String Class

An index is the position of a Char object, not a Unicode character, in a String. An index is a zero-based, nonnegative number starting from the first position in the string, which is index position zero. Consecutive index values might not correspond to consecutive Unicode characters because a Unicode character might be encoded as more than one Char object. To work with each Unicode character instead of each Char object, use the System.Globalization.StringInfo class.

Members of the String class perform either an ordinal or linguistic operation on a String object. An ordinal operation acts on the numeric value of each Char object. A linguistic operation acts on the value of the String taking into account culture-specific casing, sorting, formatting, and parsing rules. Linguistic operations execute in the context of an explicitly declared culture or the implicit current culture. For more information about the current culture, see the CultureInfo.CurrentCulture topic.

Casing rules determine how to change a Unicode character between one case and another; for example, from lowercase to uppercase.

Formatting rules determine how to convert a value to its string representation, while parsing rules determine how to convert a string representation to a value.

Sort rules determine the alphabetic order of Unicode characters and how two strings compare to each other. For example, the Compare method performs a linguistic comparison while the CompareOrdinal method performs an ordinal comparison. Consequently, if the current culture is U.S. English, the Compare method considers 'a' less than 'A' while the CompareOrdinal method considers 'a' greater than 'A'.

The .NET Framework supports word, string, and ordinal sort rules. A word sort performs a culture-sensitive comparison of strings in which certain nonalphanumeric Unicode characters might have special weights assigned to them. For example, the hyphen ("-") might have a very small weight assigned to it so that "coop" and "co-op" appear next to each other in a sorted list. A string sort is similar to a word sort, except that there are no special cases and all nonalphanumeric symbols come before all alphanumeric Unicode characters.

A culture-sensitive comparison is any comparison that explicitly or implicitly uses a CultureInfo object, including the invariant culture that is specified by the CultureInfo.InvariantCulture property. The current implicit culture is specified by the Thread.CurrentCulture property.

An ordinal sort compares strings based on the numeric value of each Char object in the string. An ordinal comparison is automatically case-sensitive because the lowercase and uppercase versions of a character have different code points. However, if case is not important in your application, you can specify an ordinal comparison that ignores case. This is equivalent to converting the string to uppercase using the invariant culture and then performing an ordinal comparison on the result.

For more information about word, string, and ordinal sort rules, see the System.Globalization.CompareOptions topic.

A culture-sensitive comparison is typically appropriate for sorting, whereas an ordinal comparison is not. An ordinal comparison is typically appropriate for determining whether two strings are equal (that is, for determining identity), whereas a culture-sensitive comparison is not.

The Remarks for comparison and search methods specify whether the method is case-sensitive and/or culture-sensitive. By definition, any string, including the empty string (""), compares as greater than a null reference, and two null references compare as equal to each other.

If your application makes a security decision about a symbolic identifier such as a file name or named pipe, or persisted data such as the text-based data in an XML file, the operation should use an ordinal comparison instead of a culture-sensitive comparison. This is because a culture-sensitive comparison can yield different results depending on the culture in effect, whereas an ordinal comparison depends solely on the binary value of the compared characters.

The String class provides members to compare String objects, return the index of a character or string within a String object, copy the value of a String object, partition a string or combine strings, modify the value of string, format numbers, dates and times, or enumeration values into a string, and normalize a string.

• Use the Compare, CompareOrdinal, CompareTo, Equals, EndsWith, and StartsWith methods for comparisons.

• Use the IndexOf, IndexOfAny, LastIndexOf, and LastIndexOfAny methods to obtain the index of a substring or Unicode character in a string.

• Use Copy and CopyTo to copy a string or substring to another string or an array of type Char.

• Use the Substring and Split methods to create one or more new strings from portions of an original string, and the Concat and Join methods to create a new string from one or more substrings.

• Use Insert, Replace, Remove, PadLeft, PadRight, Trim, TrimEnd, and TrimStart to modify all or part of a string.

• Use the ToLower and ToUpper methods to change the case of Unicode characters in a string.

• Use Format to replace one or more format item placeholders in a string with the string representation of one or more numeric, date and time, or enumeration values.

• Use the Length property to obtain the number of Char objects in a string, and the Chars property to access the actual Char objects in a string.

In the .NET Framework, a String object can include embedded nulls, which count as a part of the string's length. However, in some languages, such as C and C++, a null character indicates the end of a string, is not considered a part of the string, and is not counted as part of the string's length. This means that the following common assumptions that C and C++ programmers or that libraries written in C or C++ might make about strings are not necessarily valid when applied to String objects:

• The value returned by the strlen or wcslen functions does not necessarily equal String.Length.

• The string created by the strcpy_s or wcscpy_s functions is not necessarily identical to the string created by the String.Copy method.

You should ensure that native C and C++ code that instantiates String objects do not assume that an embedded null marks the end of the string.

The String class implements the IComparable, IConvertible, IComparable<T>, IEnumerable, IEnumerable<T>, and IEquatable<T> interfaces. Use the Convert class for conversions instead of this type's explicit interface member implementation of IConvertible.