Top 20 NuGet guard Packages

Catel.Core library.
A simple package with guard clause helper methods. See docs for how to extend using your own extension methods.
Argument validation made simple and unified. It ensures arguments conforms to simple validation rules and provides an uniformed exception throwing strategy if arguments are invalid.
Simple Guard (or DbC) implementation.
A high-performance, extensible argument validation library.
Evolved toolkit for developing applications faster and with less code. Includes general purpose extension methods, guard clauses, predefined value objects (PasswordHash,Email, Percentage etc) and specialized helper classes for specific tasks.
Static methods to simplify argument validity checking.
LeadPipe.Net is a collection of useful types, patterns, and extensions. These include a guarding type, tracking observable collection, command mediator, poller, and finite state machine. LeadPipe.Net is an open source collection of useful tools for .NET development. Read more about the ...
A collection of static classes for debugging and diagnostics of program contracts such as preconditions, postconditions, and invariants.
.NET Core port of Seterlund.CodeGuard, a Guard and Validator library. Example: Guard.That(arg1).IsNotNull().GreaterThan(100); Throws an exception if conditions are not met. Supports the ability to get a list of the failed conditions.
Lightweight guard / pre-condition / parameter validation library for .NET
A simple library that facilitates runtime checks of code and allows to define preconditions and invariants within a method.
Roslyn analyzers that aim to help security audits on .NET applications.
The Diagnostics library provides classes that help checking incoming parameters of a method using the Fluent API or not, depending on what you prefer.
A fluent, extensible interface for guard clauses and argument validation.
A simple guard library for validating method parameters without being a complete validation library. This library makes it easy to validate method arguments in a uniform way.
Deep fork of the Seterlund.CodeGuard library. Example: Guard.That(arg1).IsNotNull().GreaterThan(100); Throws an exception if conditions are not met.
Guard and Validator library. Example: Guard.That(arg1).IsNotNull().GreaterThan(100); Throws an exception if conditions are not met. Supports the ability to get a list of the failed conditions.
Provides extension methods for the Guardian source code package.
A .NET library that provides a variety of reusable functionality, including packed integers (PackedInt16, PackedInt32, and PackedInt64), guard methods, various extension methods, and classes related to the Task Parallel Library (AsyncLazy, AsyncLock, AsyncAutoResetEvent, and AsyncManualResetEvent).