Trigger ignored when style added programically

[MBursill]

I have two simple styles. One with a trigger, setting the background of a TextBox when the IsReadOnly style is true:

<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="IsReadOnly" Value="True">
<Setter Property="Background" Value="{StaticResource TextBoxReadOnlyBackgroundBrush}" />
</Trigger>
</Style.Triggers>
</Style>

The second style is setting the IsReadOnly property to true:

<Style TargetType="{x:Type TextBox}">
<Setter Property="IsReadOnly" Value="True" />
</Style>

I add the second style dynamically, like this:

public void AddReadOnlyStyles()
{
ResourceDictionary readOnlyStyles = new ResourceDictionary();
readOnlyStyles.Source = new Uri("ReadOnlyStyles.xaml");

if (Resources.MergedDictionaries.Contains(readOnlyStyles) == false)
    Resources.MergedDictionaries.Add(readOnlyStyles);

}

The read only style is added just fine, but the trigger is ignored. Why is it being ignored? Is there a way to make it so it wont be ignored? Thanks -Mike.

[MBursill]

I managed to solve this. By giving the first style (the one with the trigger) a key, calling it TextBoxStyle, and adding BasedOn="{StaticResource TextBoxStyle}" to the second style, it works. Don't fully understand why though. -Mike.