how to write validation error

please refer below one,

https://www.c-sharpcorner.com/article/validation-message-and-validation-summary-in-asp-net-mvc/

https://www.tutorialsteacher.com/mvc/htmlhelper-validationmessagefor

1. Using Validation Controls:
   ASP.NET provides built-in validation controls that you can use to display validation errors. You can place these controls on your web form and customize their error messages. Some commonly used validation controls include:
   - RequiredFieldValidator: Ensures that a specific input field is not left empty.
   - RangeValidator: Checks if the input falls within a specified range of values.
   - RegularExpressionValidator: Validates the input against a specified regular expression pattern.
   - CustomValidator: Allows you to define custom validation logic.

   To display validation errors using these controls, follow these steps:
   a. Add the required validation control to your web form, specifying the validation logic and error message.
   b. Set the `ErrorMessage` property of the validation control to the desired error message.
   c. Associate the validation control with the input control you want to validate using the `ControlToValidate` property.
   d. When the validation fails, the error message will be displayed automatically.

2. Using ModelState:
   In ASP.NET MVC, you can use the `ModelState` object to store and display validation errors. This approach allows you to perform server-side validation and display the errors next to the corresponding input fields. Here's an example:

[HttpPost]
public ActionResult MyAction(MyModel model)
{
    if (!ModelState.IsValid)
    {
        return View(model); // Return the view with validation errors
    }

    // Perform other actions if the validation passes

    return RedirectToAction("SuccessAction"); // Redirect to a success page
}

In your view, you can display validation errors using the `ValidationMessageFor` helper method:

@model MyModel

@Html.ValidationMessageFor(m => m.PropertyName)

Make sure you have included the necessary validation attributes (e.g., `[Required]`, `[Range]`, etc.) on your model properties.

3. Manual Validation:
   You can manually validate input in your code-behind or controller and add validation errors to the `ModelState` object. This approach gives you more flexibility but requires additional code. Here's an example:

[HttpPost]
public ActionResult MyAction(string inputValue)
{
    if (string.IsNullOrEmpty(inputValue))
    {
        ModelState.AddModelError("inputValue", "The input value is required.");
    }

    if (!ModelState.IsValid)
    {
        return View(); // Return the view with validation errors
    }

    // Perform other actions if the validation passes

    return RedirectToAction("SuccessAction"); // Redirect to a success page
}

In your view, you can display manual validation errors using the `ValidationSummary` helper method:

@Html.ValidationSummary()

These are some common approaches to writing validation errors in ASP.NET. The specific method you choose depends on your application's architecture and requirements.