Becaus StringFormat is required and target to is be a string type. You can get around this by either using a TextBlock instead (which has a Text property) or putting the Textblock as the child of the Label:
<Label x:Name="myLabel"> <Label.Content> <TextBlock> <TextBlock.Text> <MultiBinding StringFormat="{}{0} - {1}"> <Binding Path="Lib1" /> <Binding Path="Lib2" /> </MultiBinding> </TextBlock.Text> </TextBlock> </Label.Content></Label>
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