public class TextBoxPasteHelper:DependencyObject
    {
        public static bool GetCanPasteNewLine(DependencyObject d)
        {
            return (bool)d.GetValue(CanPasteNewLineProperty);
        }

        public static void SetCanPasteNewLine(DependencyObject d,bool value)
        {
            d.SetValue(CanPasteNewLineProperty, value);
        }

        
 
// Using a DependencyProperty as the backing store for CanPasteNewLine.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty CanPasteNewLineProperty =
            DependencyProperty.RegisterAttached("CanPasteNewLine", typeof(bool), typeof(TextBoxPasteHelper), new
 
 PropertyMetadata(default(bool),OnCanPasteNewLineChanged));

        private static void OnCanPasteNewLineChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var currentTextBox = d as TextBox;
            if(currentTextBox != null)
            {
                var
 
 cmd = new CommandBinding();
                cmd.Command = ApplicationCommands.Paste;
                cmd.Executed += Cmd_Executed;
                currentTextBox.CommandBindings.Add(cmd);
                var keyBinding = new KeyBinding();
                keyBinding.Key = Key.V;
                keyBinding.Modifiers = ModifierKeys.Control;
                keyBinding.Command = ApplicationCommands.Paste;
                currentTextBox.InputBindings.Add(keyBinding);
            }
        }

        private static void Cmd_Executed(object sender, ExecutedRoutedEventArgs e)
        {
            string copiedStr = Clipboard.GetText();
            if (string.IsNullOrEmpty(copiedStr))
                return;

            var currentTB = sender as TextBox;
            if (currentTB != null)
            {
                if (currentTB.SelectedText == currentTB.Text)
                {
                    currentTB.Text = copiedStr;
                }
                else
                {
                    if (!string.IsNullOrEmpty(currentTB.SelectedText))
                    {
                        currentTB.SelectedText = copiedStr;
                    }
                    else
                    {
                        StringBuilder currentStr = new StringBuilder(currentTB.Text);
                        currentStr.Insert(currentTB.CaretIndex, copiedStr);
                        currentTB.Text = currentStr.ToString();
                    }
                }
            }
        }
    }

<TextBox attached:TextBoxPasteHelper.CanPasteNewLine="True"/>