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string Constring = @"Data Source=JDKSOFTWARE\MSSQL2008R2; Database=SYSTEM_V10; Integrated Security=SSPI; User Instance = true;Persist Security Info=False;";

       private void button1_Click(object sender, EventArgs e)
        {
            string s;
            SqlConnection connection;
            SqlConnectionStringBuilder strB = new SqlConnectionStringBuilder(Constring);
 
            connection = new SqlConnection(strB.ConnectionString);  // hier geht's schon schief
            try
            {
                s = connection.ServerVersion;
                connection.Open();
                //textBox1.Text += connection.State.ToString() + s;
            }
            catch (Exception)
            {
                //textBox1.Text += "Nix geht..";
// Fehler:  ServerName ... invalidOperationException verursacht....
            }
            strB = null;
            connection = null;

        }					

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SqlConnectionStringBuilder sql = new SqlConnectionStringBuilder();
sql.UserID = "kjkjk";
sql.Password = "lklklkl";
usw.......					

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                s = connection.ServerVersion;  // ^^^^
                connection.Open();             // das beides muss man nur tauschen