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hugo/content/posts/2021-01-18-find-the-bug.md

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+---
+lastmod: "2021-06-05T13:10:16.0000000+01:00"
+author: patrick
+categories:
+- programming
+comments: true
+date: "2021-01-18T00:00:00Z"
+title: Find the Bug, C# edition
+summary: "A cute little exercise in bug-spotting."
+---
+
+Thanks to [Joe Roberts](https://github.com/Joey9801/) for showing me the code which I've rendered below as an exercise in bug-spotting.
+
+The following C# defines a class with a private nullable int field `_data`, initialised to `null`, and a public int property `Data`, which assumes the value of `_data` if `_data` has a value, or sets `_data` to `3` and returns `3` otherwise.
+
+```csharp
+class Foo {
+  private int? _data = null;
+  public int Data => _data ??= 3;
+}
+
+Foo f;
+// more code here...
+Console.WriteLine(f.Data);
+```
+
+It is very likely that this code contains a bug. What is the probable bug?
+
+# Hint, which you will certainly need
+
+C# compiles down to Microsoft Intermediate Language ("IL") bytecode.
+The example above compiles to essentially the following:
+
+```
+int32 local0;
+local0 = this._data.GetValueOrDefault();
+if (this._data.HasValue)
+    goto label1;
+local0 = 3;
+this._data = new Nullable<int32>(local0);
+stack.push(local0);
+goto label2;
+
+label1:
+    stack.push(local0);
+label2:
+    return stack.pop();
+```
+
+# Hint 2
+In [ROT13](https://rot13.com/):
+
+Gurer'f n enpr pbaqvgvba - jung unccraf va n zhygv-guernqrq pbagrkg?
+
+# Answer
+In ROT13:
+
+Rkrphgvba ortvaf, naq gur ybpny inevnoyr vf nffvtarq gur qrsnhyg inyhr mreb. Gur guernq vf fhfcraqrq. N arj guernq nfxf sbe gur inyhr bs Qngn, naq pbeerpgyl trgf gur inyhr guerr. Rkrphgvba bs gur bevtvany guernq erfhzrf; qngn abj unf n inyhr, fb jr whzc gb gur svefg ynory, naq erghea gur qrsnhyg inyhr bs na vag, anzryl mreb.