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  public string _label1
        {
            get { return Preis.Text; }
        }					

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         int z1 = 0 ;
            int z2 = 0;

         


            z1 = int.Parse(label3.Text);
            z2 = int.Parse(label7.Text);

            int erg = z1 - z2;

            label8.Text = erg.ToString();					

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Die Eingabezeichenfolge hat das falsche Format.					

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     SQLiteConnection connection = new SQLiteConnection("Data Source=Database.dat");
                string Query = "select * from Kasse where (Seriennumer= " + textBox1.Text + ")";
                SQLiteCommand cmd = new SQLiteCommand(Query, connection);
                SQLiteDataReader reader;

                try
                {

                    connection.Open();
                    reader = cmd.ExecuteReader();

                    while (reader.Read())
                    {
                        Artikel.Text = reader["Artikelbeschreibung"].ToString();
                        pr = reader["Preis"].ToString() + " €";
                        Preis.Text = pr;


                    }					

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reader["Preis"].ToString() + " €"