Small fixes

[rinatxp]

Some small fixes:

• using (obj is [type] objType) to get rid of an extra cast
• using StringBuilder in ToString() methods for more performance

[rejurime]

Hi! my 2 cents, I think 21 commits to change 15 files isn't great for commit history, wouldn't it be better to do a PR with a clean history to reflect what you want to change? For example, the same commit message is repeated many times and I don't think that adds value to the story.

[rinatxp]

Hi! Ty for your advice. I squashed all these commits into one

[harry-cpp]

I like small cleanup of is operator usage, but please revert the StringBuilder usage in ToString methods.

Also don't worry about making too many commits in a PR, the maintainers can easily squash them down during the merge process 😉

[rinatxp]

No problem, I reverted StringBuilder usage

[harry-cpp]

Missed reverting this one.

[mrhelmut]

These changes are C# 5+ features. We really need to restore the C# 5 check from the ConsoleCheck csproj and re-run it to make sure that none slipped in while it was disabled.

EDIT: reverted, and ran the checks.

[mrhelmut]

Like I feared, we already merged code that breaks this. I'll track that later to revert those changes.