添加转换器时缺少装配参考

Question

在App.xaml中，当我尝试添加转换器时，它会显示you are missing assembly reference

<Application.Resources>
    <converter:RssTextTrimmer xmlns:converter="clr-namespace:sdkRSSReaderCS"
        x:Key="RssTextTrimmer" />
    <converter:ImageFromRssText xmlns:converter="clr-namespace:sdkRSSReaderCS"
        x:Key="ImageFromRssText" />

</Application.Resources>