How do I check the contents of zip file before list it into combobox using C# WF

var = comboBox1.SelectedItem.ToString();
fullpath = fbd.SelectedPath + "\\"+ var;

comboBox2.Items.Clear();
if (File.Exists(fullpath))
{
ZipArchive zip = ZipFile.OpenRead(fullpath);
MessageBox.Show(Directory.EnumerateFileSystemEntries(fullpath, "*.csv").ToList<string>().Count + " csv file(s) present"); // get error here
foreach (ZipArchiveEntry entry in zip.Entries)
{
comboBox2.Items.Add(entry.FullName); // fill comboBox2 with *.csv files from selected zip file
}
zip.Dispose();
}
else
{
MessageBox.Show("no csv file(s) present");
}