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private void button1_Click_1(object sender, EventArgs e)
        {
            StreamWriter WStream = new StreamWriter("qwert.txt");
            WStream.Write(textBox1.Text, textBox2.Text);
            WStream.Close();                    

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private void button1_Click_1(object sender, EventArgs e)
        {
            StreamWriter WStream = new StreamWriter("qwert.txt");
            WStream.Write(textBox1.Text);
            WStream.Write(textBox2.Text);
            WStream.Close();                    

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... 
StreamWriter sw = new StreamWriter(filename, true)
...                    

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private void button1_Click_1(object sender, EventArgs e)
        {
            StreamWriter WStream = new StreamWriter("qwert.txt");
            StreamWriter sw = new StreamWriter(filename, true);
            WStream.Write(textBox1.Text, textBox2.Text);
            WStream.Close();                    

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using (StreamWriter wStream = new StreamWriter("qwert.txt", append))
{
  wStream.WriteLine(textBox1.Text);
  wStream.WriteLine(textBox2.Text);
}                    

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textBox1.text= //dann der string!                    

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private string XorEncrypt(string text, int key)
        {
            string newText = "";

            for (int i = 0; i < text.Length; i++)
            {
                int charValue = Convert.ToInt32(text[i]); //get the ASCII value of the character
                charValue ^= key; //xor the value

                newText += char.ConvertFromUtf32(charValue); //convert back to string
            }

            return newText;
        }                    

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str_unlesbar = XorEncrypt('Hallo Welt', 17);                    

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str_lesbar = XorEncrypt(str_unlesbar,17);                    

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string s= textBox1.text