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StreamWriter sw = File.CreateText(filename);
                sw.Write(text);
                sw.Flush();
                sw.Close();                    

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File.WriteAllText(filePath, content);                    

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private void cmdStreaming_Click(object sender, EventArgs e)
        {
                string strNameData = "text.txt";
                string strCurrentPath = @"C:\";
                string strCurrentValue = String.Empty;

                using (StreamWriter writer = new StreamWriter(strCurrentPath + strNameData))
                {

                    try
                    {
                        strCurrentValue = txtValues.Text;
                        MessageBox.Show("Auslesen des Textfeldes erfolgreich !");
                    }

                    catch (NullReferenceException ex)
                    {
                        Console.WriteLine("Throwing Null Reference Exception in Streaming Situation, Source: " + ex.Message);
                        return;
                    }
            
                    finally
                    {
                         writer.WriteLine(strCurrentValue);
                         MessageBox.Show("Serialisierung erfolgreich abgeschlossen !");
                    }

                }            
                       
         }                    

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new StreamWriter(strCurrentPath+"\\" + strNameData)                    

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OpenFileDialog openDialog = new OpenFileDialog();
openDialog.FileName = "Auswahl einer Datei";
openDialog.Filter = "Text documents (.txt)|*.txt";

Nullable<bool> result = openDialog.ShowDialog();

  if (result == true)
  {
     // Open document
     string filename = openDialog.FileName;
  }