求大神

string[] ju = null;
DirectoryInfo di = new DirectoryInfo("F:\\");
FileInfo[] file = di.GetFiles();
int b = 0;
foreach (FileInfo f in file)
{
if (f.Extension ==".jpg")
{
ju[b] = f.Name;
b++;
}
}
foreach (string fd in ju)
{
listBox1.Items.Add(fd);
}
为什么listbox.items什么都没有