如何添加验证以查看模型属性或如何实现 INotifyDataErrorInfo

Question

我有一个 ObservableCollection 类型的数据集合（比如 myClassTypes 实例）。在一些用户操作之后，这个 myClassTypes 填充了 ViewModel 中的值。在视图中，有一个文本框，用户可以在其中输入文本。我需要根据 myClassTypes 值验证文本框数据。因此，如果 myClassTypes 包含用户在文本框中插入的文本，则验证通过，否则将失败。我的代码片段是：ViewModel：

public ObservableCollection < MyClassType > ViewModelClassTypes {
    get {

        return _myClassTypes;
    }
    set {
        _myClassTypes = value;
        NotifyOfPropertyChange(() = >MyClassTypes);
    }
}

public class TestValidationRule: ValidationRule {
    public ObservableCollection < MyClassType > MyClassTypes {
        get = >(ObservableCollection < MyClassType > ) GetValue(MyClassTypesProperty);
        set = >SetValue(MyClassTypesProperty, value);
    }
}

仅供参考：MyClassTypesProperty 是一个依赖属性

我的 View.xaml 是：

<TextBox>
    <TextBox.Text>
        <Binding UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <validationRules:TestValidationRule MyClassTypes="{Binding ViewModelClassTypes}"/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

我无法在 MyClassTypes 中获得 ViewModelClassTypes 填充值。任何人都可以请建议我在做什么错吗？

Answer 1

自 .Net 4.5 以来，实现数据验证的首选方法是让您的视图模型实现INotifyDataErrorInfo（来自Technet 的示例，来自MSDN (Silverlight) 的示例）。

注意：INotifyDataErrorInfo替换过时的IDataErrorInfo.

如何INotifyDataErrorInfo工作

当 的ValidatesOnNotifyDataErrors属性Binding设置为 时true，绑定引擎将INotifyDataErrorInfo在绑定源上搜索实现并订阅该INotifyDataErrorInfo.ErrorsChanged事件。

如果ErrorsChanged绑定源的事件被引发并INotifyDataErrorInfo.HasErrors评估为true，则绑定引擎将调用INotifyDataErrorInfo.GetErrors()实际源属性的方法来检索相应的错误消息，然后将可自定义的验证错误模板应用于目标控件以可视化验证错误。
默认情况下，会在验证失败的元素周围绘制红色边框。

当该验证反馈的可视化过程仅执行Binding.ValidatesOnNotifyDataErrors被设置为true特定的数据绑定和Binding.Mode被设置为BindingMode.TwoWay或BindingMode.OneWayToSource。

如何实施 INotifyDataErrorInfo

以下示例显示使用ValidationRule（封装实际数据验证实现）和 Lambda（或委托）的默认验证。最后一个示例展示了如何使用验证属性实现数据验证。

代码没有经过测试。这些片段应该都可以工作，但由于输入错误可能无法编译。此代码旨在提供有关如何实现INotifyDataErrorInfo接口的简单示例。

视图模型.cs

视图模型负责验证其属性以确保模型的数据完整性。
从 .NET 4.5 开始，推荐的方式是让视图模型实现INotifyDataErrorInfo接口。
关键是ValidationRule对每个属性或规则有单独的实现：

public class ViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{
  // Example property, which validates its value before applying it
  private string userInput;
  public string UserInput
  { 
    get => this.userInput; 
    set 
    { 
      // Validate the value
      ValidateProperty(value);

      this.userInput = value; 
      OnPropertyChanged();
    }
  }

  // Constructor
  public ViewModel()
  {
    this.Errors = new Dictionary<string, List<string>>();
    this.ValidationRules = new Dictionary<string, List<ValidationRule>>();

    // Create a Dictionary of validation rules for fast lookup. 
    // Each property name of a validated property maps to one or more ValidationRule.
    this.ValidationRules.Add(nameof(this.UserInput), new List<ValidationRule>() {new UserInputValidationRule()});
  }

  // Validation method. 
  // Is called from each property which needs to validate its value.
  // Because the parameter 'propertyName' is decorated with the 'CallerMemberName' attribute.
  // this parameter is automatically generated by the compiler. 
  // The caller only needs to pass in the 'propertyValue', if the caller is the target property's set method.
  public bool ValidateProperty<TValue>(TValue propertyValue, [CallerMemberName] string propertyName = null)  
  {  
    // Clear previous errors of the current property to be validated 
    this.Errors.Remove(propertyName); 
    OnErrorsChanged(propertyName); 

    if (this.ValidationRules.TryGetValue(propertyName, out List<ValidationRule> propertyValidationRules))
    {
      // Apply all the rules that are associated with the current property 
      // and validate the property's value
      propertyValidationRules
        .Select(validationRule => validationRule.Validate(propertyValue, CultureInfo.CurrentCulture))
        .Where(result => !result.IsValid)
        .ToList()
        .ForEach(invalidResult => AddError(propertyName, invalidResult.ErrorContent as string));

      return !PropertyHasErrors(propertyName);
    }

    // No rules found for the current property
    return true;
  }   

  // Adds the specified error to the errors collection if it is not 
  // already present, inserting it in the first position if 'isWarning' is 
  // false. Raises the ErrorsChanged event if the Errors collection changes. 
  // A property can have multiple errors.
  public void AddError(string propertyName, string errorMessage, bool isWarning = false)
  {
    if (!this.Errors.TryGetValue(propertyName, out List<string> propertyErrors))
    {
      propertyErrors = new List<string>();
      this.Errors[propertyName] = propertyErrors;
    }

    if (!propertyErrors.Contains(errorMessage))
    {
      if (isWarning) 
      {
        // Move warnings to the end
        propertyErrors.Add(errorMessage);
      }
      else 
      {
        propertyErrors.Insert(0, errorMessage);
      }
      OnErrorsChanged(propertyName);
    } 
  }

  public bool PropertyHasErrors(string propertyName) => this.Errors.TryGetValue(propertyName, out List<string> propertyErrors) && propertyErrors.Any();

  #region INotifyDataErrorInfo implementation

  public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

  // Returns all errors of a property. If the argument is 'null' instead of the property's name, 
  // then the method will return all errors of all properties.
  public System.Collections.IEnumerable GetErrors(string propertyName) 
    => string.IsNullOrWhiteSpace(propertyName) 
      ? this.Errors.SelectMany(entry => entry.Value) 
      : this.Errors.TryGetValue(propertyName, out List<string> errors) 
        ? errors 
        : new List<string>();

  // Returns if the view model has any invalid property
  public bool HasErrors => this.Errors.Any(); 

  #endregion

  #region INotifyPropertyChanged implementation

  public event PropertyChangedEventHandler PropertyChanged;

  #endregion

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }

  protected virtual void OnErrorsChanged(string propertyName)
  {
    this.ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
  }

  // Maps a property name to a list of errors that belong to this property
  private Dictionary<String, List<String>> Errors { get; }

  // Maps a property name to a list of ValidationRules that belong to this property
  private Dictionary<String, List<ValidationRule>> ValidationRules { get; }
}

用户输入验证规则.cs

此示例验证规则扩展ValidationRule并检查输入是否以“@”字符开头。如果不是，它返回一个无效ValidationResult的错误消息：

public class UserInputValidationRule : ValidationRule
{        
  public override ValidationResult Validate(object value, CultureInfo cultureInfo)
  {
    if (!(value is string userInput))
    {
      return new ValidationResult(false, "Value must be of type string.");    
    }

    if (!userInput.StartsWith("@"))
    {
      return new ValidationResult(false, "Input must start with '@'.");    
    }

    return ValidationResult.ValidResult;
  }
}

主窗口.xaml

要启用可视化数据验证反馈，Binding.ValidatesOnNotifyDataErrors必须true在每个相关Binding. 然后 WPF 框架将显示控件的默认错误反馈。注意要使这项工作Binding.Mode必须是OneWayToSource或TwoWay（这是TextBox.Text属性的默认值）：

<Window>
    <Window.DataContext>
        <ViewModel />       
    </Window.DataContext>
    
    <!-- Important: set ValidatesOnNotifyDataErrors to true to enable visual feedback -->
    <TextBox Text="{Binding UserInput, ValidatesOnNotifyDataErrors=True}" 
             Validation.ErrorTemplate="{DynamicResource ValidationErrorTemplate}" />  
</Window>

以下是验证错误模板，以防您想自定义可视化表示（可选）。它TextBox通过附加属性Validation.ErrorTemplate（见上文）设置在经过验证的元素（在本例中为）上：

<ControlTemplate x:Key="ValidationErrorTemplate">
    <StackPanel>
        <!-- Placeholder for the element containing the -->
        <AdornedElementPlaceholder />

        <ItemsControl ItemsSource="{Binding}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding ErrorContent}" Foreground="Red"/>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </StackPanel>
</ControlTemplate>

除了我提供的链接之外，您还可以在网上找到许多示例。

我建议将 的实现INotifyDataErrorInfo移到基类中（例如BaseViewModel) together with INotifyPropertyChanged` 并让所有视图模型继承它。这使得验证逻辑可重用并保持视图模型类清洁。

您可以更改 的实现细节INotifyDataErrorInfo以满足要求。

评论

作为替代方法，ValidationRule可以用委托替换以启用 Lambda 表达式或方法组：

// Example uses System.ValueTuple
public bool ValidateProperty<TValue>(
  TValue value, 
  Func<TValue, (bool IsValid, IEnumerable<string> ErrorMessages)> validationDelegate, 
  [CallerMemberName] string propertyName = null)  
{  
  // Clear previous errors of the current property to be validated 
  this.Errors.Remove(propertyName); 
  OnErrorsChanged(propertyName); 

  // Validate using the delegate
  (bool IsValid, IEnumerable<string> ErrorMessages) validationResult = validationDelegate?.Invoke(value) ?? (true, string.Empty);

  if (!validationResult.IsValid)
  {
    // Store the error messages of the failed validation
    foreach (string errorMessage in validationResult.ErrorMessages)
    {
      // See previous example for implementation of AddError(string,string):void
      AddError(propertyName, errorMessage);
    }
  } 

  return validationResult.IsValid;
}   


private string userInput;
public string UserInput
{ 
  get => this.userInput; 
  set 
  { 
    // Validate the new property value before it is accepted
    if (ValidateProperty(value, 
      newValue => newValue.StartsWith("@") 
        ? (true, new List<string>()) 
        : (false, new List<string> {"Value must start with '@'."})))
    {
      // Accept the valid value
      this.userInput = value; 
      OnPropertyChanged();
    }
  }
}

// Alternative usage example property which validates its value 
// before applying it using a Method group.
// Example uses System.ValueTuple.
private string userInputAlternativeValidation;
public string UserInputAlternativeValidation
{ 
  get => this.userInputAlternativeValidation; 
  set 
  { 
    // Use Method group
    if (ValidateProperty(value, AlternativeValidation))
    {
      this.userInputAlternativeValidation = value; 
      OnPropertyChanged();
    }
  }
}

private (bool IsValid, string ErrorMessage) AlternativeValidation(string value)
{
  return value.StartsWith("@") 
    ? (true, string.Empty) 
    : (false, "Value must start with '@'.");
}

数据验证使用 ValidationAttribute

这是INotifyDataErrorInfo具有ValidationAttribute支持的示例实现，例如，MaxLengthAttribute。此解决方案结合了以前的 Lamda 版本，以额外支持同时使用 Lambda 表达式/委托的验证：

public class ViewModel : INotifyPropertyChanged, INotifyDataErrorInfo
{    
  private string userInputAttributeValidation;
 
  [MaxLength(Length = 5, ErrorMessage = "Only five characters allowed.")]
  public string UserInputAttributeValidation
  { 
    get => this.userInputAttributeValidation; 
    set 
    { 
      // Use only the attribute (can be combined with a Lambda or Method group)
      if (ValidateProperty(value))
      {
        this.userInputAttributeValidation = value; 
        OnPropertyChanged();
      }
    }
  }

  // Constructor
  public ViewModel()
  {
    this.Errors = new Dictionary<string, List<string>>();
  }

  // Validate properties using decorated attributes and/or a validation delegate. 
  // The validation delegate is optional.
  public bool ValidateProperty<TValue>(
    TValue value, 
    Func<TValue, (bool IsValid, IEnumerable<string> ErrorMessages)> validationDelegate = null, 
    [CallerMemberName] string propertyName = null)  
  {  
    // Clear previous errors of the current property to be validated 
    this.Errors.Remove(propertyName); 
    OnErrorsChanged(propertyName); 

    bool isValueValid = ValidatePropertyUsingAttributes(value, propertyName);
    if (validationDelegate != null)
    {
      isValueValid |= ValidatePropertyUsingDelegate(value, validationDelegate, propertyName);
    }

    return isValueValid;
  }     

  // Validate properties using decorated attributes. 
  public bool ValidatePropertyUsingAttributes<TValue>(TValue value, string propertyName)  
  {  
    // The result flag
    bool isValueValid = true;

    // Check if property is decorated with validation attributes
    // using reflection
    IEnumerable<Attribute> validationAttributes = GetType()
      .GetProperty(propertyName, BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static)
      ?.GetCustomAttributes(typeof(ValidationAttribute)) ?? new List<Attribute>();

    // Validate using attributes if present
    if (validationAttributes.Any())
    {
      var validationContext = new ValidationContext(this, null, null) { MemberName = propertyName };
      var validationResults = new List<ValidationResult>();
      if (!Validator.TryValidateProperty(value, validationContext, validationResults))
      {           
        isValueValid = false;

        foreach (ValidationResult attributeValidationResult in validationResults)
        {
          AddError(propertyName, attributeValidationResult.ErrorMessage);
        }
      }
    }

    return isValueValid;
  }       

  // Validate properties using the delegate. 
  public bool ValidatePropertyUsingDelegate<TValue>(
    TValue value, 
    Func<TValue, (bool IsValid, IEnumerable<string> ErrorMessages)> validationDelegate, 
    string propertyName) 
  {  
    // The result flag
    bool isValueValid = true;

    // Validate using the delegate
    (bool IsValid, IEnumerable<string> ErrorMessages) validationResult = validationDelegate.Invoke(value);

    if (!validationResult.IsValid)
    {
      isValueValid = false;

      // Store the error messages of the failed validation
      foreach (string errorMessage in validationResult.ErrorMessages)
      {
        AddError(propertyName, errorMessage);
      }
    } 

    return isValueValid;
  }       

  // Adds the specified error to the errors collection if it is not 
  // already present, inserting it in the first position if 'isWarning' is 
  // false. Raises the ErrorsChanged event if the Errors collection changes. 
  // A property can have multiple errors.
  public void AddError(string propertyName, string errorMessage, bool isWarning = false)
  {
    if (!this.Errors.TryGetValue(propertyName, out List<string> propertyErrors))
    {
      propertyErrors = new List<string>();
      this.Errors[propertyName] = propertyErrors;
    }

    if (!propertyErrors.Contains(errorMessage))
    {
      if (isWarning) 
      {
        // Move warnings to the end
        propertyErrors.Add(errorMessage);
      }
      else 
      {
        propertyErrors.Insert(0, errorMessage);
      }
      OnErrorsChanged(propertyName);
    } 
  }

  public bool PropertyHasErrors(string propertyName) => this.Errors.TryGetValue(propertyName, out List<string> propertyErrors) && propertyErrors.Any();

  #region INotifyDataErrorInfo implementation

  public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

  // Returns all errors of a property. If the argument is 'null' instead of the property's name, 
  // then the method will return all errors of all properties.
  public System.Collections.IEnumerable GetErrors(string propertyName) 
    => string.IsNullOrWhiteSpace(propertyName) 
      ? this.Errors.SelectMany(entry => entry.Value) 
      : this.Errors.TryGetValue(propertyName, out IEnumerable<string> errors) 
        ? errors 
        : new List<string>();

  // Returns if the view model has any invalid property
  public bool HasErrors => this.Errors.Any(); 

  #endregion

  #region INotifyPropertyChanged implementation

  public event PropertyChangedEventHandler PropertyChanged;

  #endregion

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }

  protected virtual void OnErrorsChanged(string propertyName)
  {
    this.ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
  }

  // Maps a property name to a list of errors that belong to this property
  private Dictionary<String, List<String>> Errors { get; }    
}