Show error message text box in WPF

Question

When my textbox is empty/null, i need to display "Required".

In my xaml:

  <TextBox Name="txtLastName"  Grid.Column="1"  Grid.Row="1"  Margin="3">
            <TextBox.Text>
                <Binding Path="LastName">
                    <Binding.ValidationRules>
                        <validators:Contractor
                                    MinimumLength="1" 
                                    MaximumLength="40"
                                    ErrorMessage="Required" />
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>

In my class:

public string LastName
        {
            get { return _lastName; }
            set
            {
                _lastName = value;
              NotifyPropertyChanged("LastName");               

            }
        }   

public event PropertyChangedEventHandler PropertyChanged;

    private int _minimumLength = -1;
    private int _maximumLength = -1;
    private string _errorMessage;

    public int MinimumLength
    {
        get { return _minimumLength; }
        set { _minimumLength = value; }
    }

    public int MaximumLength
    {
        get { return _maximumLength; }
        set { _maximumLength = value; }
    }

    public string ErrorMessage
    {
        get { return _errorMessage; }
        set { _errorMessage = value; }
    }

    public override ValidationResult Validate(object value,CultureInfo cultureInfo)
    {
        ValidationResult result = new ValidationResult(true, null);
        string inputString = (value ?? string.Empty).ToString();
        if (inputString.Length < this.MinimumLength || value==null ||
               (this.MaximumLength > 0 &&
                inputString.Length > this.MaximumLength))
        {
            result = new ValidationResult(false, this.ErrorMessage);
        }
        return result;
    }


  private void NotifyPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

What i get is the textbox turns into red border when the data is null/empty and i am not able to see the "Required" error message, any thoughts?

Answer 1

(The red border is the default behavior of a TextBox when the attached property Validation.HasError is true. In order to display the error messsage you'll have to do that yourself by binding to Validation.Errors . Validation.Errors is a list of error from each validator applied to the TextBox . Now in your case you only have one validator so in order to get the error message you need to bind to Validation.Errors[0].ErrorContent

Example

<StackPanel Orientation="Horizontal">
    <TextBox Name="txtLastName" Width="100">
        <TextBox.Text>
            <Binding Path="LastName">
                <Binding.ValidationRules>
                    <validators:Contractor
                                MinimumLength="1" 
                                MaximumLength="40"
                                ErrorMessage="Required" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
    <!-- Show error message tot the right of the TextBox-->
    <TextBlock Text="{Binding (Validation.Errors)[0].ErrorContent, ElementName=txtLastName}"/>
</StackPanel>