WPF Validation Control - wpf

Question

I am new to WPF and trying to implement validation control on submit form.
Can anyone help me. My code doen't show up any error message even if I enter invalid data infect it does nothing.
Here is my code,
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
}
}
public class UserName : INotifyPropertyChanged, IDataErrorInfo
{
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(PropertyChangedEventArgs e)
{
if (PropertyChanged != null)
PropertyChanged(this, e);
}
private string username;
public string _UserName
{
get { return username; }
set
{
username = value;
OnPropertyChanged(new PropertyChangedEventArgs("_UserName"));
}
}
public string this[string propertyName]
{
get
{
if (propertyName == "_UserName")
{
bool valid = true;
foreach (char c in _UserName)
{
if (!Char.IsLetterOrDigit(c))
{
valid = false;
break;
}
}
if (!valid)
return "The Username can only contain letters and numbers.";
}
return null;
}
}
public string Error
{
get { return null; }
}
}
}
My XAML code is,
<Grid>
<Label Content="User Name" Height="28" HorizontalAlignment="Left" Margin="27,37,0,0" Name="UserNameLB" VerticalAlignment="Top" Width="96" />
<TextBox Height="23" HorizontalAlignment="Left" Margin="135,37,0,0" Name="UserNameTB" VerticalAlignment="Top" Width="189">
<TextBox.Text>
<Binding Path="_UserName">
<Binding.ValidationRules>
<DataErrorValidationRule></DataErrorValidationRule>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
</Grid>

Answer 1

All you need is to put all logic about your validation in a separate class that inherits from class ValidationRule and override method Validate. Then you can set the message you want to see about the reason of validation failure, like:
public override ValidationResult Validate(object value, CultureInfo cultureInfo)
{
try
{
if (!Char.IsLetterOrDigit(c))
{
return new ValidationResult(false,
"The Username can only contain letters and numbers.");
}
else
{
return new ValidationResult(true, null);
}
}
catch (Exception e)
{
return new ValidationResult(false, "Illegal characters or " + e.Message);
}
}

Answer 2

Try this:
EDIT: (here's a style I define which shows errors for all TextBox controls)
(put it in Window.Resources)
This style will then show the error message in a ToolTip
<Style x:Key="{x:Type TextBox}" TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip"
Value="{Binding RelativeSource={x:Static RelativeSource.Self},
Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>
<Grid>
<Label Content="User Name" Height="28" HorizontalAlignment="Left" Margin="27,37,0,0" Name="UserNameLB" VerticalAlignment="Top" Width="96" />
<TextBox Height="23" HorizontalAlignment="Left" Margin="135,37,0,0"
Name="UserNameTB" VerticalAlignment="Top" Width="189"
Text={Binding Path=_UserName, UpdateSourceTrigger=LostFocus,
ValidatesOnDataErrors=true, NotifyOnValidationError=true} />
</Grid>
Source