Getting CS0843 (backing field must be assigned) when calling constructor of struct

Question

I am trying to replicate .NET's Color struct , which lets you call colors using Color.Black , Color.White etc. Here's my code:

    struct Material
    {
        public string FilePath { get; set; }

        public Material(string filepath) { FilePath = filepath; }

        public static Material Sand
        {
            get
            {
                return new Material("images/sand");
            }
        }

        public static Material Conrete
        {
            get
            {
                return new Material("images/conrete");
            }
        }
    }

I'm getting an error saying I cannot use a constructor in a struct . I am effectively copying from the .NET source code (Color.cs) and this is how it does it, although it doesn't use a constructor. The static properties do return a new Material() however.

Full error message appearing on constructor CS0843 :

backing field for automatically implemented property must be fully assigned before it is returned to the caller

Answer 1

You can simply "chain" this() , as in:

public Material(string filepath)
  : this()
{
  FilePath = filepath;
}

and this is the most common solution.

Of course you can do the same in other ways:

public Material(string filepath)
{
  this = default(Material);
  FilePath = filepath;
}

and

public Material(string filepath)
{
  this = new Material { FilePath = filepath, };
}

and so on.

Answer 2

You CAN have a constructor, just not one with zero arguments.

However, isn't it clearer to write

new Material { FilePath = "images/concrete" }

instead of

new Material("images/concrete");

A custom constructor can always be bypassed by invoking the framework-provided parameterless constructor, so it can't be relied on for setting up invariants. And fields and properties can be set using member-initialization syntax. So struct constructors aren't terribly useful.