How to implement data validation in wpf by clicking submit button in a mvvm pattern

Question

Hi

I would like to get some thoughts on how to get the button in the wpf to show message when the textbox is empty . How do I go about doing it, theoretical advice would be good, I plan to code myself. I just am getting confused with should I use INotifyDataErrorInfo or INotifyPropertyChanged in this case.

Answer 1

Consider using data annotations.

For a simple example, validate user name and password (along with password confirmation)

using System.ComponentModel.DataAnnotations;  
using ValidateLoginCore.Classes.ValidationRules;  
  
namespace ValidateLoginCore.Classes  
{  
    public class CustomerLogin  
    {  
        /// <summary>  
        /// User name  
        /// </summary>  
        /// <returns>User name for login</returns>  
        [Required(ErrorMessage = "{0} is required"), DataType(DataType.Text)]  
        [StringLength(10, MinimumLength = 6)]  
        public string UserName { get; set; }  
        /// <summary>  
        /// User password which must match PasswordConfirmation using  
        /// PasswordCheck attribute  
        /// </summary>  
        /// <returns>plain text password</returns>  
        [Required(ErrorMessage = "{0} is required"), DataType(DataType.Text)]  
        [StringLength(20, MinimumLength = 6)]  
        [PasswordCheck(ErrorMessage = "Must include a number and symbol in {0}")]  
        public string Password { get; set; }  
        /// <summary>  
        /// Confirmation of user password  
        /// </summary>  
        /// <returns>plain text password</returns>  
        [Compare(  
             "Password",   
             ErrorMessage = "Passwords do not match, please try again"),   
         DataType(DataType.Text)]  
        [StringLength(20, MinimumLength = 6)]  
        public string PasswordConfirmation { get; set; }  
        public override string ToString() => UserName;  
  
    }  
}  

Custom attribute

using System.ComponentModel.DataAnnotations;  
using System.Text.RegularExpressions;  
  
namespace ValidateLoginCore.Classes.ValidationRules  
{  
    public class PasswordCheck : ValidationAttribute  
    {  
        public override bool IsValid(object value)  
        {  
            var validPassword = false;  
            var reason = string.Empty;  
            var password = (value == null) ? string.Empty : value.ToString();  
  
            if (string.IsNullOrWhiteSpace(password) || password.Length < 6)  
            {  
                reason = "new password must be at least 6 characters long. ";  
            }  
            else  
            {  
                var pattern = new Regex("((?=.*\\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{6,20})");  
                if (!pattern.IsMatch(password))  
                {  
                    reason += "Your new password must contain at least 1 symbol character and number.";  
                }  
                else  
                {  
                    validPassword = true;  
                }  
            }  
  
            if (validPassword)  
            {  
                return true;  
            }  
            else  
            {  
                return false;  
            }  
  
        }  
  
    }  
}  

Here when there is an issue we present a message box. For bonus points we could have this information show up in labels which for WPF requires a tad bit more code.

Say we want to validate an SSN passed the usual validation on length

using System;  
using System.ComponentModel.DataAnnotations;  
using System.Text.RegularExpressions;  
  
namespace ValidationCoreLibrary.CommonRules  
{  
    /// <summary>  
    /// Is a valid SSN  
    /// </summary>  
    /// <returns>True if valid, false if invalid SSN</returns>  
    /// <remarks>  
    ///  
    /// Handle some commonly encountered “fake” values or ones that might easily be forged in  
    /// an area that might require a user to enter one.  
    ///   
    /// ^                                       #Start of expression  
    /// (?!\b(\d)\1+-(\d)\1+-(\d)\1+\b)         #Don't allow all matching digits for every field  
    /// (?!123-45-6789|219-09-9999|078-05-1120) #Don't allow "123-45-6789", "219-09-9999" or "078-05-1120"  
    /// (?!666|000|9\d{2})\d{3}                 #Don't allow the SSN to begin with 666, 000 or anything between 900-999  
    /// -                                       #A dash (separating Area and Group numbers)  
    /// (?!00)\d{2}                             #Don't allow the Group Number to be "00"  
    /// -                                       #Another dash (separating Group and Serial numbers)  
    /// (?!0{4})\d{4}                           #Don't allow last four digits to be "0000"  
    /// $                                       #End of expression  
    /// </remarks>  
    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]  
    public class SocialSecurityAttribute : ValidationAttribute  
    {  
        //public string SocialValue { get; set; }  
  
        public override bool IsValid(object value) =>  
            value is string && Regex.IsMatch(value.ToString().Replace("-", ""),  
                @"^(?!\b(\d)\1+\b)(?!123456789|219099999|078051120)(?!666|000|9\d{2})\d{3}(?!00)\d{2}(?!0{4})\d{4}$");  
    }  
}  

In a class usage

public class Person  
{  
    /// <summary>  
    /// First name  
    /// </summary>  
    [Required(ErrorMessage = "{0} is required"), DataType(DataType.Text)]  
    [StringLength(20, MinimumLength = 6)]  
    public string FirstName { get; set; }  
      
    /// <summary>  
    /// Last name  
    /// </summary>  
    [Required(ErrorMessage = "{0} is required"), DataType(DataType.Text)]  
    [StringLength(10)]  
    public string LastName { get; set; }  
      
    /// <summary>  
    /// Phone number  
    /// </summary>  
    [Required(ErrorMessage = "{0} is required"), DataType(DataType.Text)]  
    [Phone]  
    public string Phone { get; set; }  
      
    /// <summary>  
    /// Social Security number  
    /// </summary>  
    [SocialSecurity] public string SSN { get; set; }  
  
    public override string ToString() => $"{FirstName} {LastName} {SSN}";  
  
}