Covarsky 1.2.1

There is a newer version of this package available.
See the version list below for details.
dotnet add package Covarsky --version 1.2.1
NuGet\Install-Package Covarsky -Version 1.2.1
This command is intended to be used within the Package Manager Console in Visual Studio, as it uses the NuGet module's version of Install-Package.
<PackageReference Include="Covarsky" Version="1.2.1">
  <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add Covarsky --version 1.2.1
#r "nuget: Covarsky, 1.2.1"
#r directive can be used in F# Interactive and Polyglot Notebooks. Copy this into the interactive tool or source code of the script to reference the package.
// Install Covarsky as a Cake Addin
#addin nuget:?package=Covarsky&version=1.2.1

// Install Covarsky as a Cake Tool
#tool nuget:?package=Covarsky&version=1.2.1

Licensed under the MIT License NuGet


Covarsky is a tool that brings co(ntra)variant types to F# (or any other language that does not support them). It runs an MSBuild task that modifies assemblies after compilation.

How to install

  1. Add the Covarsky NuGet package to your project.

  2. Create a new source file with two attributes like that (F# example shown):

namespace global

open System

type internal CovariantOutAttribute() =
    inherit Attribute()

type internal ContravariantInAttribute() =
    inherit Attribute()

Warning: The attributes must be declared in the global namespace, and must be internal.

  1. Add the following line inside a PropertyGroup in your project file:
  1. You are good to go! Let's see an example.
type MyType<[<CovariantOut>] 'T> =
    abstract GetObject: unit -> 'T

// [...]

let cast (x: MyType<string>): MyType<obj> =
    unbox x


These two attributes will be ignored if used anywhere but in the generic parameters of an interface or a delegate.

Using these two attributes in a generic parameter that is already co(ntra)variant will raise a warning but will be ignored as well.

Using both attributes on the same time will raise an error and fail the build (unless something above hasn't already happened).

DISCLAIMER: Using the attributes in any other wrong way (such as a contravariant interface with a method that returns the generic type) will not be checked by Covarsky and may break your assembly.


There are no supported framework assets in this package.

Learn more about Target Frameworks and .NET Standard.

This package has no dependencies.

NuGet packages

This package is not used by any NuGet packages.

GitHub repositories

This package is not used by any popular GitHub repositories.

Version Downloads Last updated
1.4.2 1,992 9/3/2021
1.4.1 395 7/19/2021
1.4.0 371 4/19/2021
1.3.1 597 11/2/2020
1.3.0 403 10/8/2020
1.2.1 460 9/21/2020
1.2.0 591 3/17/2020
1.0.0 566 2/1/2020

Fixed occasional NullReferenceExceptions.
Exceptions during Covarsky's execution will show a stacktrace.