C# 9 Records and Init Only Settings Without .NET 5

7 minute read

This blog is one of The December 11th entries on the 2020 C# Advent Calendar. Thanks for having me again Matt!

Some of the biggest new features in C# 9 have got to be Records and Init only setters. But when can you safely use these features? Officially, you can only use them if you are targeting .NET 5. However, many developers may not be ready to move to .NET 5 yet, especially since it isn’t an LTS release. And what about NuGet packages targeting .NET Standard or other versions of .NET?


For internal projects and/or types, you can safely use records and init only setters to target any modern version of .NET. I’ve tested .NET 4.8, .NET Core 2.1, and .NET Core 3.1. However, for any public members exposed via NuGet packages there are lots of caveats.

Note: Most of the other C# 9 features are fine to use, regardless of target frameworks or consumer C# versions:

  • Top-level statements
  • Switch expression pattern matching enhancements
  • Native sized integers (syntactic sugar for IntPtr/UIntPtr)
  • Target-typed new expression
  • Static anonymous functions
  • GetEnumerator extension methods
  • Lambda discard patterns
  • Attributes on local functions
  • New features for partial methods

What are Records?

There are plenty of blog posts out there that describe records and what they do, so I won’t dig in too deep here. In simple terms, they provide a lot of boilerplate code that allows a developer to create immutable reference types with value equality, easy methods to clone the types with slightly different values, and more.

public namespace MyProgram
  public record Person
    public string? FirstName { get; init; }
    public string? LastName { get; init; }

  public record PersonWithHeight : Person
    public int HeightInInches { get; init; }

    public PersonWithHeight Grow(int inches) =>
      this with { HeightInInches = HeightInInches + inches };

PersonWithHeight person = new()
  FirstName = "Brant",
  LastName = "Burnett",
  HeightInInches = 69

Under the covers, records are really classes with a lot of boilerplate code already added to deal with equality comparisons, cloning, ToString, etc.

What are init only setters?

Init only setters are not really specific to records, though they are quite powerful when combined. You can see an example of init only setters above in the example for records. Instead of a set keyword, the property uses an init keyword. These properties can only be set as part of an object initializer combined with the constructor, after which they may not be modified.

Person person = new()
  FirstName = "Brant" // Allowed

person.LastName = "Burnett"; // This will cause a compiler error

Under the covers, C# does this by making the setter method have a slightly different signature. Normally, the signature would be void set_FirstName(string value), but an init only setter has the signature void modreq(System.Runtime.CompilerServices.IsExternalInit) set_FirstName(string value). Note that modreq can’t be directly added to a method signature in C#, this is an IL construct. C# adds it for you in this case.

Requirements to Create Records and Init Only Setters

LangVersion 9

Your project must be using C# 9 or later. If you’re targeting .NET 5, this should be the case already. However, if you target other runtimes, you may need to manually enable C# 9 in your .csproj file.


You must also be using Visual Studio 2019 16.8 or later, or MSBuild 2019 16.8 or later, or the .NET Core SDK 5.0.100 or later. These are the versions that include the C# 9 compiler.

Creating Init Only Properties on Older Frameworks

The key to init-only properties is the IsExternalInit class, which is basically nothing but a placeholder (somewhat like an empty attribute) that is applied to the void return type of the setter. This type is defined as part of .NET 5, but if you’re not targeting .NET 5 then it’s not available for the compiler to reference.

CS0518 Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported

To fix this, you must include the type yourself in your code. I recommend a copy/paste of the following into a file in your project. It includes conditional compilation directives which work if you’re multi-targeting so that it isn’t included unless necessary.

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

#if NETSTANDARD2_0 || NETSTANDARD2_1 || NETCOREAPP2_0 || NETCOREAPP2_1 || NETCOREAPP2_2 || NETCOREAPP3_0 || NETCOREAPP3_1 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48

using System.ComponentModel;

// ReSharper disable once CheckNamespace
namespace System.Runtime.CompilerServices
    /// <summary>
    /// Reserved to be used by the compiler for tracking metadata.
    /// This class should not be used by developers in source code.
    /// </summary>
    internal static class IsExternalInit


Creating Records on Older Frameworks

Records actually work fine on older frameworks without any special changes, so long as you aren’t using init only setters.

However, records do transparently apply init only setters if you use this syntax to define the properties:

public record Person(string FirstName, string LastName)

In that case, be sure to add the IsExternalInit class above.

Consuming Records and Init Only Properties

What about including records and init only properties in things like NuGet packages? What are the requirements for package consumers to be able to use these features?

Private Members

It is safe to include private or internal records or properties with init only setters in public facing assemblies. Records are basically just syntactic sugar on top of classes, and modreq used by init only setters have been around a long time.

Public Init Only Setters

To use a public or protected property with an init only setter, the consumer must be using a C# compiler that supports C# 9. Even if they aren’t using LangVersion 9 in their csproj, the compiler must be capable of understanding the modreq. If the consumer doesn’t understand the init only setter, it will be unable to access the setter at all and will appear as a read-only property.

Similarly, other languages like VB.NET don’t like init only setters. In fact, VB.NET won’t recognize the property at all, even as read-only, and even with the latest version of the VB.NET compiler.

As a result, I do not recommend using init only setters for public members unless it is for an internal project with a known list of consumers. All of these consumers must be C# projects, and all developers must be using the latest C# compiler.

Public Records

The compatibility of records across different versions of C# is a bit more confusing. First of all, if you’re using init only setters on your records (you probably are) then see those rules above.

C# consumers can get most of the other features of records, except cloning. The clone method is hidden under the special name <Clone>$ and you can’t reach it directly in C#, you must use the with operator which is only available in C# 9.

The same limitation applies in VB.NET, except even on the latest version there is no equivalent of the with operator. This makes cloning completely unavailable.


Okay, all of that was pretty convoluted. Variables include your target framework, your consumer’s target framework, your LangVersion, and the version of MSBuild or .NET Core SDK installed on the consumer’s development machine and build agents.

For developers of NuGet packages which must be consumed by many users, here’s a matrix that will hopefully make things a bit clearer. These are my recommendations, some of these listed as Not Available could be considered partially available. But in my opinion the limitations aren’t worth it and it should be avoided.

Private/Internal Members .NET 5.0 Target Older Targets
C# .NET 5.0 Consumer Available Available (*)
VB .NET 5.0 Consumer Available Available (*)
C# 9 Older Framework Consumer n/a Available (*)
C# <9 Older Framework Consumer n/a Available (*)
VB Older Framework Consumer n/a Available (*)
Public/Protected Members .NET 5.0 Target Older Targets
C# .NET 5.0 Consumer Available Available (*)
VB .NET 5.0 Consumer Available Available (*)
C# 9 Older Framework Consumer n/a Available (*)
C# <9 Older Framework Consumer n/a Not Available
VB Older Framework Consumer n/a Not Available

(*) = Must add IsExternalInit class snippet above


To boil it all down even more simply, here are my overall recommendations for when you should and should not use records and init only setters.

Project Type Public Records Internal/Private Records Public/Protected Init Only Setters Internal/Private Init Only Setters
Internal Use Yes Yes Yes Yes
NuGet No Yes No Yes
NuGet .NET 5 Target Only Yes Yes Yes Yes