Your code is working fine for me. it close dialog when I press enter. You can write e.Handled = true; line after your search functionality in tbxSearchString_PreviewKeyDown event. So it will not close dialog.

<Grid>        <TextBox Name="tbxSearchString" HorizontalAlignment="Left" Width="100" Height="30" Grid.Row="0" PreviewKeyDown="tt_PreviewKeyDown"></TextBox>        <StackPanel Orientation="Horizontal" Grid.Row="1"  Height="45" VerticalAlignment="Bottom" HorizontalAlignment="Right" Width="190">            <Button Content="OK"                 Height="25" Margin="10,10,10,10" Width="75" Name="btnOK" TabIndex="1600" IsDefault="True" Click="btnOK_Click"                                        VerticalContentAlignment="Center" HorizontalContentAlignment="Center" />            <Button Content="Cancel"                 Height="25" Margin="10,10,10,10" Width="75" Name="btnCancel" TabIndex="1700" IsCancel="True"                 VerticalContentAlignment="Center" HorizontalContentAlignment="Center" Click="btnCancel_Click" />        </StackPanel>    </Grid>

Code behind

private void btnOK_Click(object sender, RoutedEventArgs e)        {            DialogResult = true;         }        private void btnCancel_Click(object sender, RoutedEventArgs e)        {            this.Close();        }        private void tbxSearchString_PreviewKeyDown(object sender, KeyEventArgs e)        {           if (e.Key == Key.Enter)           {               this.Search();               e.Handled = true;           }        }

there is no built-in way to close the dialog window in wpf. what you have to do, is to set the DialogResult for your default button. so all you need is the following:

xaml

<Button Content="OK"             Height="25" Margin="10,10,10,10" Width="75" Name="btnOK" TabIndex="1600" IsDefault="True" Click="btnOK_Click"                                    VerticalContentAlignment="Center" HorizontalContentAlignment="Center" />

codebehind:

    private void btnOK_Click(object sender, RoutedEventArgs e)    {        DialogResult = true;    }

You shouldn't be calling Close() or handling PreviewKeyDown yourself.

The proper way to do this is to have Ok/Cancel buttons, and use Button.IsDefault, Button.IsCancel, and Window.DialogResult. If the 'enter' press is unhandled in your textbox, the keypress will propagate to the Window and the default button will be pressed.

MyForm.xaml:

<Button x:Name="btnOk" Content="Ok" Click="btnOk_Click" IsDefault="True"/><Button x:Name="btnCancel" Content="Cancel" Click="btnCancel_Click" IsCancel="True"/>

MyForm.xaml.cs:

private void btnOk_Click(object sender, RoutedEventArgs e){    DialogResult = true;}private void btnCancel_Click(object sender, RoutedEventArgs e){    DialogResult = false;}

Now hitting enter or escape on any textbox in the form will close the form (with the proper result)