Differences in string compare methods in C

Question

Comparing string in C  is pretty simple   In fact there are several ways to do it   I have listed some in the block below   What I am curious about are the differences between them and when one should be used over the others   Should one be avoided at all costs   Are there more I haven t listed     string testString    Test   string anotherString    Another    if  testString CompareTo anotherString     0     if  testString Equals anotherString      if  testString    anotherString        Note  I am looking for equality in this example  not less than or greater than but feel free to comment on that as well

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s1 CompareTo s2    Do NOT use if primary purpose is to determine whether two strings are equivalent s1    s2   Cannot ignore case s1 Equals s2  StringComparison    Throws NullReferenceException if s1 is null String Equals s2  StringComparison    By process of eliminiation  this static method is the WINNER  assuming a typical use case to determine whether two strings are equivalent

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Using  Equals is also a lot easier to read

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with  Equals  you also gain the StringComparison options  very handy for ignoring case and other things   btw  this will evaluate to false  string a    myString   string b    myString    return a  b   Since    compares the values of a and b  which are pointers  this will only evaluate to true if the pointers point to the same object in memory   Equals dereferences the pointers and compares the values stored at the pointers   a Equals b  would be true here   and if you change b to   b    MYSTRING     then a Equals b  is false  but  a Equals b  StringComparison OrdinalIgnoreCase     would be true  a CompareTo b  calls the string s CompareTo function which compares the values at the pointers and returns  lt 0 if the value stored at a is less than the value stored at b  returns 0 if a Equals b  is true  and  0 otherwise  However  this is case sensitive  I think there are possibly options for CompareTo to ignore case and such  but don t have time to look now   As others have already stated  this would be done for sorting  Comparing for equality in this manner would result in unecessary overhead   I m sure I m leaving stuff out  but I think this should be enough info to start experimenting if you need more details

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If you are ever curious about differences in BCL methods  Reflector is your friend  -   I follow these guidelines   Exact match  EDIT  I previously always used    operator on the principle that inside Equals string  string  the object    operator is used to compare the object references but it seems strA Equals strB  is still 1-11  faster overall than string Equals strA  strB   strA    strB  and string CompareOrdinal strA  strB    I loop tested with a StopWatch on both interned non-interned string values  with same different string lengths  and varying sizes  1B to 5MB    strA Equals strB    Human-readable match  Western cultures  case-insensitive    string Compare strA  strB  StringComparison OrdinalIgnoreCase     0   Human-readable match  All other cultures  insensitive case accent kana etc defined by CultureInfo    string Compare strA  strB  myCultureInfo     0   Human-readable match with custom rules  All other cultures    CompareOptions compareOptions   CompareOptions IgnoreCase                                 CompareOptions IgnoreWidth                                 CompareOptions IgnoreNonSpace  string Compare strA  strB  CultureInfo CurrentCulture  compareOptions     0

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In the forms you listed here  there s not much difference between the two   CompareTo ends up calling a CompareInfo method that does a comparison using the current culture  Equals is called by the    operator   If you consider overloads  then things get different   Compare and    can only use the current culture to compare a string   Equals and String Compare can take a StringComparison enumeration argument that let you specify culture-insensitive or case-insensitive comparisons   Only String Compare allows you to specify a CultureInfo and perform comparisons using a culture other than the default culture   Because of its versatility  I find I use String Compare more than any other comparison method  it lets me specify exactly what I want

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From MSDN       The CompareTo method was designed primarily for use in sorting or   alphabetizing      operations  It should not be used when the primary   purpose of the method call is to     determine whether two strings are   equivalent  To determine whether two strings are    equivalent  call   the Equals method     They suggest using  Equals instead of  CompareTo when looking solely for equality   I am not sure if there is a difference between  Equals and    for the string class   I will sometimes use  Equals or Object ReferenceEquals instead of    for my own classes in case someone comes along at a later time and redefines the    operator for that class

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Here are the rules for how these functions work   stringValue CompareTo otherStringValue    null comes before a string it uses CultureInfo CurrentCulture CompareInfo Compare  which means it will use a culture-dependent comparison  This might mean that    will compare equal to SS in Germany  or similar   stringValue Equals otherStringValue    null is not considered equal to anything unless you specify a StringComparison option  it will use what looks like a direct ordinal equality check  i e     is not the same as SS  in any language or culture   stringValue    otherStringValue   Is not the same as stringValue Equals    The    operator calls the static Equals string a  string b  method  which in turn goes to an internal EqualsHelper to do the comparison  Calling  Equals   on a null string gets null reference exception  while on    does not    Object ReferenceEquals stringValue  otherStringValue   Just checks that references are the same  i e  it isn t just two strings with the same contents  you re comparing a string object with itself     Note that with the options above that use method calls  there are overloads with more options to specify how to compare   My advice if you just want to check for equality is to make up your mind whether you want to use a culture-dependent comparison or not  and then use  CompareTo or  Equals  depending on the choice

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Not that performance usually matters with 99  of the times you need to do this  but if you had to do this in a loop several million times I would highly suggest that you use  Equals or    because as soon as it finds a character that doesn t match it throws the whole thing out as false  but if you use the CompareTo it will have to figure out which character is less than the other  leading to slightly worse performance time   If your app will be running in different countries  I d recommend that you take a look at the CultureInfo implications and possibly use  Equals  Since I only really write apps for the US  and don t care if it doesn t work properly by someone   I always just use

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One BIG difference to note is  Equals   will throw an exception if first string is null  Whereas    will not          string s   null          string a    a             Throws   Object reference not set to an instance of an object            if  s Equals a               Console WriteLine  s is equal to a              no Exception         if s  a              Console WriteLine  s is equal to a

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Good explanation and practices about string comparison issues may be found in the article New Recommendations for Using Strings in Microsoft  NET 2 0 and also in Best Practices for Using Strings in the  NET Framework     Each of mentioned method  and other  has particular purpose  The key difference between them is what sort of StringComparison Enumeration they are using by default  There are several options    CurrentCulture CurrentCultureIgnoreCase InvariantCulture InvariantCultureIgnoreCase Ordinal OrdinalIgnoreCase   Each of above comparison type targets different use case    Ordinal   Case-sensitive internal identifiers Case-sensitive identifiers in standards like XML and HTTP Case-sensitive security-related settings  OrdinalIgnoreCase   Case-insensitive internal identifiers Case-insensitive identifiers in standards like XML and HTTP File paths  on Microsoft Windows  Registry keys values Environment variables Resource identifiers  handle names  for example  Case insensitive security related settings  InvariantCulture or InvariantCultureIgnoreCase   Some persisted linguistically-relevant data Display of linguistic data requiring a fixed sort order  CurrentCulture or CurrentCultureIgnoreCase   Data displayed to the user Most user input    Note  that StringComparison Enumeration as well as overloads for string comparison methods  exists since  NET 2 0     String CompareTo Method  String   Is in fact type safe implementation of IComparable CompareTo Method  Default interpretation  CurrentCulture   Usage      The CompareTo method was designed primarily for use in sorting or alphabetizing operations   Thus     Implementing the IComparable interface will necessarily use this method   String Compare Method  A static member of String Class which has many overloads  Default interpretation  CurrentCulture      Whenever possible  you should call an overload of the Compare method that includes a StringComparison parameter    String Equals Method  Overriden from Object class and overloaded for type safety  Default interpretation  Ordinal  Notice that      The String class s equality methods include the static Equals  the static operator     and the instance method Equals      StringComparer class  There is also another way to deal with string comparisons especially aims to sorting      You can use the StringComparer class to create a type-specific comparison to sort the elements in a generic collection  Classes such as Hashtable  Dictionary  SortedList  and SortedList use the StringComparer class for sorting purposes

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As Ed said  CompareTo is used for sorting   There is a difference  however  between  Equals and         resolves to essentially the following code   if object ReferenceEquals left  null   amp  amp      object ReferenceEquals right  null       return true  if object ReferenceEquals left  null       return right Equals left   return left Equals right     The simple reason is the following will throw an exception   string a   null  string b    foo    bool equal   a Equals b     And the following will not   string a   null  string b    foo    bool equal   a    b

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