String.IndexOf() and NewLine '\n'

[robloo]

Really basic yet interesting question for me.

Why does the following code return -1 when running on EN-US culture? I would expect it to find the newline character and return index 1.

"\r\n".IndexOf("\n");

Of course this is bad practice and we should be specifying StringComparison.Ordinal or StringComparison.OrdinalIgnoreCase. However, I genuinely don't understand why this doesn't work without specifying the StringComparison.

Note that the following work just fine:

"\r\n".IndexOf("\r\n");
"\r\n".IndexOf(Environment.NewLine);

[stephentoub]

See #43736

[robloo]

Thanks, that explains the issue and why it wasn't noticed earlier.

This was found after moving some tests to .NET 5. I'm glad the main codebase hasn't been ported yet. I wouldn't expect such a breaking change to occur especially one that doesn't seem to make much sense.