.NET strings are not always immutable!

Strings are immutable. If you want to modify a sequence of characters, use StringBuilder. At least, that’s whats officially said. But in the framework there is at least one method that does modify a string:

TextRenderer.MeasureText() with ModifyString and EndEllipses will modify your string to match the ellipsed text if ellipsing happens. You can look at this VB# example on codeproject using TextRenderer.MeasureText() for trimming text on how it is used.

The string seems to be modified directly in native code by DrawTextEx from user32.dll. Additionally to the scary fact that strings are not immutable, the length of the string is not updated, regardless if the resulting string is shorter!

For instance if you have a string “aaaaaaa” which will be truncated to “aa...“, the Length property will still return 7 for the shortened string. The debugger shows that the string will in fact be “aa…\0a” after the operation. So maybe it might be right that the string is still 7 characters long but most outputting functionality like Console.Out.WriteLine() gets confused sometimes and stops any further output to the debugger or console under certain conditions.

A very quick investigation of the System.Drawing assembly using Lutz Roeder’s fabulous .NET Reflector showed that at least there should be no memory corruption in case “WW” would get ellipsed to “W...“, as DrawTextEx takes the length of the buffer and should result only in “W.“.

Summing up, I find the corruption of an immutable string by an official Microsoft API very troubling.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.