someString.IndexOf(someString) returns 1 instead of 0 under .NET 4

Question

We have recently upgraded all our projects from .NET 3.5 to .NET 4. I have come across a rather strange issue with respect to string.IndexOf().

My code

Answer 1

It seems be a bug in .NET4, and new changes reverted in .NET4 Beta1 to previous version same as .NET 2.0/3.0/3.5.

What's New in the BCL in .NET 4.0 CTP (MSDN blogs):

String Security Changes in .NET 4

The default partial matching overloads on System.String (StartsWith, EndsWith, IndexOf, and LastIndexOf) have been changed to be culture-agnostic (ordinal) by default.

This change affected the behavior of the String.IndexOf method by changing them to perform an ordinal (byte-for-byte) comparison by default an will be changed to use CultureInfo.InvariantCulture instead of CultureInfo.CurrentCulture.

UPDATE for .NET 4 Beta 1

In order to maintain high compatibility between .NET 4 and previous releases, we have decided to revert this change. The behavior of String's default partial matching overloads and String and Char's ToUpper and ToLower methods now behave the same as they did in .NET 2.0/3.0/3.5. The change back to the original behavior is present in .NET 4 Beta 1.

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To fix this, change the string comparison method to an overload that accepts the System.StringComparison enumeration as a parameter, and specify either Ordinal or OrdinalIgnoreCase.

// string contains 'unicode dash' \x2D
string text = "\xAD\x2D"; 

// woks in .NET 2.0/3.0/3.5 and .NET 4 Beta 1 and later
// but seems be buggy in .NET 4 because of 'culture-sensitive' comparison        
int index = text.IndexOf(text); 

// fixed version
index = text.IndexOf(text, StringComparison.Ordinal);