value doesn't fall within the expected range

<pre lang="c#">
namespace pharmcy
{
public partial class MDIParent3 : Form
{
// private int childFormNumber = 0;
SqlConnection con=new SqlConnection ();
SqlCommand cmd=new SqlCommand ();
int i=1;

string a = "l.bak";
string x = @"c:\SQLBackup\";

public MDIParent3()
{
InitializeComponent();
}

private void ShowNewForm(object sender, EventArgs e)
{
Sale s = new Sale();
s.MdiParent = this;
s.Show();
}

private void OpenFile(object sender, EventArgs e)
{
deleteproduct s = new deleteproduct();
s.MdiParent = this;
s.Show();
}

private void SaveAsToolStripMenuItem_Click(object sender, EventArgs e)
{
SaveFileDialog saveFileDialog = new SaveFileDialog();
saveFileDialog.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.Personal);
saveFileDialog.Filter = "Text Files (*.bak)|*.bak|All Files (*.*)|*.*";
if (saveFileDialog.ShowDialog(this) == DialogResult.OK)
{
string FileName = saveFileDialog.FileName;
}
}

private void ExitToolsStripMenuItem_Click(object sender, EventArgs e)
{
this.Close();
}

private void CutToolStripMenuItem_Click(object sender, EventArgs e)
{
}

private void CopyToolStripMenuItem_Click(object sender, EventArgs e)
{
}

private void PasteToolStripMenuItem_Click(object sender, EventArgs e)
{

}

private void CascadeToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.Cascade);
}

private void TileVerticalToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.TileVertical);
}

private void TileHorizontalToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.TileHorizontal);
}

private void ArrangeIconsToolStripMenuItem_Click(object sender, EventArgs e)
{
LayoutMdi(MdiLayout.ArrangeIcons);
}

private void CloseAllToolStripMenuItem_Click(object sender, EventArgs e)
{
foreach (Form childForm in MdiChildren)
{
childForm.Close();
}
}

private void backupToolStripMenuItem_Click(object sender, EventArgs e)
{
backup b = new backup();
b.ShowDialog();
}


public void backup()
{
string iii=".bak";
string xxx = dateTimePicker4.Text;
string y = Convert.ToString(i);
string format = ("yyyy-MM-dd");
DateTime f = Convert.ToDateTime(xxx);
string yy = f.ToString(format);
bool ii = true;
int number = 1;
x += yy;
x += iii;
bool bBackUpStatus = true;
Cursor.Current = Cursors.WaitCursor;
try
{
con.Open();
if (Directory.Exists(@"c:\SQLBackup"))
{
while (ii)
{
if (!File.Exists(x))
{
ii = false;
bBackUpStatus = true;
break;
}
else
{
y = (number + 1).ToString();
x += y;
bBackUpStatus = false;
}
}
/*if (File.Exists(@"c:\SQLBackup\wcBackUp1.bak"))
{
if (MessageBox.Show(@"Do you want to replace it?", "Back