SQL connection string is not recognised and throws and exception in C#

CONTEXT: I am writing a WPF app. It works with a SQL Server database in which I put some data, concretely the strings Titulo and Descripcion, the int XP and the string fotoMision. When I click a button the program is supposed to save this data in the database.

PROBLEM: when I click the button it throws me an exception in string connection’s line showing that the object is not instanced. If I put these first lines right below the InitializeComponent(); line the second one doesn’t recognise the miConexion string. Why does that happen and how can I fix it?

CODE:

static string miConexion = ConfigurationManager.ConnectionStrings["myProgress.Properties.Settings.DatosHabilidades"].ConnectionString;

SqlConnection miConexionSql = new SqlConnection(miConexion);

private void Button_Click(object sender, EventArgs e)
{
    string consulta = "INSERT INTO datosMisiones (Titulo, Descripcion, XP, fotoMision) VALUES (tituloMision, descripcionMision, xpMision, nuestroIconoMision";

    SqlCommand miSqlCommand = new SqlCommand(consulta, miConexionSql);

    miConexionSql.Open();

    miSqlCommand.Parameters.AddWithValue("@Titulo", tituloMision);

    miSqlCommand.ExecuteNonQuery();
    miConexionSql.Close();
}

Answer

Connections are meant to be short-lived, i.e. you should create one when the button is clicked and then dispose it right after you have executed the query. Using a using statement implicitly disposes the IDisposable:

private void Button_Click(object sender, EventArgs e)
{
    const string Consulta = "INSERT INTO datosMisiones (Titulo, Descripcion, XP, fotoMision) VALUES (tituloMision, descripcionMision, xpMision, nuestroIconoMision";
    string miConexion = ConfigurationManager.ConnectionStrings["myProgress.Properties.Settings.DatosHabilidades"].ConnectionString;
    using (SqlConnection miConexionSql = new SqlConnection(miConexion))
    using (SqlCommand miSqlCommand = new SqlCommand(Consulta, miConexionSql))
    {
        miConexionSql.Open();

        miSqlCommand.Parameters.AddWithValue("@Titulo", tituloMision);

        miSqlCommand.ExecuteNonQuery();
        miConexionSql.Close();
    }
}