From 52dec9b278867239dad2e14bcd1e9847169c6888 Mon Sep 17 00:00:00 2001
From: Kiril Markov <krivil@gmail.com>
Date: Fri, 30 Jul 2021 17:17:45 +0300
Subject: [PATCH] C# records immutability

---
 README.md                    |  5 ++-
 RecordImmutability/README.md | 68 ++++++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+), 1 deletion(-)
 create mode 100644 RecordImmutability/README.md

diff --git a/README.md b/README.md
index 1dda9bb..f2e4a93 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,5 @@
-# pages
+# Some thoughts on programming
 
+## [C# records and immutability](./RecordImmutability/README.md)
+
+Possibly unwanted side effects with records.
diff --git a/RecordImmutability/README.md b/RecordImmutability/README.md
new file mode 100644
index 0000000..28a3735
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+++ b/RecordImmutability/README.md
@@ -0,0 +1,68 @@
+# Shallow copy
+
+When C# records were intrduced, thy were created with immutability in mind.
+This introduced `with` keyword.
+
+``` cs
+    var topLeft = new Point(0, 0);
+    var adjacent = topLeft with { X = 1 };
+
+    public record Point(int X, int Y);
+```
+
+Even though from developer's point of view `Point` is immutable, that is not the case for compliler. Here is excerpt from code:
+
+``` cs
+    Point point = new Point(0, 0);
+    Point point2 = point.<Clone>$();
+    point2.X = 1;
+    Point point3 = point2;
+```
+
+`<Clone>$()` is automatically generated method of `Point`.
+
+
+``` cs
+    public virtual Point <Clone>$()
+    {
+        return new Point(this);
+    }
+
+    protected Point(Point original)
+    {
+        <X>k__BackingField = original.<X>k__BackingField;
+        <Y>k__BackingField = original.<Y>k__BackingField;
+    }
+```
+
+You can see full decompiled code [here](https://sharplab.io/#v2:CYLg1APgbghgTgAgC4HsAOAZApgMyQgXgQDssB3BABRQEtikAKABgBoEmBKAbgFgAoWIhjAAVjADGWeoWTpseBGRpIAFggDeCABoyAjAgC+vPvwACAZgRws4lHGBVa9BnXxa2rhAE1uQA===).
+
+While this code looks like it can be optimized further (overwriting X and entire existence of point2 can be completely skipped if only contructor and decomposition is used), it is necessary in case `Point` is being inherited. Generational garbage collector of .net is quite efficient with such short-lived objects.
+
+This approach has an obvious disadvantage in terms of immutability since `<Clone>$()` is making shallow copy of the object. Having an example `Path` that consists of series of `Point`s will illustrate this:
+
+``` cs
+    public record Path(Point[] Points);
+```
+
+Here `Points` can be shared between "immutable" instances of `Path`. Modifying n<sup>th</sup> element of the array will affect both instances.
+
+# Enter immutability
+
+``` cs
+    using System.Collections.Immutable;
+
+    var topLeft = new Point(0, 0);
+    var adjacent = topLeft with { X = 1 };
+
+    var path = new Path(ImmutableArray.Create(topLeft, adjacent));
+    var closedPath = path with { Points = path.Points.Add(topLeft)};
+
+    public record Point(int X, int Y);
+    public record Path(ImmutableArray<Point> Points);
+```
+
+You can see full decompiled code [here](https://sharplab.io/#v2:CYLg1APgAgDABFAjAOgMIHsA2mCmBjAFwEt0A7AZ2QEkBbGgVwIEMAjXAbgFgAoHgNyYAnOAXQAHADI4AZgTgBeOKRwB3OAAV0RUgQAUMADRwYASi7cBwpsABWTPDh0KR4qbLgqiBABZwA3nAAGs6IcAC+5vxCcGJMPs7Kaupx3rq0DMxsOACCgoJMAJ5ogjhxOLqikjIERtZ2DjomZlHCeJjo5DjAyfGKsfGe8QGa2gTkzv3eyCM6lNnAwBWu1SYRPDxQAMxwJXjogsAaWjq6o0FGZwCazdxbO/j7hz2p6YysuLn5BQA8MwQAfEdRuQzEA=).
+
+
+`System.Collections.Immutable` solves all that and even with shallow copy, modifications will not be shared.
\ No newline at end of file