Convert expression to string in a reversible manner

Question

What is the best way to convert an expression to a string in a reversible manner?

I need a toStr function so that it is always true that

ToExpression@toStr[expr] === expr

ToString is not satisfactory because it may use special, irreversible formatting. Example:

ToString[Graphics[{Circle[{0,0}]}]]
(* "-Graphics-" *)

In this case the problem is that it uses OutputForm by default. Requesting InputForm would solve this. But then consider

Format[x, InputForm] = "foo"

ToString[x, InputForm]
(* "foo" *)

x does exist as a proper expression in-memory. It can be written to disk by exporting to MX and then reliably re-imported. I am looking for the same functionality, but through strings.

---

Note: I am aware that there are some expressions which can't be safely cycled through a string, or even through Compress or through MathLink. An example would be

asc = <| a -> 1|>
a = 5;

asc
(* <| a -> 1|> *)

ToExpression@ToString[asc]  (* or use Uncompress@Compress[...] if you like *)
(* <| 5 -> 1 |> *)

I am not aiming to solve these types of difficulties, which are separate from my main problem.

Answer 1

This may fail in some obvious way, but it works on the example you give

toStr = ToString@*FullForm

For example with the following

testcases = {Graphics[{Circle[{0, 0}]}], Series[Sin[x], {x, 0, 3}], Integrate[f[x], x]}

I have

strs = toStr/@testcases
results = ToExpression /@ strs
testcases === results
(* True *)