Auto-default struct property values

Daily Knowledge Drop

Coming with C# 11 (being released later this year, coinciding with the .NET 7 release) the compiler will now ensure that fields on a struct will be initialized to their default value if not explicitly set.

With the current, and prior C# versions, all fields on a struct need to explicitly be set when a struct instance is initialized using a defined constructor.

C# 10 and prior

Consider a Song struct, which contains a number of properties related to a Song:

public struct Song
{
    public int Id { get; init; }

    public string Name { get; init; }

    // this property is required to have
    // a default value
    public string Artist { get; init; } = "Unavailable";

    // this property is required to have
    // a default value
    public int SongLength { get; init; } = 0;

    // As this constructor doesn't set all properties
    // those unset properties need an explicit default value
    public Song(int id, string name)
    {
        Id = id;
        Name = name;
    }

    public Song(int id, string name, string artist, int length)
    {
        Id = id;
        Name = name;
        Artist = artist;
        SongLength = length;
    }

    public override string ToString() => $"{Id}: Song with name '{Name}' " +
        $"by '{Artist}' is {SongLength} seconds long";
}

In the above, the Artist and SongLength properties need to explicitly have a default value set, as there is a constructor which does not set them.

If the default values are removed:

public string Artist { get; init; } 

public int SongLength { get; init; }

then a compiler error will occur:

Auto-implemented property 'Song.Artist' must be fully assigned before 
    control is returned to the caller. Consider updating to language 
    version '11.0' to auto-default the property.	
Auto-implemented property 'Song.SongLength' must be fully assigned 
    before control is returned to the caller. Consider updating to language 
    version '11.0' to auto-default the property.	

The error even has a suggestion on how to resolve the issue (assuming you have C# 11 preview version installed).

For completeness, here is an example of using the above struct:

var song = new Song(1, "Everlong", "Foo Fighters", 469);
Console.WriteLine(song);

var halfSong = new Song(1, "Everlong");
Console.WriteLine(halfSong);

var defaultSong = default(Song);
Console.WriteLine(defaultSong);

var blankSong = new Song();
Console.WriteLine(blankSong);

with the output:

1: Song with name 'Everlong' by 'Foo Fighters' is 469 seconds long
1: Song with name 'Everlong' by 'Unavailable' is 0 seconds long
0: Song with name '' by '' is 0 seconds long
0: Song with name '' by '' is 0 seconds long

From the output, one can see that when calling one of the defined constructors, all the properties need to be explicitly set - either in the constructor itself, or with a default value. However when using one of the other techniques to instantiate, the property values will be set to the type's default value.

C# 11

When using C# 11 (currently this means having a preview version of .NET7 installer, and updating the csproj file to contain <LangVersion>preview</LangVersion>), the struct can be updated to be as follows:

public struct Song
{
    public int Id { get; init; }

    public string Name { get; init; }

    // now these properties don't require
    // a default value
    public string Artist { get; init; }

    public int SongLength { get; init; }

    public Song(int id, string name)
    {
        Id = id;
        Name = name;
    }

    public Song(int id, string name, string artist, int length)
    {
        Id = id;
        Name = name;
        Artist = artist;
        SongLength = length;
    }

    public override string ToString() => $"{Id}: Song with name '{Name}' " +
        $"by '{Artist}' is {SongLength} seconds long";
}

Now, when instantiated, not all properties of the struct need to be explicitly set.

Executing the same sample code as above:

var song = new Song(1, "Everlong", "Foo Fighters", 469);
Console.WriteLine(song);

var halfSong = new Song(1, "Everlong");
Console.WriteLine(halfSong);

var defaultSong = default(Song);
Console.WriteLine(defaultSong);

var blankSong = new Song();
Console.WriteLine(blankSong);

with the output:

1: Song with name 'Everlong' by 'Foo Fighters' is 469 seconds long
1: Song with name 'Everlong' by '' is 0 seconds long
0: Song with name '' by '' is 0 seconds long
0: Song with name '' by '' is 0 seconds long

When calling a defined constructor which doesn't set all properties, the unset properties will be set to the type's default value.

Notes

A minor update to the language, but which will result in a more familiar, intuitive and expected experience for the experienced C# developer, which have worked with previous versions of the the runtime, and are familiar with the behavior of classes.

References

Auto-default struct

Daily Drop 160: 14-09-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.