C# 6 Preview: Parameterless struct Constructors

Update 20th October 2015: As Karl Fenech pointed out, this feature has been dropped in the final version of C# 6.0.

Up until C# 5, you couldn’t have a parameterless constructor in a struct. So if you try something like this:

    public struct Point
    {
        public int x;
        public int y;

        public Point()
        {
            this.x = 0;
            this.y = 0;
        }

        public Point(int x, int y)
        {
            this.x = x;
            this.y = y;
        }
    }

…then it’s not quite going to work:

struct-parameterless-constructors-vs2012

At the time of writing this article, using a parameterless constructor in a struct as above is now supported, but as an experimental feature. This means you need to edit your project’s .csproj file and add the Experimental language version as shown below:

  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <LangVersion>Experimental</LangVersion>
  </PropertyGroup>

Parameterless struct constructors still require you to initialise all members of the struct, just like any other struct constructor.

Additionally, parameterless struct constructors must be public:

struct-parameterless-constructors-vs14ctp4

The reason for this is explained in the C# Design Notes for Aug 27, 2014:

C#, VB and F# will all call an accessible parameterless constructor if they find one. If there is one, but it is not accessible, C# and VB will backfill default(T) instead. (F# will complain.)

It is problematic to have successful but different behavior of new S() depending on where you are in the code. To minimize this issue, we should make it so that explicit parameterless constructors have to be public. That way, if you want to replace the “default behavior” you do it everywhere.

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