Proposal: Allow optional property initialization with fallback to the default value

[nrhoffmann]

Proposal

Can we get this syntax [Property Name] ?= [Some Value], when using the object initializer, to only override a property's default value if [Some Value] is not null or default?

Example / Explanation

Currently, you can do this...

public class Connection
{
  public string Name { get; set; }
  public string Server { get; set; } = "127.0.0.1";
  public int Port { get; set; } = 80;
}

This is really nice, because it allows you to initialize a property with a default value. For example...

 var localhost = new Connection
 {
   Name = "localhost"   
 };

connections[localhost.Name] = localhost;

Suppose, that while refactoring my code I notice that I frequently make connections and add them to some dictionary that maps their name to themselves. To clean things up a bit I might do the following...

public Connection MapNewConnection(string name, string server = default, int port = default)
{
  var connection = new Connection
  {
    Name = name,
    Server = server,
    Port = port 
  };
  connections[connection.Name] = connection;
  return connection;
}

However, now I've lost the defaults set in Connection...

Console.WriteLine(MapNewConnection("localhost"));
// Connection { Name = localhost, Server = null, Port = 0 }
Console.WriteLine(MapNewConnection("google", "172.217.10.46"));
// Connection { Name = localhost, Server = "172.217.10.46", Port = 0 }

I am proposing the using the following syntax to solve this issue...

public Connection MapNewConnection(string name, string server = default, int port = default)
{
  var connection = new Connection
  {
    Name = name,
    Server ?= server,
    Port ?= port 
  };
  connections[connection.Name] = connection;
  return connection;
}

[vladd]

Why not just use Server = server ?? default?

[jnm2]

I like ??= because it follows the existing patterns we have for converting an operator to an assignment shorthand operator, but ?? has always been about null and not default. Both of these things have been covered in previous issues #34 and #328.

[nrhoffmann]

@jnm2
Are they planning on making #34 work in object initializers?

What I mean is...
According to #34 x ??= y would be syntactic sugar for x = (y ?? x). However, in the above example Server = (server ?? Server) doesn’t compile.

My understanding was that [Property Name] = [Some Value] in an object initializer wasn't exactly a regular assignment.

Additionally, I would prefer default because properties are by default initialized to default. Thus, when a programmer writes public string Server { get; set; } = "127.0.0.1", he is conceptually overriding the default.

So I see it as "initialize the property to this value unless this value is the default value, because the programmer doesn't want that property to be initialized with that default value."

[YairHalberstadt]

??= has now shipped, but is not supported in object initializers. If this still interests you, I would suggest opening a new issue allowing it in object initializers.