Proposal: Modify a struct-type property directly, and allow intialization from existing instance

[ghost]

Suggestion:

Use special symbol (such as <=) to modify struct-type properties directly.

Example:

Suppose we have this struct:

struct Person
{
    public int ID;
    public string Name;
    public int Points;
}

and this property:

public Person User { get; set; }

We can't directly use:
User.Points += 1;
It will not compile.

So we have to do this:

Person p = User;
p.Points += 1;
User = p;

I suggest to use this syntax instead to do the same job as the previous code:

User <= Points += 1;

Another approach:

it will be easier if the Type initializer can initialize the new instance by the values of another instance:

User = User {Points += 1};

In this syntax we allow two new things:
1- use an existing instance instead of a "new Person ()".
2- use the += inside the {} because ID have a previous value and can be modified.

The last syntax can be re-written as:

User += {Points += 1};

[svick]

Is using ref-returning property not acceptable for you?

[ghost]

How can I use it with existing properties in framework?
If the problem is about programmer-defined properties and structures, one can find many other ways to solve the problems.

[HaloFour]

Value copy semantics is the distinguishing characteristic of structs. You're not updating a property of that struct, you are overwriting the entire thing. I think that glossing over this would encourage using structs in ways that are discouraged.

[svick]

How can I use it with existing properties in framework?

You can't. But what framework properties return mutable value types that you want to modify often?

[ghost]

The one that I recall, although it's not about structs is:
textBox1.Lines[0] = "abc";
This will compile, but will not affect the textbox!
acourding to my suggesion it can be re-written as:
textBox1.Lines<=[0] = "abc";
Or:
textBox1.Lines = textBox1.Lines {[0] = "abc"};

Note that this matter affects third-party codes as well: COM, NuGet .... etc.

[ghost]

Addition: Any method that returns a Point, a Size, or a Rectangle in framework face the same problem, and it is widely used. There are many similar structs like Padding and others.

[orthoxerox]

Withers (#162) could be extended to work not only with immutable types, but with struct properties as well: User = User with { .Points += 1 }

[HaloFour]

@MohammadHamdyGhanem

Indeed, that has nothing to do with structs. It will happen with any property that returns a disconnected value.

These situations should be sufficiently rare and since you'd need a separate syntax anyway I don't see why it's such a problem to explicitly have to reassign the property. Again, in the case with TextBox.Lines, you are rewriting the entire contents of the textbox based on the array, regardless of whether you only modified a single element. I think it's important for the developer to understand that.

[jnm2]

The only times I can recall having to deal with setting struct properties:

• Once in a great while obj.Size.Width = 42 which I resolve with extension methods, so obj.Size = obj.Size.WithWidth(42). I like that because it doesn't gloss over the fact (like HaloFour said) that there is no way to set just the width; set_Size must be called. I wouldn't mind a with syntax to save me the trouble of maintaining extension methods.

• Slightly more commonly I'll have builder structs in a dictionary and have to do an extra hash lookup to store the modified struct back into the dictionary. To svick's point, we now have https://github.com/dotnet/corefx/issues/25189 in 2.1 (https://github.com/dotnet/corefx/issues/20684 for ordinary Dictionary still open) which addresses that.

[ghost]

@orthoxerox

Withers (#162#162, Champion "pattern-based with expressions") could be extended to work not only with immutable types, but with struct properties as well: User = User with { .Points += 1 }

I agree. But it can be more compact like this:
User += { .Points += 1 }

[orthoxerox]

@MohammadHamdyGhanem Or User with= { ... }?

[ghost]

With= does not imply that the object should keep its old values.

[jnm2]

@MohammadHamdyGhanem Yes, it does. If the word with appears, it means all other previous values are preserved.

[AustinBryan]

Why not use <- or something else instead of <= which already means something and could cause conflicts.

Besides that, just use a class..