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Hauptform:
String suchen;

public void Suchroutine(String search)
{
    suchen = search;         
}


Form2:
private void rtfsuchen_KeyPress(object sender, KeyPressEventArgs e)
{            
  if (e.KeyChar.Equals((char)13))
 {             
   ((Form1)Owner).Suchroutine("Testtext");
 }
}					

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private Form1 mainForm;

public Form2(Form1 mainForm)
{
  InitializeComponent(); 
  this.mainForm = mainForm;  
}					

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this.mainForm..Suchroutine("Testtext");