Data Annotation to validate confirm password

If you are using ASP.Net MVC 3, you can use System.Web.Mvc.CompareAttribute.

If you are using ASP.Net 4.5, it’s in the System.Component.DataAnnotations.

[Required(ErrorMessage = "Password is required")]
[StringLength(255, ErrorMessage = "Must be between 5 and 255 characters", MinimumLength = 5)]
[DataType(DataType.Password)]
public string Password { get; set; }

[Required(ErrorMessage = "Confirm Password is required")]
[StringLength(255, ErrorMessage = "Must be between 5 and 255 characters", MinimumLength = 5)]
[DataType(DataType.Password)]
[Compare("Password")]
public string ConfirmPassword { get; set; }

EDIT: For MVC2 Use the below Logic, Use PropertiesMustMatch instead Compare attribute [Below code is copied from the default MVCApplication project template.]

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
public sealed class PropertiesMustMatchAttribute : ValidationAttribute
{
    private const string _defaultErrorMessage = "'{0}' and '{1}' do not match.";
    private readonly object _typeId = new object();

    public PropertiesMustMatchAttribute(string originalProperty, string confirmProperty)
        : base(_defaultErrorMessage)
    {
        OriginalProperty = originalProperty;
        ConfirmProperty = confirmProperty;
    }

    public string ConfirmProperty { get; private set; }
    public string OriginalProperty { get; private set; }

    public override object TypeId
    {
        get
        {
            return _typeId;
        }
    }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString,
            OriginalProperty, ConfirmProperty);
    }

    public override bool IsValid(object value)
    {
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(value);
        object originalValue = properties.Find(OriginalProperty, true /* ignoreCase */).GetValue(value);
        object confirmValue = properties.Find(ConfirmProperty, true /* ignoreCase */).GetValue(value);
        return Object.Equals(originalValue, confirmValue);
    }
}