WPF Binding - StringFormat - Not Formatting

A toolTip is a content control, which means it doesn't really have a display model. This is demonstrated in the earlier answer by @deccyclone that sets the content to a TextBlock. Since the TextBox is designed to display text, the StringFormat binding property works as advertised. Button is another example of this. (Both derive from ContentControl)

If you set the Content of a ToolTip to a string, the string is displayed because the ToolTip has a built in converter if the dataType is string. If you want to take advantage of that built in string converter, you need to set the format using the ContentStringFormat property.

<ToolTip Content="{Binding Path=Value}" ContentStringFormat="{}{0:F2} M" /> 

BTW, the tip off for when to use StringFormat or ContentStringFormat is by which property the control supplies for setting the displayed text. Text property -> use StringFormat Content property -> use ContentStringFormat

For anyone else that winds up here in a slightly different situation this was desired for setting a tooltip StringFormat via Style:

<DataGridTextColumn Header="Amount" CanUserSort="True" Binding="{Binding Amount,Mode=OneWay}"> <DataGridTextColumn.CellStyle> <Style TargetType="DataGridCell"> <Setter Property="ToolTip"> <Setter.Value> <TextBlock Text="{Binding JournalEntryId, StringFormat='Reserved (Equity Share: \{0\}%)'}" /> </Setter.Value> </Setter> </Style> </DataGridTextColumn.CellStyle> </DataGridTextColumn> 

You don't need to escape the brackets. Try this (i like to put the format in single quotes):

Value="{Binding Path=DataItem.EquitySold, StringFormat='Reserved (Equity Share: {0}%)'}" 

<Button.ToolTip> <TextBlock Text="{Binding Path=ToggleText, StringFormat={}{0} Text}"/> </Button.ToolTip> 

Button inside DataGridTemplateColumn

I assume it is what your datatype supports - as far as I know it is passed on as arguments to IFormattable.

Try

StringFormat=Reserved (Equity Share: {0:P0})