Here's one way to do validation in silverlight.

This is a pretty straight method using exceptions (if you're from the camp that allows exceptions in your Data objects )

[code:c#]

public string Name
{ 
 get 
 {
     return _name; 
 } 
 set 
 {   
  if ( value.Length >= 10 )
    throw new ArgumentException( "Name is too long" );   
  _name = value;
 }
}

<TextBox x:Name="name"
  <TextBox.Text>
   <Binding Path="name"
     NotifyOnValidationError="True"
      ValidatesOnExceptions="True" />  
 </TextBox.Text>
</TextBox>

[/code]