string.LastIndexOf() Bug?

Question

does anyone know why:

"  <exception>".LastIndexOf("<",0) returns -1 (wrong)

while

"  <exception>".LastIndexOf("<") returns 2 (right)

and

"<exception>".LastIndexOf("<",0) returns 0 (right)

Is this a bug or am I misunderstanding the LastIndexOf-Method?

Answer 1

You are misunderstanding that particular overload of the LastIndexOf method.

The docs state the following:

The search starts at a specified character position and proceeds backward toward the beginning of the string.

Note that it says backward. So, if you start at position 0, there is no "<" substring at that position or in front of that position and hence the result is -1.

In contrast, if you use the overload that takes only the substring, the search will start at the end of the string and hence correctly find the indicated substring.

Answer 2

The string, int32 overload of LastIndexOf says, in the description, "Reports the zero-based index position of the last occurrence of a specified Unicode character within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string."

Thus if you pass in 0, it will only check the first character, not check the whole string from 0.

Answer 3

The second parameter does not do what you seem to think it does:

LastIndex(char value, int startIndex)

the startIndex is the char to start searching backwards through the string, so if you pass a 0 then only the first char is checked...

To check the whole string from the end you would have to pass the length of the string -1.

see MSDN String.LastIndex

Answer 4

The docs (http://msdn.microsoft.com/en-us/library/bc3z4t9d.aspx#Y0) say:

The search begins at the startIndex character position of this instance and proceeds backward toward the beginning until either value is found or the first character position has been examined. For example, if startIndex is Length - 1, the method searches every character from the last character in the string to the beginning.

(My emphasis)

So this:

"  <exception>".LastIndexOf("<",0)

is beginning at 0 and working backwards, and therefore correctly finding no result and returning -1.

I think the confusion is that LastIndexOf counts backwards, where IndexOf counts forwards.