源BLog https://kmatyaszek.github.io/wpf%20validation/2019/03/06/wpf-validation-using-idataerrorinfo.html
    示例工程 https://github.com/JustMySpace/DemoCollections.WPF/tree/main/%E5%B1%9E%E6%80%A7%E9%AA%8C%E8%AF%81/INotifyDataErrorInfo%E5%AE%9E%E7%8E%B0%E5%B1%9E%E6%80%A7%E9%AA%8C%E8%AF%81

    优点在于支持复杂的验证逻辑，但是要与Attribute 如 MaxLength、Required等结合需要自己补充一些逻辑，且在现有的Mvvm框架中较难以动态代理的方式添加这些逻辑。

    INotifyDataErrorInfo是.NET 4.5中增加的一个接口,可是实现自定义的规则验证并反馈结果到界面,该接口包含三个接口:

    • HasErrors 标识是否有验证错误
    • GetErrors 返回IEnumerable类型的错误集合,当propertyName 不为空时,返回特定属性的验证错误,当propertyName 为空字符串或Null时,戴白哦整个实体的验证错误
    • ErrorsChanged 当验证错误发生变化时,必须触发这个事件

    一个简单的示例
    界面:

    1. <TextBox Text="{Binding UserName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnNotifyDataErrors=True}">
    2.     <Validation.ErrorTemplate>
    3.         <ControlTemplate>
    4.             <StackPanel>
    5.                 <AdornedElementPlaceholder x:Name="textBox" />
    6.                 <ItemsControl ItemsSource="{Binding}">
    7.                     <ItemsControl.ItemTemplate>
    8.                         <DataTemplate>
    9.                             <TextBlock Text="{Binding ErrorContent}" Foreground="Red" />
    10.                         </DataTemplate>
    11.                     </ItemsControl.ItemTemplate>
    12.                 </ItemsControl>
    13.             </StackPanel>
    14.         </ControlTemplate>
    15.     </Validation.ErrorTemplate>
    16. </TextBox>

    VM:

    1. public class MainViewModel : BindableBase, INotifyDataErrorInfo
    2. {
    3.     private string _userName;
    4.     private readonly Dictionary<string, List<string>> _errorsByPropertyName = new Dictionary<string, List<string>>();
    5.     public MainViewModel()
    6.     {
    7.         UserName = null;
    8.     }
    9.     public string UserName
    10.     {
    11.         get => _userName;
    12.         set
    13.         {
    14.             _userName = value;
    15.             ValidateUserName();
    16.             RaisePropertyChanged();
    17.         }
    18.     }
    19.     public bool HasErrors => _errorsByPropertyName.Any();
    20.     public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
    21.     public IEnumerable GetErrors(string propertyName)
    22.     {
    23.         return _errorsByPropertyName.ContainsKey(propertyName) ?
    24.             _errorsByPropertyName[propertyName] : null;
    25.     }
    26.     private void OnErrorsChanged(string propertyName)
    27.     {
    28.         ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
    29.     }
    30.     private void ValidateUserName()
    31.     {
    32.         ClearErrors(nameof(UserName));
    33.         if (string.IsNullOrWhiteSpace(UserName))
    34.             AddError(nameof(UserName), "Username cannot be empty.");
    35.         if (string.Equals(UserName, "Admin", StringComparison.OrdinalIgnoreCase))
    36.             AddError(nameof(UserName), "Admin is not valid username.");
    37.         if (UserName == null || UserName?.Length <= 5)
    38.             AddError(nameof(UserName), "Username must be at least 6 characters long.");
    39.     }
    40.     private void AddError(string propertyName, string error)
    41.     {
    42.         if (!_errorsByPropertyName.ContainsKey(propertyName))
    43.             _errorsByPropertyName[propertyName] = new List<string>();
    44.         if (!_errorsByPropertyName[propertyName].Contains(error))
    45.         {
    46.             _errorsByPropertyName[propertyName].Add(error);
    47.             OnErrorsChanged(propertyName);
    48.         }
    49.     }
    50.     private void ClearErrors(string propertyName)
    51.     {
    52.         if (_errorsByPropertyName.ContainsKey(propertyName))
    53.         {
    54.             _errorsByPropertyName.Remove(propertyName);
    55.             OnErrorsChanged(propertyName);
    56.         }
    57.     }
    58. }