Object validation

The Validator class is commonly used for validating data annotations on objects. Validation attributes are defined in the System.ComponentModel.DataAnnotations namespace. To validate an object, first it must be annotated:

public class Person
{
    [Required(ErrorMessage = "Name is required.")]
    [StringLength(100, ErrorMessage = "Name cannot be longer than 100 characters.")]
    public string Name { get; set; }

    [Range(1, 120, ErrorMessage = "Age must be between 1 and 120.")]
    public int Age { get; set; }

    [EmailAddress(ErrorMessage = "Invalid Email Address.")]
    public string Email { get; set; }
}

Validation code:

//person instance with invalid data
var person = new Person
{
    Name = null, //Invalid: Required
    Age = 150, //Invalid: Out of Range
    Email = "invalid-email" //Invalid: Not an email format
};

// Create a list to hold validation results
var validationResults = new List<ValidationResult>();

// Validate the object
var validationContext = new ValidationContext(person, null, null);
bool isValid = Validator.TryValidateObject(person, validationContext, validationResults, true);

// Output the validation results
if (!isValid)
{
    Console.WriteLine("Validation failed for the following reasons:");

    foreach (var validationResult in validationResults)
    {
        Console.WriteLine($"- {validationResult.ErrorMessage}");
    }
}
else
{
    Console.WriteLine("Validation succeeded.");
}

Defining a custom validation attribute

You can create a custom attribute by deriving from ValidationAttribute class:

public class FutureDateAttribute : ValidationAttribute
{
    public FutureDateAttribute()
    {
        ErrorMessage = "The date must be in the future.";
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        if (value == null)
        {
            return new ValidationResult(ErrorMessage);
        }

        DateTime dateValue;
        if (DateTime.TryParse(value.ToString(), out dateValue))
        {
            if (dateValue > DateTime.Now)
            {
                return ValidationResult.Success;
            }
        }

        return new ValidationResult(ErrorMessage);
    }
}