How to add validation to view model properties or how to implement INotifyDataErrorInfo

Solution 1:

The preferred way since .Net 4.5 to implement data validation is to let your view model implement INotifyDataErrorInfo (example from Technet, example from MSDN (Silverlight)).

Note: INotifyDataErrorInfo replaces the obsolete IDataErrorInfo.

How INotifyDataErrorInfo works

When the ValidatesOnNotifyDataErrors property of Binding is set to true, the binding engine will search for an INotifyDataErrorInfo implementation on the binding source and subscribe to the INotifyDataErrorInfo.ErrorsChanged event.

If the ErrorsChanged event of the binding source is raised and INotifyDataErrorInfo.HasErrors evaluates to true, the binding engine will invoke the INotifyDataErrorInfo.GetErrors(propertyName) method for the actual source property to retrieve the corresponding error message and then apply the customizable validation error template to the target control to visualize the validation error.
By default a red border is drawn around the element that has failed to validate.

This validation feedback visualization procedure only executes when Binding.ValidatesOnNotifyDataErrors is set to true on the particular data binding and the Binding.Mode is set to either BindingMode.TwoWay or BindingMode.OneWayToSource.

How to implement INotifyDataErrorInfo

The following examples show three variations of property validation using

• a ValidationRule (class to encapsulate the actual data validation implementation)
• Lambdas (or delegates)
• validation attributes (used to decorate the validated property).

The code is not tested. The snippets should all work, but may not compile due to typing errors. This code is intended to provide a simple example on how the INotifyDataErrorInfo interface could be implemented.

ViewModel.cs

The view model is responsible for validating its own properties to ensure the data integrity of the model.
Since .NET 4.5, the recommended way is to let the view model implement the INotifyDataErrorInfo interface.
The key is to have separate ValidationRule implementations for each property or rule.

Extending ValidationRule is optional. I chose to extend ValidationRule because it already provides a complete validation API and because the implementations can be reused with binding validation if necessary.
Basically, the result of the property validation should be a bool to indicate fail or success of the validation and a message that can be displayed to the user to help him to fix his input.

All we have to do in case of a validation error is to generate an error message, add it to a private string collection to allow our INotifyDataErrorInfo.GetErrors(propertyName) implementation to return the proper error messages from this collection and raise the INotifyDataErrorInfo.ErrorChanged event to notify the WPF binding engine about the error:

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);
    } 
  }

  // Optional method to check if a certain property has validation errors
  public bool PropertyHasErrors(string propertyName) => this.Errors.TryGetValue(propertyName, out List<string> propertyErrors) && propertyErrors.Any();

  #region INotifyDataErrorInfo implementation

  // The WPF binding engine will listen to this event
  public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;

  // This implementatio of GetErrors returns all errors of the specified property. 
  // If the argument is 'null' instead of the property's name, 
  // then the method will return all errors of all properties.
  // This method is called by the WPF binding engine when ErrorsChanged event was raised and HasErrors retirn true
  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 'true' 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; }
}

UserInputValidationRule.cs

This example validation rule extends ValidationRule and checks if the input starts with the '@' character. If not, it returns an invalid ValidationResult with an error message that can be displayed to the user to help him to fix his input.

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;
  }
}

MainWindow.xaml

To enable the visual data validation feedback, the Binding.ValidatesOnNotifyDataErrors property must be set to true on each relevant Binding i.e. where the source of the Binding is a validated property. The WPF framework will then show the control's default error feedback.
Note to make this work the Binding.Mode must be either OneWayToSource or TwoWay (which is the default for the TextBox.Text property):

<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>

The following is an example of a custom validation error template.
The default visual error feedback is a simple red border around the validated element. In case you like to customize the visual feedback e.g., to allow showing error messages to the user, you can define a custom ControlTemplate and assign it to the validated element (in this case the TextBox) via the attached property Validation.ErrorTemplate (see above).
The following ControlTemplate enables showing a list of error messaages that are associated with the validated property:

<ControlTemplate x:Key="ValidationErrorTemplate">
  <StackPanel>
    <Border BorderBrush="Red" 
            BorderThickness="1">
      
      <!-- Placeholder for the DataGridTextColumn itself -->
      <AdornedElementPlaceholder x:Name="AdornedElement"  />
    </Border>

    <Border Background="White" 
            BorderBrush="Red" 
            Padding="4"
            BorderThickness="1,0,1,1" 
            HorizontalAlignment="Left">
      <ItemsControl ItemsSource="{Binding}" HorizontalAlignment="Left">
        <ItemsControl.ItemTemplate>
          <DataTemplate>
            <TextBlock Text="{Binding ErrorContent}" 
                       Foreground="Red"/>
          </DataTemplate>
        </ItemsControl.ItemTemplate>
      </ItemsControl>
    </Border>
  </StackPanel>
</ControlTemplate>

I recommend moving the implementation of INotifyDataErrorInfo into a base class (e.g. an abstract ViewModel class) together with the INotifyPropertyChanged implementation and let all your view models inherit it. This makes the validation logic reusable and keeps your view model classes clean.

You can change the example's implementation details of INotifyDataErrorInfo to meet requirements.

Remarks

As an alternative approach the ValidationRule can be replaced with delegates to enable the use of Lambda expressions or Method Groups:

// 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 '@'.");
}

Data validation using ValidationAttribute

This is an example implementation of INotifyDataErrorInfo with ValidationAttribute support e.g., MaxLengthAttribute. This solution combines the previous Lamda version to additionally support validation using a Lambda expression/delegate simultaneously:

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; }    
}