Using Records in legacy .NET Frameworks

Recently we’ve been seeing an increased activities on various blogs due to upcoming .NET 5 release date and one of it’s hottest feature - C# 9.0 records. A lot was written about this feature, starting with MSDN.

What was not clear however, was whether one can use records in older frameworks - like .NET 4.8.

<PropertyGroup>
    <LangVersion>9.0</LangVersion>
</PropertyGroup>

Anatomy of record positional properties

Let’s settle our attention on the following example

record Vertebrate(string Name)
{
    public Vertebrate() : this("") { }
}

public enum Habitat { Terrestrial, Aquatic, Amphibian }

public record Reptile(Habitat Habitat) : Vertebrate { }

Upon compilation under net5.0 framework moniker, everything works as expected. Change it to however to net48 and you will not be able to compile it. This compiler feature fortunatelly works like opt-in member resolution (similarly to string interpolation). What compiler needs in this case is an accessible class of the following structure so that init-only properties are accessible

//TODO: use appropriate compiler directives for legacy targets - it's not needed in net5.0+
#if NETSTANDARD2_0
namespace System.Runtime.CompilerServices
{
    [System.ComponentModel.EditorBrowsable(System.ComponentModel.EditorBrowsableState.Never)]
    internal static class IsExternalInit { }
}
#endif

Check if property is init-only

After quick research one can spot that init-only setter has special structure:

.set instance void modreq([System.Runtime]System.Runtime.CompilerServices.IsExternalInit)
  DotnetCommunityDemoNet5.Records/Vertebrate::set_Name(string)

To determine that setter is init-only one just needs to query the existence of required modifier initialized with aforementioned IsExternalInit type - this code helper should do the trick:

public static class RecordsHelper
{
    public static bool IsInitOnly(this PropertyInfo property) =>
        property.CanWrite && property.SetMethod is var setter
            && setter.ReturnParameter.GetRequiredCustomModifiers() is var reqMods 
            && reqMods.Length > 0
            && Array.IndexOf(reqMods, typeof(System.Runtime.CompilerServices.IsExternalInit)) > -1;
}

Consuming records in older frameworks

Due to this phenomenon, consuming records is quite straightforward - you are using them as normal classes. So even older C# versions will be able to use it straight away.

If you’d like to use with keyword then you need to use C# 9.0+ for instance by specifying that in *.csproj file of your target project:

<PropertyGroup>
    <LangVersion>9.0</LangVersion>
</PropertyGroup>

and you’ll be able to use all record features upon consumption:

Sources

• Records producer/consumer