C# Using External Config File

app.config

<?xml version='1.0' encoding='utf-8'?>
<configuration>
  <connectionStrings configSource="connections.config"/>
</configuration>

connections.config

<connectionStrings>
  <add name="Name" 
   providerName="System.Data.ProviderName" 
   connectionString="Valid Connection String;" />
</connectionStrings>

References:

• https://msdn.microsoft.com/en-us/library/ms254494(v=vs.110).aspx