Fall back to other formatters when IArgumentValueFormatter.GetArgumentValueAsString throws an exception or returns null

[blairconrad]

From a conversation in #1683. At that point I suggested a throwing a UserCallbackException, but the documentation for that class says it's "An exception thrown when a user-provided callback throws an exception", so if we want to throw, we should either consider a different exception or update the docs for that class.
However, Maybe throwing isn't the best course of action. We're already in an error condition, so derailing that just to tell the user about a different error could confuse things. Perhaps we should just construct a non-null message in its place. Clients who turn nullability warnings on in their test projects will be alerted that null isn't the best return value anyhow.

[blairconrad]

Perhaps we should just replace the null with an indication that the argument value was an object of whatever type it was.

[thomaslevesque]

Perhaps we should just replace the null with an indication that the argument value was an object of whatever type it was.

Which is the default behavior if there's no custom formatter, no?

[blairconrad]

No. If the value isn't null and there's no custom formatter for the value's type, we will

• format strings as either "string.Empty" or the string with quotes around it,
• format enumerables like a comma-separated list of values in "[]"s, where each internal value is formatted according to its type, possibly using custom formatters,
• format fakes as the FakeDisplayName, and
• format anything else using ToString

[thomaslevesque]

So, can't we just do as if there were no custom formatter?

[blairconrad]

We can. I'm in.

[blairconrad]

Marking as breaking, as it's a change in behaviour, but I do not think there's any downside.

[blairconrad]

Late-breaking change! Using best-available fallback formatter whenenver the preferred formatter throws an exception or returns null.

[afakebot]

This change has been released as part of FakeItEasy 6.0.0-beta.1.