Bug with IndexOf in .NET 7.0 with non-spacing characters?

Question

I'm seeing a weird issue with .NET 7 with the string APIs IndexOf and Contains. I'm using C#. To repro:

("Foo" + (char)1618).Contains("Foo") returns true .... yet

("Foo" + (char)1618).IndexOf("Foo") returns -1

1618 is a non-spacing Unicode character.

However, when I add a space before the Unicode character like this:

("Foo" + " " + (char)1618).IndexOf("Foo")

It returns 0 as expected.

Any help would be much appreciated.

Dan

Answer 1

string.Contains(string):

This method performs an ordinal (case-sensitive and culture-insensitive) comparison. The search begins at the first character position of this string and continues through the last character position.

While string.IndexOf(string) is using CurrentCulture:

This method performs a word (case-sensitive and culture-sensitive) search using the current culture. The search begins at the first character position of this instance and continues until the last character position.

Provide StringComparison parameter so they are equivalent:

Console.WriteLine(("Foo" + (char)1618).Contains("Foo", StringComparison.CurrentCulture)); // False
Console.WriteLine(("Foo" + (char)1618).IndexOf("Foo")); // -1 

Or

Console.WriteLine(("Foo" + (char)1618).Contains("Foo")); // True
Console.WriteLine(("Foo" + (char)1618).IndexOf("Foo", StringComparison.Ordinal)); // 0

But the best option is to always pass the StringComparison parameter for consistency.

Also Behavior changes when comparing strings on .NET 5+ article can be useful.