Accessing database connection string using app.config in C# winform
This is all you need:
System.Configuration.ConfigurationManager.ConnectionStrings["MyDBConnectionString"].ConnectionString;
Use ConfigurationManager instead of ConfigurationSettings. It has a ConnectionStrings property that you should use for connection strings in the connectionStrings section:
ConfigurationManager.ConnectionStrings["MyDBConnectionString"].ConnectionString;
You are using the ConnectionStrings collection, not the AppSettings.
ConfigurationManager.ConnectionStrings["MyDbConnectionString"].ConnectionString;