Dynamic setting of App.config

Question

I have App.config in my project.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="OSConnection"
providerName="System.Data.SqlClient"
connectionString="data source=My-PC\SQLTEST;initial catalog=productorder;User ID=sa;Password=1327221;integrated security=True;"/>
</connectionStrings>
</configuration>
I hard coded in the App.config but I want to set some parameters dynamically by reading some external file.
For example, I want to read "sa" and "1327221" from the external file and pass to User ID and Password of this App.config.
How do I implement that?

Answer 1

For the specific case where the DB file is in the same directory as your app then you probably don't need to specify the path, just the filename.  However if that doesn't work then there is 1 substitution string that was added for ASP.NET called DataDirectory. 
However to use that outside ASP.NET you need to set it using the code provided
here.
For more elaborate changes to a connection string you are better off loading the base string at runtime and then using a connection string builder to adjust the settings.  For example if your app normally uses Win auth but you need to be able to switch
to SQL auth dynamically at runtime then that would be a case where the base connstring comes from the config but a builder will allow you to adjust it before actually passing it onto the connection object.
Michael Taylor
http://blogs.msmvps.com/p3net