FluentValidation by: Jeremy Skinner

A validation library for .NET that uses a fluent interface to construct strongly-typed validation rules.

FastMember by: Marc Gravell

In .NET reflection is slow... well, kinda slow. If you need access to the members of an arbitrary type, with the type and member-names known only at runtime - then it is frankly hard (especially for DLR types). This library makes such access easy and fast.

Fhir.Metrics by: Martijn Harthoorn

Conversion system for physical units, with UCUM implementation - The Unified Code for Units of Measure

FastEndpoints by: FastEndpoints

A light-weight REST Api framework for ASP.Net 6 and newer that implements REPR (Request-Endpoint-Response) Pattern.

ExcelNumberFormat by: ExcelNumberFormat developers

.NET library to parse ECMA-376 number format strings and format values like Excel and other spreadsheet softwares.

FastExpressionCompiler by: Maksim Volkau

FastExpressionCompiler is 10-40x times faster than Expression.Compile()

DotNetAnalyzers.PublicApiAnalyzer by: Sam Harwell et. al.

An analyzer for packages with public APIs.

Elmah.Io.AspNetCore by: elmah.io

An elmah.io provider for ASP.NET Core

CsvHelper by: Josh Close

A library for reading and writing CSV files. Extremely fast, flexible, and easy to use. Supports reading and writing of custom class objects.

Flurl.Http by: Todd Menier

A fluent, portable, testable HTTP client library.

FastMember.Signed by: Marc Gravell

In .NET reflection is slow... well, kinda slow. If you need access to the members of an arbitrary type, with the type and member-names known only at runtime - then it is frankly hard (especially for DLR types). This library makes such access easy and fast.

DocumentFormat.OpenXml by: Microsoft

The Open XML SDK provides tools for working with Office Word, Excel, and PowerPoint documents. It supports scenarios such as: - High-performance generation of word-processing documents, spreadsheets, and presentations. - Populating content in Word files from an XML data source. - Splitting up (shredding) a Word or PowerPoint file into multiple files, and combining multiple Word/PowerPoint files into a single file. - Extraction of data from Excel documents. - Searching and replacing content in Word/PowerPoint using regular expressions. - Updating cached data and embedded spreadsheets for charts in Word/PowerPoint. - Document modification, such as removing tracked revisions or removing unacceptable content from documents.

Flurl by: Todd Menier

A fluent, portable URL builder. To make HTTP calls off the fluent chain, check out Flurl.Http.

Dapper by: Sam Saffron Marc Gravell Nick Craver

A high performance Micro-ORM supporting SQL Server, MySQL, Sqlite, SqlCE, Firebird etc..

EPPlus by: Jan Källman

A library to manage Excel spreadsheets, using OOXML.

DeviceId by: Matthew King

Provides functionality to generate a 'device ID' that can be used to uniquely identify a computer.

FluentMigrator by: Josh Coffman Tom Marien

FluentMigrator is a database migration framework for .NET written in C#. The basic idea is that you can create migrations which are simply classes that derive from the Migration base class and have a Migration attribute with a unique version number attached to them. Upon executing FluentMigrator, you tell it which version to migrate to and it will run all necessary migrations in order to bring your database up to that version. In addition to forward migration support, FluentMigrator also supports different ways to execute the migrations along with selective migrations called profiles and executing arbitrary SQL.

Elmah.Io.Client by: elmah.io

.NET client for communicating with the elmah.io REST API

DotNetAnalyzers.PublicApiAnalyzer.Unstable by: Sam Harwell et. al.

An analyzer for packages with public APIs.

DotLiquid by: Tim Jones Alessandro Petrelli

DotLiquid is a templating system ported to the .NET framework from Ruby’s Liquid Markup.