How can I connect C# with Db with App.config?

I need to connect C# with SQL Server database using app.config.

My code is:

public partial class Form1 : Form
{
    string  connectionString = ConfigurationManager.ConnectionStrings["Conn"].ConnectionString;     

    public Form1()
    {
            InitializeComponent();
    }

    public void InsertDeleteUpdate(string query)
    {
            try
            {
                SqlConnection Conn = new SqlConnection(connectionString);
                Conn.Open();
                SqlCommand comm = new SqlCommand(query, Conn);
                comm.ExecuteNonQuery();
                Conn.Close();
                MessageBox.Show("Succeeded");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);   
            }
        }

and in app.config I have:

<connectionStrings>
    <add name="Test1" 
         providerName="System.Data.SqlClient"
         connectionString="Data Source=KSPR1WS126;Initial Catalog=Test1;User ID=sa" />
</connectionStrings>

but I get an error and I can’t fix it:

ConfigurationErrorExeption was Unhandled Configuration system failed to initialize

Can anyone help me please ?

Answer

Try:

var connectionString = ConfigurationManager.ConnectionStrings["Test1"].ConnectionString; 

You’re referencing Conn as the connection string name but referenced it as Test1 in the App.Config