute:

public class NotZeroAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value is int i) return i != 0;
        return true;
    }
}

5. Use Cases

• Protecting endpoints from invalid or malicious input.
• Enforcing business rules before database writes.
• Returning detailed errors for API clients.

6. Mini Practice Task

1. Add server-side validation to a checkout form ensuring Quantity >= 1 and Price >= 0.
2. Implement a custom attribute [MaxWords(50)] that limits a description to a number of words.