C# structure and sorting

Question

I had an exam this morning i couldn't figure it out.  it looks like doesn't matter now because the exam was this
morning but i still want to know how to do it....

i have 2 button: Add(btn_ADD) and Sort(btn_SORT)
  1 textbox ( textbox1 )and  1 listbox (listbox1)

Everything is working except for 1 problem:

The teacher add 3 string: 12345
  12
  abc
(my code is sorting like this, and this is wrong)
Then click the sort buttin: abc
    12345
  12
(how to sort like this?)
the correct sorting would be:  abc
  12
  12345

  struct SString
  {
  public string m_Sname;

  public SString(string s)
  {
  m_Sname = s;
  }
  public override string ToString()
  {
  return string.Format(m_Sname.ToString());
  }
  }
  List<SString> m_list = new List<SString>();
  public Form1()
  {
  InitializeComponent();
  }

  private void btn_ADD_Click(object sender, EventArgs e)
  {
  try
  {
  SString temp = new SString(textBox1.Text);
  m_list.Add(temp);
  listBox1.Items.Add(temp.ToString());
  }
  catch (Exception ex)
  {
  MessageBox.Show(ex.Message);
  }
  }

  private void btn_SORT_Click(object sender, EventArgs e)
  {
  SString[] sort;
  sort = m_list.ToArray();
  BubbleSort_string(sort);
  listBox1.Items.Clear();
  m_list.Clear();
  foreach (SString n in sort)
  {
  m_list.Add(n);
  listBox1.Items.Add(n.ToString());

  }
  }
  private void BubbleSort_string(SString[] ArrayOfString)
  {
  SString tempS = new SString();
  //number of passes through array equals number of elements - 1
  for (int iPass = 0; iPass < ArrayOfString.Length - 1; iPass++)
  {
  //scan through the array looking for swaps
  for (int iScan = 0; iScan < ArrayOfString.Length - 1; iScan++)
  {
  char a = string.Format(ArrayOfString[iScan].m_Sname).ToCharArray()[0];
  char b = string.Format(ArrayOfString[iScan + 1].m_Sname).ToCharArray()[0];
  int x = (int)a;
  int y = (int)b;
  if (x < y)
  {
  tempS = ArrayOfString[iScan];
  ArrayOfString[iScan] = ArrayOfString[iScan + 1];
  ArrayOfString[iScan + 1] = tempS;
  }
  }
  }
  return;
  }