Learn how to implement and leverage custom validation in ASP.NET Core to handle complex scenarios and ensure robust data validation.
Introduction
In modern web applications, validating user input is crucial to ensure data integrity, prevent errors, and enhance security. While ASP.NET Core provides a rich set of built-in validation attributes, there are scenarios where custom logic is required. This is where custom validation comes into play, allowing developers to implement specific business rules seamlessly.
The following diagram illustrates Custom Validation in ASP.NET Core:
Understanding Custom Validation
Custom validation refers to the process of creating custom attributes or logic to validate data that cannot be handled using built-in attributes like [Required] or [Range]. For example, you might need to validate a username for uniqueness or enforce a specific format for a custom ID field.
When to Use Custom Validation?
Custom validation is required when:
- Complex business rules cannot be handled by built-in attributes.
- You need to perform validation based on external data (e.g., database checks).
- Validation rules depend on multiple model properties.
Creating Custom Validation Attributes
To create a custom validation attribute, inherit from the ValidationAttribute class and override the IsValid method:
// Custom validation attribute to validate age
public class AgeValidationAttribute : ValidationAttribute
{
private readonly int _minAge;
private readonly int _maxAge;
public AgeValidationAttribute(int minAge, int maxAge)
{
_minAge = minAge;
_maxAge = maxAge;
}
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
if (value == null)
{
return new ValidationResult("Age is required.");
}
int age = (int)value;
if (age < _minAge || age > _maxAge)
{
return new ValidationResult($"Age must be between {_minAge} and {_maxAge}.");
}
return ValidationResult.Success;
}
}
Applying Custom Validation to Models
Once the custom validation attribute is created, you can apply it to model properties like this:
public class UserViewModel
{
[Required]
public string Name { get; set; }
[AgeValidation(18, 60, ErrorMessage = "Age must be between 18 and 60.")]
public int Age { get; set; }
}
Handling Client-Side Validation
ASP.NET Core supports client-side validation for custom attributes when paired with a validation adapter. You need to register the adapter in the Startup.cs file and implement JavaScript for client-side validation.
Real-World Example
Consider a scenario where you need to validate that a product's price is higher than a minimum threshold only if the product is marked as premium:
// Custom validation attribute for conditional validation
public class PremiumProductPriceValidation : ValidationAttribute
{
protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
var product = (ProductViewModel)validationContext.ObjectInstance;
if (product.IsPremium && product.Price < 100)
{
return new ValidationResult("Premium products must have a price of at least $100.");
}
return ValidationResult.Success;
}
}
Best Practices
- Keep custom validation attributes reusable and modular.
- Use clear and user-friendly error messages.
- Test validation logic thoroughly for edge cases.
- Combine custom validation with built-in attributes for comprehensive validation.
Conclusion
Custom validation in ASP.NET Core allows developers to enforce complex business rules and ensure data integrity efficiently. By leveraging custom validation attributes, you can create flexible and reusable validation logic, enhancing your application's overall reliability and user experience.