foo1.cs

while (ReferenceNameNavigator.GetByQualifier(referenceName) != null)
  referenceName = ReferenceNameNavigator.GetByQualifier(referenceName);

foo2.cs

while (ReferenceNameNavigator.GetByQualifier(referenceName) is var byQualifier && byQualifier != null)
  referenceName = byQualifier;

Looks like the same

Q qualifier;
while ((qualifier = ReferenceNameNavigator.GetByQualifier(referenceName)) != null)
  referenceName = qualifier;

but more easy..?

@ViIvanov yep, and with a less-scoped variable

"Less-scoped" is nice; the minimalist aesthetic should be more widely celebrated.

In this vein, why not just eliminate the excess variable entirely?

while (referenceName != (referenceName = ReferenceNameNavigator.GetByQualifier(referenceName) ?? referenceName))
    ;