Format of the initialization string does not conform to specification
starting at index 0
Web.config :
<connectionStrings>
<add name="TestDataConnectionString" connectionString="Data Source=.\SQLExpress;Initial Catalog=TestData;User ID=satest;Password=satest"
/>
</connectionStrings>
In aspx.cs Page the code must be written in the below format :
SqlConnection con = new
SqlConnection(ConfigurationManager.ConnectionStrings["TestDataConnectionString"].ToString());