ASP.Net application hangs due to string.IndexOf

I had an interesting case recently. We discovered a hang in ASP.Net application caused by string.IndexOf method.

Problem description:

 Frequently, this ASP.Net application would hang and the IIS worker process needs to be restarted for the application to function correctly. This happens even when the load is low.


Same as before, we captured hang dumps for investigation. From the dump, we could see that almost requests were blocked by GC.  Here is a sample calling stack.







Some customer code here

System_Web_ni!System.Web.Hosting.ISAPIRuntime.ProcessRequest(IntPtr, Int32)


However, if we look at the GC threads, they were not in GC status at all. Then, we found that thread 48 disabled PreEmptive GC, not allowing GC to suspend it. The GC had to wait this thread to finish.

                                              PreEmptive                                                Lock

       ID OSID        ThreadOBJ     State   GC     GC Alloc Context                  Domain           Count APT Exception

  24    1 1e48 000000000015f280   1808220 Enabled  00000000902c9e50:00000000902c9e70 00000000001bbbe0     1 MTA (Threadpool Worker)


  48    8 2070 00000000091043e0   180b222 Disabled 0000000000000000:0000000000000000 00000000001bbbe0     1 MTA (Threadpool Worker)



Thread 48 was replacing strings.





And thread 48 ate most of the CPU.

0:048> !runaway

User Mode Time

  Thread       Time

  48:2070      0 days 0:01:02.468

  52:20b8      0 days 0:00:16.359

  34:1ef0      0 days 0:00:16.171



If we dump out the string, it is really small string which contains ~200 chars only. Replacing something inside this string should not take such long time. The code running looks like below, it tries to replace all continuous spaces with single space. Actually, there were no continuous spaces in the string already. In addition, we don’t need the while loop here cause String.Replace returns a new string in which all occurrences of a specified string in the current instance are replaced with another specified string. At this moment, I suspect that the while loop was not able to terminate as expected.

    if ((str != null) && (str.Length != 0))


        while (str.IndexOf("  ") != -1)  // as long as there are two continuous spaces


            str = str.Replace("  ", " "); //replace it with single space



Look at the hex value of this string, I found something abnormal. Inside the string, there is one special char 0x fffd.

0:048> dd 801fc6d8

00000000`801fc6d8  0041004d 00200058 0020fffd 00680054


Then, I was able to reproduce this problem with follow code. IndexOf always returns 0x0 instead of the expected -1 for this special string.

            char[] test= {'\u0020', '\ufffd', '\u0020'};

            string testStr = new string(test);

            int i = testStr.IndexOf("  ");


0xFFFD is a special char for Unicode. U+FFFD(“ ​”) is replacement character used to replace an unknown or unprintable character. For security purpose, invalid bytes inside a Unicode text are replaced with U+FFFD, the Unicode replacement character. For example, the string “Ad\xD800min\xDC00istrator” would change to “Ad\xFFFDmin\xFFFDistrator” This string is decoded to "Administrator," where the is the Unicode replacement character 0xFFFD.

The reason is  “String.Replace” performs an ordinal (culture-insensitive) search, where a character is considered equivalent to another character only if their Unicode scalar values are the same. However, “IndexOf(string)” method performs a word (case-sensitive and culture-sensitive) search using the current culture. To resolve the endless loop, we should use string.IndexOf("AA", StringComparison.Ordinal);   

        static void Main(string[] args)


            char[] test = { '\u0020', '\ufffd', '\u0020' };  // A string ""

            string testStr = new string(test);

            int i = testStr.IndexOf("  ", StringComparison.Ordinal);    //This returns 0x0 which means “  ”(two spaces) was found at position 0x0

            while (i != -1)


                    testStr = testStr.Replace("  ", " ");  //replace two continuous spaces with single space.

                    i = testStr.IndexOf("  ", StringComparison.Ordinal);




For more information, please see the “Remarks” section of these two methods.

String::IndexOf Method (String)


String.Replace Method (String, String)


See you next time.

Wei Zhao

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