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The documentation and language varies between VS 2008 and 2010:
Internally, the text is stored as a readonly collection of Char objects, each of which represents one Unicode character encoded in UTF-16. ... The length of a string represents the number of characters regardless of whether the characters are formed from Unicode surrogate pairs or not. To access the individual Unicode code points in a string, use the StringInfo object. - http://msdn.microsoft.com/en-us/library/ms228362%28v=vs.90%29.aspx
Internally, the text is stored as a sequential read-only collection of Char objects. ... The Length property of a string represents the number of Char objects it contains, not the number of Unicode characters. To access the individual Unicode code points in a string, use the StringInfo object. - http://msdn.microsoft.com/en-us/library/ms228362%28v=VS.100%29.aspx
The language used in both cases doesn't clearly differentiate between "character", "Unicode character", "Char class", "Unicode surrogate pair", and "Unicode code point".
The language in the VS2008 documentation stating that a "string represents the number of characters regardless of whether the characters are formed from Unicode surrogate pairs or not" seems to be defining "character" as as object that may be the result of a Unicode surrogate pair, which suggests that it may represent a 4-byte sequence rather than a 2-byte sequence. It also specifically states at the beginning that a "char" object is encoded in UTF-16, which suggests that it could represent a surrogate pair (being 4 bytes instead of 2). I'm fairly certain that is wrong though.
The VS2010 documentation is a little more precise. It draws a distinction between "char" and "Unicode character", but not between "Unicode character" and "Unicode code point". If a code point refers to half a surrogate pair, and a "Unicode character" represents a full pair, then the "Char" class is named incorrectly, and does not refer to a "Unicode character" at all (which they state it does not), and it's really a Unicode code point.
So are both of the following statements true? (Yes, I think.)
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String. Length does not account for surrogate pairs, it only counts UTF-16 chars (i.e. chars are always 2 bytes) - surrogate pairs are counted as 2 chars.
String Length in C# is a property on the String object that returns the number of characters in a string. The returned length value is of type Int32. The Length property of a string is the number of Char objects it contains, not the number of Unicode characters.
The Length function returns the number of characters in an input string. For example, using the Length function on the string "MicroStrategy" would return a value of 13. Length is often used to manipulate strings with the help of other string functions.
String.Length does not account for surrogate pairs; however, the StringInfo.LengthInTextElements method does.
StringInfo.SubstringByTextElements is similar to String.Substring, but it operates on "Text Elements", such as surrogate pairs and combining characters, as well as normal characters. The functionality of both these methods are based on the StringInfo.ParseCombiningCharacters method, which extracts the starting index of each text element and stores them in a private array.
"The .NET Framework defines a text element as a unit of text that is displayed as a single character, that is, a grapheme. A text element can be a base character, a surrogate pair, or a combining character sequence." - http://msdn.microsoft.com/en-us/library/system.globalization.stringinfo.aspx
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