Add contracts verification in all methods #51
Description
Is your feature request related to a problem? Please describe.
We should add contracts verification according to existing docs. If there are no docs in wiki about some conditions and remarks, we must create wiki pages with detailed explanation.
Describe the solution you'd like
Clarify all methods contracts and implement verifications for them. We can use our extensions helpers to avoid code duplication or use some third-party library to work with contracts.
Describe alternatives you've considered
We also can write XML docs in the code and generate HTML version of the docs by special tools.
Additional context
Add any other context or screenshots about the feature request here.