connection = new SqlConnection(ConfigurationManager.AppSettings["myconnectionstring"])

<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConfigurationManager.AppSettings["myconnectionstring"] %>"
SelectCommand="SELECT * from table1"></asp:SqlDataSource>

<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:myconnectionstring%>"
SelectCommand="SELECT * from table1"></asp:SqlDataSource>

<connectionStrings>
  <add name="myconnectionstring" connectionString="server=localhost;database=Northwind;user=north;password=north;" providerName="System.Data.SqlClient" />
</connectionStrings>