Conversion in ASP.NET expressions

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Hello,

I have created my own simple ASP.NET expression. It looks like <%$
InstanceSettings:Name %>. It access NameValueCollection and returns string
value. At the moment I need to convert this string to bool for one case
because I use it to evalueate Visibility of column in templated grid view.

I am trying to do something like this:
<asp:BoundField Visible="<%$ InstanceSettings:UseOrg %>" ....
but it fails because no implicit conversion between string and bool exists.

How can I use conversion in ASP.NET expressions?

Thank you in advance,
Ladislav Mrnka