C# combobox item counts error

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C#

combobox

I am working with comboxbox and facing issue.

I have 75 or 50 files listed in comboBox2, when selecetd index reaches 75th or 50th file it throws error.

I am trying to change selected index of comboBox1 when selected index reached last file of combobox2.

private void btnaccept_Click(object sender, System.EventArgs e)
{
if (comboBox2.SelectedIndex < comboBox2.Items.Count - 1)
                               {
                                  comboBox2.SelectedIndex = comboBox2.SelectedIndex + 1;
                               }

if (comboBox2.SelectedIndex > comboBox2.Items.Count -1)
                               {
                     comboBox1.SelectedIndex = comboBox1.SelectedIndex + 1;

                               }
}

I am getting error as:

InvalidArgument=Value of 75 is not valid for 'SelectedIndex
Parameter name: SelectedIndex
Any help and suggestions appreciated.

Thanks.