Top 20 NuGet precondition Packages

Code Contracts and common functionalities used by other PostSharp pattern libraries. An official PostSharp pattern library.
Design by contract programming support
Package Description
A no frills library for validating arguments.
No fuss, KISS contract library for .NET.
Simple Design by contract library supporting PreConditions and PostConditions
Pineapple is a set of core libraries that includes utilities, primitives, helper classes for .NET! The inspiration for Pineapple comes from Google's Guava framework.
A simple code-contract class for validating method argument preconditions.
A fluent library of wonderful extensions and utilities that can make your code short and clean.