C# sql connection string

            string connetionString = null;
            SqlConnection cnn;
            cnn = new SqlConnection(ConfigurationManager.ConnectionStrings["connection"].ConnectionString);
            try
            {
                cnn.Open();
                MessageBox.Show("Connection Open ! ");
                cnn.Close();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Can not open connection ! ");
            }



<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="connection" connectionString="Data Source='OFFICE\SQLEXPRESS';Initial Catalog=market_data;Persist Security Info=True;User ID=sa;Password=xxxxx" providerName="System.Data.SqlClient" />
    </connectionStrings>
    <startup>
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
    </startup>
</configuration>