The 'as' cast operator throws if its operand does not have the specified type.
That makes it useless without a prior type test (or implicit knowledge), but in that case, it doing an explicit test is redundant. I find myself not using it because I don't want to introduce an extra, unnecessary, test.

I suggest changing it to make it either simpler or useful on its own.
That is, either:

1. Don't do a test in unchecked mode, but simply change the static type of the expression. I.e., make exp as Foo equivalent to assigning exp to a Foo variable and reading it back, but as an inline cast. In checked mode it would throw, just as assignment would. Or ...
2. Let exp as Foo evaluate to null if the value of exp is not a Foo (as in C#). That is, return null where it currently throws. This makes 'as' useful on its own, without a prior type check. Instead of doing 'var tmp = exp; Foo foo = tmp is Foo ? tmp : null;' you just do 'Foo foo = exp as Foo;'.