How to disable button in WPF during validation

How to disable button in WPF during validation

i created WPF Validation form, in it i am taking name and password of
password. I want that "Login" Button should only enabled when both
password and username has entered correctly. I wrote this code below but
it only works for name field and when capslock is set to oN for password.
Here code is:
<Grid x:Name="validationContainer"
dxe:ValidationService.IsValidationContainer="True">
<dxe:PasswordBoxEdit HorizontalAlignment="Left"
Height="25" Width="173" ValidateOnTextInput="True"
VerticalAlignment="Top" ShowCapsLockWarningToolTip="True"
Name="passBox" Validate="passBox_Validate"
Margin="138,95,0,0" NullText="Enter Password Here">
<dxe:PasswordBoxEdit.CapsLockWarningToolTipTemplate>
<DataTemplate>
<TextBlock Text="Caps-Lock is Off"
Foreground="Red"/>
</DataTemplate>
</dxe:PasswordBoxEdit.CapsLockWarningToolTipTemplate>
</dxe:PasswordBoxEdit>
<TextBlock HorizontalAlignment="Left" Margin="57,133,0,0"
TextWrapping="Wrap" Text="Password strength: "
VerticalAlignment="Top" Name="textBlock"/>
<dxe:TextEdit x:Name="dxTextEdit" Height="25" Width="173"
ValidateOnTextInput="True" HorizontalAlignment="Left"
VerticalAlignment="Top"
Validate="dxTextEdit_Validate" NullText="Enter Name Here"
MaskType="RegEx" Mask="[a-zA-Z]+" MaskIgnoreBlank="True"
Margin="138,58,81,175" >
</dxe:TextEdit>
<Label Content="User Name" HorizontalAlignment="Left"
Margin="64,57,0,0" VerticalAlignment="Top"/>
<Label Content="Password" HorizontalAlignment="Left"
Margin="64,94,0,0" VerticalAlignment="Top"/>
<Button Content="Login" MinWidth="100" HorizontalAlignment="Right"
Margin="0,229,287,0" Width="105" Click="Button_Click_1">
<Button.IsEnabled>
<Binding Path="(dxe:ValidationService.HasValidationError)"
ElementName="validationContainer">
<Binding.Converter>
<dx:NegationConverterExtension />
</Binding.Converter>
</Binding>
</Button.IsEnabled>
</Button>
Need your suggestion. Thank you! NOTE:I am using Devexpress and WPF