how to write code for entering text is not available in combobox List it will show messagebox in c#windows forms

string text1 = textBox1.Text;
boolean isFound = false;

foreach(object item in comboBox1.Items)
{
   DataRowView row = item as DataRowView;

   if(row != null && text1 == row["ColumnName"].ToString())
   {
        isFound = true;
        break;
   }
}

if (!isFound)
{
    MessageBox.Show("Text is not found in the combo box");
}