How to get connection string from app.config in C#

Make sure that your project references the System.Configuration assembly.

You can add System.Configuration to your project by right-clicking the "References" node in Solution Explorer and selecting "Add Reference...". Then, choose System.Configuration.

In your app.config file, define a connection string inside the <connectionStrings> element.

For example:

<configuration>
  <connectionStrings>
    <add name="cn" connectionString="Server=.;Database=myDb;User Id=sa;Password=123@qaz;" providerName="System.Data.SqlClient"/>
  </connectionStrings>
</configuration>

Use ConfigurationManager.ConnectionStrings to access the connection string.

string connectionString = ConfigurationManager.ConnectionStrings["cn"].ConnectionString;

Ensure that the connection string name in the ConfigurationManager.ConnectionStrings["cn"] matches the name attribute in the app.config.