How to add a tooltip using IValueConverter - wpf

Question

I am looking for a solution to add a tooltip for a button using value converter.
This is because some part/text of this tooltip is calculated dynamically.
For instance i have an application with some "launchable" tasks in it in such a way that clicking any/all of these tasks launches another application.
In this task list, there is a button "Launch Tasks" with count depending upon the no. of tasks selected to launch.I need to show a tooltip for this button only.Like:
"Launch Tasks(2)", this is for two tasks selected.
Can someone please help in writing a converter for the same and then how to bind the same in XAML?
Update:
Earlier it is something like as below, but problem is tooltip shown is not upto the standards and not clearly visible, so i thought of removing the same and used converter instead.
<ToolTipService.ToolTip>
<ToolTip>
<StackPanel Orientation="Horizontal">
<TextBlock x:Name="ttlblActivityCount" Text="{Binding LaunchTaskButtonTitle, Source={StaticResource ResourceData}}" />
<TextBlock x:Name="ttlblActivityCountStart"
Margin="2,0,0,0"
Text="("/>
<TextBlock x:Name="tttxtActivityCount" Text="{Binding Path=SelectedActivitiesCount}"/>
<TextBlock x:Name="ttlblActivityCountEnd"
FontWeight="Bold"
Text=")" />
</StackPanel>
</ToolTip>
</ToolTipService.ToolTip>

Answer 1

Using a StackPanel to artificially combine a string is yes pretty much something you don't want to do. However you don't have to use an IValueConverter for this
If you have a property for SelectedActivitiesCount and you want to bind it's tooltip with some static text and the property a simple example could be:
<Button Content="Launch Tasks">
<Button.ToolTip>
<TextBlock Text="{Binding SelectedActivitiesCount, StringFormat=Launch Tasks({0})}" />
</Button.ToolTip>
</Button>
^^ With this output would look like: "Launch Tasks(2)" where SelectedActivitiesCount -> 2
If the text("Launch Tasks") is also dynamic:
<Button Content="Launch Tasks">
<Button.ToolTip>
<TextBlock>
<TextBlock.Text>
<MultiBinding StringFormat="{}{0} ({1})">
<Binding Path="LaunchTaskButtonTitle" Source="{StaticResource ResourceData}" />
<Binding Path="SelectedActivitiesCount" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>
</Button.ToolTip>
</Button>
^^ With this output would look like: "Launch Tasks(2)" where SelectedActivitiesCount -> 2, LaunchTaskButtonTitle -> Launch Tasks
Choose whichever method is applicable for you.