How can I implement custom validation attributes in ASP.NET?

How can I implement custom validation attributes in ASP.NET?

You can create custom validation attributes by inheriting from the ValidationAttribute class and overriding the IsValid method.

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

Sure! Here's a simple example:

public class AgeValidationAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value is int age && age >= 18)
        {
            return ValidationResult.Success;
        }
        return new ValidationResult("Age must be at least 18.");
    }
}

How do I apply this attribute to a model property?

You can apply it like this:

public class User
{
    [AgeValidation]
    public int Age { get; set; }
}

What if I want to add a custom error message?

You can set a custom error message in the constructor of your validation attribute:

public class AgeValidationAttribute : ValidationAttribute
{
    public AgeValidationAttribute() : base("You must be at least 18 years old.") {}
    // ... rest of the code
}