Am I misunderstanding this method?

Elements C#
1

I have this C# code snippet:

ulong codePoint = Convert.TryHexStringToUInt64(hexString);
if (codePoint == null) {
  // Invalid code point.
  Error("Invalid Unicode codepoint `" + hexString + "`.");
} else {
  lex += codePoint.ToUTF16String();
}

The compiler is warning me that if (codePoint == null) { will always be False. Why is this? I thought that if I passed TryHexStringToUInt64() an invalid codepoint it would return null?

2

because code point is defined as a ulong which, is a value type and =thus never nil.

now, yes, Convert.TryHexStringToUInt64 returns a nullable ulong (aka UInt64), true, but hardcoding a type in the bvafriable declaration will override type inference. iff you were to write

ulong? codePoint = Convert.TryHexStringToUInt64(hexString);

or (better)

var codePoint = Convert.TryHexStringToUInt64(hexString);

then this code would work as you expect. but as written, you “force” the result into a non-nullable UInt64, and as a consequence, the following if statement will always be false.

3

hmm.

Changing ulong codePoint = Convert.TryHexStringToUInt64(hexString); to ulong? codePoint = Convert.TryHexStringToUInt64(hexString); fixes the warning but breaks the line lex += codePoint.ToUTF16String() saying there is “no such member” for ToUTF16String().

Changing ulong codePoint = Convert.TryHexStringToUInt64(hexString); to var codePoint = Convert.TryHexStringToUInt64(hexString); fixes the warning but agin breaks the line lex += codePoint.ToUTF16String() saying there is “no such member” for ToUTF16String().

If I use var and try to cast code point to a ulong (since I know it will be) then I still get the “no such member” error for codePoint.ToUTF16String ():

var codePoint = Convert.TryHexStringToUInt64(hexString);
if (codePoint == null) {
  // Invalid code point.
  Error("Invalid Unicode codepoint `" + hexString + "`.");
} else {
  lex += (ulong)codePoint.ToUTF16String(); // <-- no such member
}
4

Hmm. what platform is this? unfortunately there’s some inconsistency with how nullables auto-unwrap (or don’t) in C#, for compatibility with VC# — but that should only affect .NET.

two options, either you need to call .value (eg codePoint.value.ToUTF16String, or change your if clause around like this:

if (codePoint != null) {
  lex += (ulong)codePoint.ToUTF16String(); // inside the IF, its unwrapped.
} else {
  // Invalid code point.
  Error("Invalid Unicode codepoint `" + hexString + "`.");
}

This has been an area of frustration for me too, and I’m going back and forth within he compiler team on addressing this, for consistency over VC# compatibility…

5

Yep that fixes it @mh, thanks. This is coding on macOS with Fire, testing on Island (a console application).

Does that imply that unwrapping doesn’t happen in the else clause? Seems odd.

6

Curious,. maybe Island behaves same as .NET then. I’ll get this reviewed/reconsidered again, as promised.

it’s a special case that after “if x != null” and "if assigned(x)', a nullable x will be considered known non-null. Think of it was the equivalent if if let x = in Swift.

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