How to constrain a TextBox

Question

How to set up a TextBox to get only certain values. e.g. DateTime input box with defined format settings.

Answer 1

How about using the Binding Validation that comes with the WPF Framework.

Create a ValidationRule like so

public class DateFormatValidationRule : ValidationRule
{
  public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
  {
     var s = value as string;
     if (string.IsNullOrEmpty(s))
        return new ValidationResult(false, "Field cannot be blank");
     var match = Regex.Match(s, @"^\d{2}/\d{2}/\d{4}$");
     if (!match.Success)
        return new ValidationResult(false, "Field must be in MM/DD/YYYY format");
     DateTime date;
     var canParse = DateTime.TryParse(s, out date);
     if (!canParse)
        return new ValidationResult(false, "Field must be a valid datetime value");
     return new ValidationResult(true, null);
  }
}

Then add it to your binding in xaml as well as a style to handle when the field is invalid. (You can also use Validation.ErrorTemplate if you're inclined to completely change the control.) This one puts the ValidationResult text as a tooltip and the box to red.

    <TextBox x:Name="tb">
        <TextBox.Text>
            <Binding Path="PropertyThatIsBoundTo" UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <val:DateFormatValidationRule/>
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
        <TextBox.Style>
            <Style TargetType="{x:Type TextBox}">
                <Style.Triggers>
                    <Trigger Property="Validation.HasError" Value="true">
                        <Setter Property="ToolTip"
                                Value="{Binding RelativeSource={RelativeSource Self},
                            Path=(Validation.Errors)[0].ErrorContent}"/>
                        <Setter Property="Background" Value="Red"/>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TextBox.Style>
    </TextBox>

A recommendation would be to take the style and put it into a resource dictionary so any textbox you want to have the same appearance when its own validation fails. Makes the XAML a lot cleaner too.

Answer 2

You can also override the input methods on the textbox and evaluate the input at that point. It all depends on your architecture.

Some I've overridden before for this kind of task:

• OnPreviewTextInput
• OnTextInput
• OnPreviewKeyDown

Answer 3

http://www.codeproject.com/KB/WPF/wpfvalidation.aspx

<TextBox Text="{Binding Path=Name}" />

And a function. This one just checks the string has content. Yours will be more complex depending on the exact format you want to enforce:

public string Name
{
    get { return _name; }
    set
    {
        _name = value;
        if (String.IsNullOrEmpty(value))
        {
            throw new ApplicationException("Customer name is mandatory.");
        }
    }
}

Answer 4

Try using a MaskedTextBox.

It has things like DateTime defined format, and some more.