Question

Recently a compiler warning and (very useful) hint prompted me to write the code below.

I had no idea you could do this, but it is perfectly legal, and also convenient in that I can declare a managed struct with public properties similar to public fields of an unmanaged struct, and also initialize it with an object instead of having to pass all the fields as parameters.

What confuses me is that this appears to call the explicit parameterless constructor, which would of course be illegal for this struct.

What’s going on here, and has this syntax always been supported?

internal struct IconEntry
{
    public byte Width { get; set; }
    public byte Height { get; set; }
    public byte ColorCount { get; set; }
    public byte Reserved { get; set; }
    public short Planes { get; set; }
    public short BitCount { get; set; }
    public int BytesInRes { get; set; }
    public int ImageOffset { get; set; }

    public IconEntry(BinaryReader reader)
        : this()
    {
        Width = reader.ReadByte();
        Height = reader.ReadByte();
        ColorCount = reader.ReadByte();
        Reserved = reader.ReadByte();
        Planes = reader.ReadInt16();
        BitCount = reader.ReadInt16();
        BytesInRes = reader.ReadInt32();
        ImageOffset = reader.ReadInt32();
    }
}

Answer 1

A struct always has a public parameterless constructor which can’t be overriden: http://msdn.microsoft.com/en-us/library/aa288208%28v=vs.71%29.aspx

This means that a user still would be able to create an instance of this struct that is not initialized according to your logic but with default values for all properties:

var s = new IconEntry();