Cache contains data, but can't retrieve data

Question

Here's my bit of code:

List<Sale> sales = new List<Sale>();

if (Cache["Sales"] != null)
{
    sales = (List<Sale>)Cache["Sales"];
}
else
{
    ...
    Cache.Add("Sales", sales, null, DateTime.Now.AddMinutes(20),
        Cache.NoSlidingExpiration, CacheItemPriority.Normal, null);
}

When I try to pull the data from the cache, my "sales" object is null. Wondering why that code is hit at all, I ran the debugger in VS to see what was in the Cache object.

The Cache contains the data I need, but when it gets the data from cache, "sales" still comes out as null.

Is there something I'm doing wrong here?

EDIT:

I'm getting this error on casting:

[A]System.Collections.Generic.List1[controls_mySales+Sale] cannot be cast to [B]System.Collections.Generic.List1[controls_mySales+Sale]. Type A originates from 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\WINDOWS\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll'. Type B originates from 'mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' in the context 'LoadNeither' at location 'C:\WINDOWS\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll'

Answer 1

MSDN's page on the as keyword states that:

The as operator is like a cast except that it yields null on conversion failure instead of raising an exception.

Looks this is what's happening here -- the cast to type List<Sale> is failing, and returning null. Are you sure this is the type of the object in your cache?

EDIT:

In response to your edit, it seems like some sort of assembly-related serialization/deserialization issue possible related to binding contexts that honestly is a little over my head. I checked around and found the following two questions here on SO that may be able to point you in the right direction:

Question 1
Question 2

Hopefully those links prove helpful.