Why does string.StartsWith("\u2D2D") always return true?

Question

I was fiddling around with parsing in C# and found that for every string I tried, string.StartsWith("\u2D2D") will return true. Why is that?

It seems it works with every char. Tried this code with .Net 4.5 the Debugger did not break.

for (char i = char.MinValue; i < char.MaxValue; i++)
{
    if(!i.ToString().StartsWith("\u2d2d"))
    {
        Debugger.Break();
    }
}

Kine

here's a line of code to check a bunch. returns false for me: Enumerable.Range(0, 10000).Any(x => !((char)x).ToString().StartsWith("\u2D2D")) – Blintz

Weird. Probably related. – Conventicle

It must be because all strings are Georgian :D – Bubble

@CodeCaster: ideone.com/0QffbT maybe related? – Sweepstake

@Sweepstake probably, but U+2D2D is supposed to have a length of 1. – Conventicle

True. Ordinal culture seems to give false, so I'm looking into that now. – Sweepstake

Same for 2D2A and the like. – Splashy

Might be useful to look at the reference source for String.StartsWith here: referencesource.microsoft.com/#mscorlib/system/… – Disembody

U+2D2D was added in Unicode version 6.1 and it seems all 4 length characters of 6.1 have the same issue. From a few quick tests this seems the case for all versions > 6.0 – Frowst

Following the framework source, the comparison for non empty strings comes down to this line in CompareInfo.cs: referencesource.microsoft.com/#mscorlib/system/globalization/…, and its all native from there on. I don't understand the win32 API sufficiently to be able to debug what is going on with the windows native code. – Disembody

A lot of the UnicodeCategory.OtherNotAssigned content matches this strange case. Can be checked with CultureInfo.CurrentCulture.CompareInfo.Compare("", "" + (char)c, CompareOptions.None) == 0, of which CurrentCulture = "en-US" – Sweepstake

Answer 1

I think I'll have a try.

From what I get, is that U+2D2D was added in Unicode v6.1 (source / source).

The .NET framework, or the native calls rather, support a lower version:

The culture-sensitive sorting and casing rules used in string comparison depend on the version of the .NET Framework. In the .NET Framework 4.5 running on the Windows 8 operating system, sorting, casing, normalization, and Unicode character information conforms to the Unicode 6.0 standard. On other operating systems, it conforms to the Unicode 5.0 standard. (source)

Thus it is required to mark it as an ignorable character, which behaves just as if the character wasn't even there.

Character sets include ignorable characters, which are characters that are not considered when performing a linguistic or culture-sensitive comparison. (source)

Example:

var culture = new CultureInfo("en-US");
int result = culture.CompareInfo.Compare("", "\u2D2D", CompareOptions.None);
Assert.AreEqual(0, result);

string.StartsWith uses a similar implementation, but uses CompareInfo.IsPrefix(string, string, CompareOptions) instead.

Sweepstake

This makes more sense than the other answer, +1. – Iyar

I just tested .StartsWith() with a blank string value, and it indicates true. I.e. "abc".StartsWith("") is true. Which makes sense, as it's basically saying that the underlying string starts with at least a blank string. This would indicate that when the underlying Kernel32.dll method FindNLSStringEx sees values it deems invalid, it simply strips them. (Or some other, lower-level method does.) The result is string.StartsWith("") when run. – Iyar

@stakx: Thanks, but I don't mind if I don't. I was just wondering why this would happen in the first place, and maybe it would help me next time I face something like this myself. :) – Sweepstake

@EBrown: In the case of an empty prefix (""), FindNLSStringEx doesn't actually get called. The special case of 0-length prefixes is taken care of on the .NET side. That aside, according to this answer this it is the logical reason why anyString.StartsWith("\u2D2D") yields true. – Sheugh

I just tested .StartsWith() with a blank string value, and it indicates true. Which is the defined (and logical) result. It does so if the string length == 0 – Shrift