Required Validation

In this sample, Entering nothing in the product name or price and click the submit button, an error message will be displayed.

RequiredValidation.cshtml.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace WorkStandard.Pages.validation
{
    public class RequiredValidationModel : PageModel
    {
        [BindProperty]
        [DisplayName("Product Name")]
        [Required(ErrorMessage = "{0} is required.")]
        public string? productName { get; set; }

        [BindProperty]
        [DisplayName("Price")]
        [Required(ErrorMessage = "Enter a number for {0}.")]
        public int price { get; set; }

        public void OnPost()
        {
            //Server side validation check
            if (!ModelState.IsValid)
            {
                return;
            }

            System.Diagnostics.Debug.WriteLine($"No errors.{productName} {price}");
        }
    }
}

→ Where Debug.WriteLine outputs to

RequiredValidation.cshtml

@page
@model WorkStandard.Pages.validation.RequiredValidationModel

<form method="post" >
    <div class="mb-3">
        <label asp-for="productName"></label>
        <input asp-for="productName" class="form-control" />
        <span asp-validation-for="productName" class="text-danger"></span>
    </div>
    <div class="mb-3">
        <label asp-for="price"></label>
        <input asp-for="price" class="form-control" />
        <span asp-validation-for="price" class="text-danger"></span>
    </div>
    <div>
        <input type="submit" class="btn btn-primary" />
    </div>
</form>

@section Scripts {
    @{
        //Enable client side validation.
        await Html.RenderPartialAsync("_ValidationScriptsPartial");
    }
}

Note: As in this example, you must check validation result at server side. Malicious users can easily bypass client side validation to attack the server.

The CSS classes...

• mb-3 Set little margin at bottom.→ Spacing
• form-control Make the input a Bootstrap look.→ Forms
• text-danger The string become red to indicate danger.→ Theme colors
• btn btn-primary Give the button a Bootstrap look to indicate that it's a key feature.→ Buttons