A typical web.config file in v2.0 could have the following section which is placed directly under the root <configuration> section.

<connectionStrings>
    <
remove name
="LocalSqlServer" />
    <
add name="LocalSqlServer" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true" providerName
="System.Data.SqlClient"/>
    <
add name="MainConnStr" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|main.mdf;User Instance=true" providerName
="System.Data.SqlClient"/>
</
connectionStrings>


connectionStrings>
    <
remove name=
"LocalSqlServer" />
    <
add name="LocalSqlServer" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true" providerName="System.Data.SqlClient"
/>
    <
add name="MainConnStr" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|main.mdf;User Instance=true" providerName
="System.Data.SqlClient"/>
</
connectionStrings>


You can reference this directly from code using:

[C#]
string connStr = ConfigurationManager.ConnectionStrings["MainConnStr"].ConnectionString;


[VB]
Dim connStr As String = ConfigurationManager.ConnectionStrings("MainConnStr").ConnectionString


Note that the namespace for this is System.Configuration so for a console application the full namespace is required.

Or you can reference this declaratively within the ConnectionString property of a SqlDataSource:

<asp:SqlDataSource ID="SqlDataSource1" runat="server"
 
ConnectionString
="<%$ ConnectionStrings:MainConnStr %>"
 
SelectCommand="SELECT [au_id], [au_lname], [au_fname], [state] FROM [authors]" />