Remember the Casting Restrictions we discussed a while back, let us settle that now. So we have some code like this: int i = 100; object obj = i; long l = (long)obj; And an invalid cast exception while casting 'obj' to long. It is obvious that we are not changing the value held by obj, but just reading it. Then why restrict such casting. Let us disassemble and see what we got. .locals init ( [0] int32 i, [1] object obj, [2] int64 l) L_0000: nop L_0001: ldc.i4.s 100 L_0003: stloc.0 L_0004: ldloc.0 L_0005: box int32 L_000a: stloc.1 L_000b: ldloc.1 L_000c: unbox.any int64 L_0011: stloc.2 L_0012: ret Oh, there we see something interesting - unbox. So the C# compiler uses the unbox instruction to retrieve the value from obj while casting; it does not use Convert . ToInt64 or similar mechanism. That is why the exception was thrown. From MSDN: Unboxing is an explicit conversion from the type object to a value type o
Another effective [debugging] technique is to explain your code to someone else. This will often cause you to explain the bug to yourself. Sometimes it takes no more than a few sentences, followed by an embarrassed "Never mind. I see what's wrong. Sorry to bother you." This works remarkbly well; you can even use non-programmers as listeners. - From "The Practice of Programming" by Brian W Kernighan & Rob Pike.