<TextBlock>
            <TextBlock.Style>
                <Style TargetType="TextBlock">
                    <Setter Property="Text" Value="{DynamicResource mytext}" />
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding TextProperty}" Value="{x:Null}">
                            <Setter Property="Text" Value="It's NULL Baby!" />
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </TextBlock.Style>
        </TextBlock>

不要写成如下代码:
<TextBlock Text="{......}">
 ..................