DTD Entity in Web.config

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Hi, I would like to use DTD entity in web.config. For example

Here is my DTD
<?xml version="1.0" encoding="us-ascii" ?>
<!ENTITY SessionTimeout "30" >

In the Web.config
Add the following line below the xml declaration
<!DOCTYPE configuration SYSTEM "Environment.dtd">

Then in the appSettings Section i am referencing the entity

<appSettings>
<add key="SessionTimeout" value="&SessionTimeout;"/>
</appSettings>

If i mouse hoover on "&SessionTimeout " it shows the actual value. This also
works fine in XML file without no issues when your open it in IE.

But when i use ConfigurationManager to get the value it is returning the
variable instead of actual value. Do you know how to solve this issue? I
appreciate your help.

Thanks
Sen