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);
    }
}

You can use the compare annotation to compare two values, and if you need to make sure it isn't persisted anywhere downstream (For example if you are using EF) You can also add NotMapped to ignore any entity->DB mapping

For a full list of data annotations available, see here:

https://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations(v=vs.110).aspx

[Required]
[DataType(DataType.Password)]
public string Password { get; set; }

[Required]
[DataType(DataType.Password)]
[Compare("Password")]
[NotMapped]
public string ConfirmPassword { get; set; }