Why does this code have different results on different OS

[crystal-lz]

string strText = "1123\r\ntest\tabc";
Console.WriteLine($"TEXT[{strText}]\r\n");
Console.WriteLine($"IndexOf [\\r] {strText.IndexOf("\r")}");
Console.WriteLine($"IndexOf [\\n] {strText.IndexOf("\n")}");
Console.WriteLine($"IndexOf [\\t] {strText.IndexOf("\t")}");

//==output=====[mac or ubuntu]=======

TEXT:[1123
test    abc]

IndexOf [\r] 4
IndexOf [\n] -1
IndexOf [\t] 10
//---------------------------------------
-1  ?????????? 

[carlossanlop]

@GrabYourPitchforks I think you may have some context on this question. Can you please help?

[GrabYourPitchforks]

Related to dotnet/runtime#43736.
(Edit: this isn't a dupe, but the issue has the same explanation. I've summarized it below.)

In a nutshell, the overload of string.IndexOf that's being called is performing a culture-sensitive check, when an ordinal check is intended.

There are two suggested workarounds:

1. If you're searching for a single char (e.g., '\n'), change the code from strText.IndexOf("\n") to strText.IndexOf('\n') instead. Searching for single chars will be ordinal.
2. If you're searching for a literal substring, change the code from strText.IndexOf("text") to strText.IndexOf("text", StringComparison.Ordinal) instead.

The code analyzer rules CA1307 and CA1309 can assist with this. See dotnet/docs#21249, section "Option 1: Enable code analyzers to help detect possibly-buggy call sites" for more information.