Data validation is a crucial aspect of web applications, ensuring that the data submitted by users meets specific criteria before it is processed or stored. In ASP.NET MVC, data validation can be implemented using data annotations, custom validation attributes, and model validation techniques. This guide will explain how to implement data validation in ASP.NET MVC with sample code.

1. Using Data Annotations

Data annotations are attributes that can be applied to model properties to enforce validation rules. ASP.NET MVC provides several built-in validation attributes that you can use to validate user input. Below is an example of a model class with data annotations:

        
        using System.ComponentModel.DataAnnotations;

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

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

            [Range(0.01, double.MaxValue, ErrorMessage = "Price must be greater than zero")]
            public decimal Price { get; set; }

            [StringLength(100, ErrorMessage = "Description cannot exceed 100 characters")]
            public string Description { get; set; }
        }
            

In this example:

• The [Required] attribute ensures that the Name property must have a value.
• The [Range] attribute specifies that the Price must be greater than zero.
• The [StringLength] attribute limits the length of the Description property to a maximum of 100 characters.

2. Using Validation in Controllers

When a user submits a form, you can validate the model in the controller action method. If the model is invalid, you can return the same view with validation messages. Below is an example of a controller that uses the Product model:

        
        using System.Collections.Generic;
        using System.Web.Mvc;

        public class ProductController : Controller
        {
            private static List<Product> products = new List<Product>();

            [HttpGet]
            public ActionResult Create()
            {
                return View();
            }

            [HttpPost]
            public ActionResult Create(Product product)
            {
                if (ModelState.IsValid)
                {
                    products.Add(product); // Add the product to the list
                    return RedirectToAction("Index"); // Redirect to the index action
                }
                return View(product); // Return the view with validation errors
            }

            public ActionResult Index()
            {
                return View(products); // Pass the list of products to the view
            }
        }
            

In this example:

• The Create action method checks if the model state is valid using ModelState.IsValid.
• If the model is valid, the product is added to the list, and the user is redirected to the index action.
• If the model is invalid, the same view is returned with the product model, which contains validation error messages.

3. Displaying Validation Messages in Views

To display validation messages in the view, you can use HTML helpers that automatically render validation messages based on the model state. Below is an example of a view named Create.cshtml that uses the Product model:

        
        @model Product

        <h2>Create New Product</h2>
        @using (Html.BeginForm())
        {
            <div>
                @Html.LabelFor(m => m.Name)
                @Html.TextBoxFor(m => m.Name)
                @Html.ValidationMessageFor(m => m.Name)
            </div>
            <div>
                @Html.LabelFor(m => m.Price)
                @Html.TextBoxFor(m => m.Price)
                @Html.ValidationMessageFor(m => m.Price)
            </div>
            <div>
                @Html.LabelFor(m => m.Description)
                @Html.TextBoxFor(m => m.Description)
                @Html.ValidationMessageFor(m => m.Description)
            </div>
            <button type="submit">Create</button>
        }
            

In this example:

• The @model directive specifies that the view expects a model of type Product.
• Form elements are created using HTML helpers, which bind to the properties of the model and automatically display validation messages for user feedback.

4. Custom Validation Attributes

In addition to built-in validation attributes, you can create custom validation attributes to enforce specific rules. Below is an example of a custom validation attribute that checks if a product's price is a multiple of 5:

        
        using System;
        using System.ComponentModel.DataAnnotations;

        public class MultipleOfFiveAttribute : ValidationAttribute
        {
            protected override ValidationResult IsValid(object value, ValidationContext validationContext)
            {
                if (value is decimal decimalValue && decimalValue % 5 != 0)
                {
                    return new ValidationResult("Price must be a multiple of 5.");
                }
                return ValidationResult.Success;
            }
        }
            

You can then use this custom attribute in your model:

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

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

            [MultipleOfFive(ErrorMessage = "Price must be a multiple of 5.")]
            public decimal Price { get; set; }

            [StringLength(100, ErrorMessage = "Description cannot exceed 100 characters")]
            public string Description { get; set; }
        }
            

Conclusion

Implementing data validation in ASP.NET MVC is essential for ensuring data integrity and providing a better user experience. By using data annotations, validation in controllers, and custom validation attributes, you can effectively manage user input and display meaningful feedback. This approach not only enhances the robustness of your application but also helps maintain a clean and maintainable codebase.