How can I implement custom validation in ASP.NET?

How can I implement custom validation in ASP.NET?

You can implement custom validation in ASP.NET by creating a custom validation attribute that inherits from ValidationAttribute.

Can you show me an example of a custom validation attribute?

Sure! Here's a simple example:

public class MyCustomValidationAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null || (value is string str && str.Length < 5))
        {
            return new ValidationResult("The field must be at least 5 characters long.");
        }
        return ValidationResult.Success;
    }
}

How do I apply this custom validation attribute to a model property?

You can apply it like this:

public class MyModel
{
    [MyCustomValidation]
    public string MyProperty { get; set; }
}

What if I want to add error messages in different languages?

You can override the FormatErrorMessage method in your custom validation attribute to provide localized error messages.