Assuming you have an array of your class elements:
MyClass[] mca = new MyClass[100];
for (int i = 0; i < 100; i++)
{
mca[i] = new MyClass(i);
}
Then storing it into the Session is trivial:
Session["MyArray"] = mca;
And retrieving it is not complex either:
MyClass[] mca = Session["MyArray"] as MyClass[];
if (mca != null)
{
...
}
Lists, arrays, class instances - all the same technique.