When $DisplayExpression is set by assigning a value to the attribute, such as in a stamp, needs to have the right side enclosed in parentheses:
$DisplayExpression="$Name + ': ' + $ChildCount";
Not enclosing the right side of the action code in the stamp is a common error – we’ve all made that mistake. The right hand side of the action code is the expression that you want to store in $DisplayExpression, which is why it is enclosed in parentheses. The code you wrote is telling Tinderbox to evaluate the terms on the right side, and then store the result of that evaluation in $DisplayExpression, which is why you end up with the content of $DisplayExpression as a string instead of an expression.
The problem here — and this is both complicated and one of the most profound beauties of computer science — is that $DisplayExpression holds an expression, which is to say, the formula or recipe for how to calculate what should be displayed.
