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;