(ASP.NET) Can you do a String.Format on a web.config key/value?

Question

This might be a god awful question but I'm not sure why it won't let me do this.

I have a URL I need to store in Web.config, which has a dynamic parameter pulled from the web page.

So I want to store:

        <add key="TestURL" 
value="https://test/subscribe?msisdn={0}&amp;code=1&amp;pass=2"/&gt;

It doesn't let me do this. After the {0} it errors at the "&".

Can anyone let me know what I'm doing wrong here? Do I need to escape the character?

Answer 1

Try this instead,

<add key="TestURL" value="https://test/subscribe?msisdn={0}&amp;amp;code=1&amp;amp;pass=2"/&gt;

Notice the escaped ampersands.

Answer 2

Config files are XML, and as such, require XML entities to be escaped. The problem isn't your {0} for use in formatting, it's the & that must be escaped as

&