You can use the Clipboard class. For example:
Copying
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Clipboard.SetDataObject(MyTextbox.SelectedText, true);
Cutting
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Clipboard.SetDataObject(MyTextbox.SelectedText, true);
MyTextbox.Text = string.Empty;
Pasting
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IDataObject data = Clipboard.GetDataObject();
if (data.GetDataPresent(DataFormats.Text))
MyTextBox.Text = (string) data.GetData(DataFormats.Text);
Regards,