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private void hotsearch_Click(object sender, EventArgs e)
        {
            string searchString = hotsearchtextbox.Text;
            {   
                hottopicListbox.SelectionMode = SelectionMode.MultiExtended; // Set the SelectionMode property of the ListBox to select multiple items.
                int x = -1; // Set our intial index variable to -1.                    
                if (searchString.Length != 0) // If the search string is empty exit.
                {   // Loop through and find each item that matches the search string.
                    do
                    {   // Retrieve the item based on the previous index found. Starts with -1 which searches start.
                        x = hottopicListbox.FindString(searchString, x);
                        // If no item is found that matches exit.
                        if (x != -1)
                        {
                            // Since the FindString loops infinitely, determine if we found first item again and exit.
                            if (hottopicListbox.SelectedIndices.Count > 0)
                            {
                                if (x == hottopicListbox.SelectedIndices[0])
                                    return;
                            }
                            hottopicListbox.SetSelected(x, true); // Select the item in the ListBox once it is found.
                        }
                    } while (x != -1);
                }
            }
}                    

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string s = hotsearchtextbox.Text;
      if (s.Length != 0) // If the search string is empty exit.
                {
                    foreach (string str in hottopicListbox.Items)
                    {
                        if (str.Contains(s))
                        { 
                            MessageBox.Show(str);
                            int i = hottopicListbox.Items.IndexOf(str);
                            hottopicListbox.SetSelected(i, true);
                        }
                    }                    

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        private void hotsearch_Click(object sender, EventArgs e)
        {
            try
            {
                string s = hotsearchtextbox.Text;
                hottopicListbox.SelectionMode = SelectionMode.MultiExtended; // multimode selection
                //int i = -1;
                if (s.Length != 0) // wenn net dann net.
                {
                    List<int> selections = new List<int>(); // array fuer die index eintraege
                    foreach (string str in hottopicListbox.Items)
                    {
                        if (str.Contains(s))
                        { 
                            MessageBox.Show(str);
                            selections.Add(hottopicListbox.Items.IndexOf(str));
                        }
                    }
                    foreach (int index in selections)
                    { hottopicListbox.SetSelected(index, true); }
                }                
            }
            catch (Exception upsala)
            { MessageBox.Show(upsala.Message); }
        }                    

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string search = hotsearchtextbox.Text;
if (search == "")
  return;

for (int i = 0; i < hottopicListbox.Items.Count; i++) {
  var item = (string)hottopicListbox.Items[i];
  if (item.Contains(search))
    hottopicListbox.SetSelected(i, true);
}                    

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                hottopicListbox.SelectionMode = SelectionMode.MultiExtended;
                string search = hotsearchtextbox.Text;
                if (search == "")
                    return;

                for (int i = 0; i < hottopicListbox.Items.Count; i++)
                {
                    var item = (string)hottopicListbox.Items[i];
                    if (item.Contains(search))
                        MessageBox.Show(item); // MessageBox mit Ergebniszeigen
                        hottopicListbox.SetSelected(i, true); //Ergebniss in der Listbox markieren
                }