Most efficient way to concatenate strings

Question

What s the most efficient way to concatenate strings

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It would depend on the code   StringBuilder is more efficient generally  but if you re only concatenating a few strings and doing it all in one line  code optimizations will likely take care of it for you  It s important to think about how the code looks too  for larger sets StringBuilder will make it easier to read  for small ones StringBuilder will just add needless clutter

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Try this 2 pieces of code and you will find the solution    static void Main string   args                StringBuilder s   new StringBuilder            for  int i   0  i  lt  10000000  i                          s Append  i ToString                       Console Write  End            Console Read            Vs  static void Main string   args                string s               for  int i   0  i  lt  10000000  i                          s    i ToString                      Console Write  End            Console Read            You will find that 1st code will end really quick and the memory will be in a good amount   The second code maybe the memory will be ok  but it will take longer    much longer  So if you have an application for a lot of users and you need speed  use the 1st  If you have an app for a short term one user app  maybe you can use both or the 2nd will be more  natural  for developers   Cheers

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Here is the fastest method I ve evolved over a decade for my large-scale NLP app  I have variations for IEnumerable lt T gt  and other input types  with and without separators of different types  Char  String   but here I show the simple case of  concatenating all strings in an array into a single string  with no separator  Latest version here is developed and unit-tested on C  7 and  NET 4 7   There are two keys to higher performance  the first is to pre-compute the exact total size required  This step is trivial when the input is an array as shown here  For handling IEnumerable lt T gt  instead  it is worth first gathering the strings into a temporary array for computing that total  The array is required to avoid calling ToString   more than once per element since technically  given the possibility of side-effects  doing so could change the expected semantics of a  string join  operation    Next  given the total allocation size of the final string  the biggest boost in performance is gained by building the result string in-place  Doing this requires the  perhaps controversial  technique of temporarily suspending the immutability of a new String which is initially allocated full of zeros  Any such controversy aside  however           note that this is the only bulk-concatenation solution on this page which entirely avoids an extra round of allocation and copying by the String constructor    Complete code        lt summary gt      Concatenate the strings in  rg   none of which may be null  into a single String       lt  summary gt  public static unsafe String StringJoin this String   rg        int i      if  rg    null     i   rg Length     0          return String Empty       if  i    1          return rg 0        String s  t      int cch   0      do         cch    rg --i  Length      while  i  gt  0       if  cch    0          return String Empty       i   rg Length      fixed  Char   p    s   new String default Char   cch                  Char  pDst    p   cch          do             if   t   rg --i   Length  gt  0                  fixed  Char  pSrc   t                      memcpy pDst -  t Length  pSrc   UIntPtr  t Length  lt  lt  1            while  pDst  gt   p             return s      DllImport  MSVCR120 CLR0400   CallingConvention   CallingConvention Cdecl   static extern unsafe void  memcpy void  dest  void  src  UIntPtr cb     I should mention that this code has a slight modification from what I use myself  In the original  I call the cpblk IL instruction from C  to do the actual copying  For simplicity and portability in the code here  I replaced that with P Invoke memcpy instead  as you can see  For highest performance on x64  but maybe not x86  you may want to use the cpblk method instead

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From Chinh Do - StringBuilder is not always faster   Rules of Thumb   When concatenating three dynamic string values or less  use traditional string concatenation  When concatenating more than three dynamic string values  use StringBuilder  When building a big string from several string literals  use either the   string literal or the inline   operator    Most of the time StringBuilder is your best bet  but there are cases as shown in that post that you should at least think about each situation

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If you re operating in a loop  StringBuilder is probably the way to go  it saves you the overhead of creating new strings regularly  In code that ll only run once  though  String Concat is probably fine   However  Rico Mariani   NET optimization guru  made up a quiz in which he stated at the end that  in most cases  he recommends String Format

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Adding to the other answers  please keep in mind that StringBuilder can be told an initial amount of memory to allocate   The capacity parameter defines the maximum number of characters that can be stored in the memory allocated by the current instance  Its value is assigned to the Capacity property  If the number of characters to be stored in the current instance exceeds this capacity value  the StringBuilder object allocates additional memory to store them  If capacity is zero  the implementation-specific default capacity is used   Repeatedly appending to a StringBuilder that hasn t been pre-allocated can result in a lot of unnecessary allocations just like repeatedly concatenating regular strings  If you know how long the final string will be  can trivially calculate it  or can make an educated guess about the common case  allocating too much isn t necessarily a bad thing   you should be providing this information to the constructor or the Capacity property  Especially when running performance tests to compare StringBuilder with other methods like String Concat  which do the same thing internally  Any test you see online which doesn t include StringBuilder pre-allocation in its comparisons is wrong  If you can t make any kind of guess about the size  you re probably writing a utility function which should have its own optional argument for controlling pre-allocation

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Another solution   inside the loop  use List instead of string   List lt string gt  lst  new List lt string gt      for int i 0  i lt 100000  i                         lst Add         return String Join     lst ToArray        it is very very fast

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The most efficient is to use StringBuilder  like so   StringBuilder sb   new StringBuilder    sb Append  string1    sb Append  string2       etc    String strResult   sb ToString       jonezy  String Concat is fine if you have a couple of small things  But if you re concatenating megabytes of data  your program will likely tank

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Rico Mariani  the  NET Performance guru  had an article on this very subject   It s not as simple as one might suspect   The basic advice is this      If your pattern looks like       x   f1        f2        f3        f4           that s one concat and it s zippy  StringBuilder probably won t help       If your pattern looks like         if       x    f1        if       x    f2        if       x    f3        if       x    f4             then you probably want StringBuilder    Yet another article to support this claim comes from Eric Lippert where he describes the optimizations performed on one line   concatenations in a detailed manner

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It really depends on your usage pattern  A detailed benchmark between string Join  string Concat and string Format can be found here  String Format Isn t Suitable for Intensive Logging   This is actually the same answer I gave to this question

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The StringBuilder Append   method is much better than using the   operator   But I ve found that  when executing 1000 concatenations or less  String Join   is even more efficient than StringBuilder   StringBuilder sb   new StringBuilder    sb Append someString     The only problem with String Join is that you have to concatenate the strings with a common delimiter   Edit  as  ryanversaw pointed out  you can make the delimiter string Empty   string key   String Join      new String       Customers Contacts   customerID  database  SessionID

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Following may be one more alternate solution to concatenate multiple strings   String str1    sometext   string str2    some other text    string afterConcate      str1  str2      string interpolation

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From this MSDN article      There is some overhead associated with   creating a StringBuilder object  both   in time and memory  On a machine with   fast memory  a StringBuilder becomes   worthwhile if you re doing about five   operations  As a rule of thumb  I   would say 10 or more string operations   is a justification for the overhead on   any machine  even a slower one    So if you trust MSDN go with StringBuilder if you have to do more than 10 strings operations concatenations - otherwise simple string concat with     is fine

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System String is immutable  When we modify the value of a string variable then a new memory is allocated to the new value and the previous memory allocation released  System StringBuilder was designed to have concept of a mutable string where a variety of operations can be performed without allocation separate memory location for the modified string

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There are 6 types of string concatenations    Using the plus     symbol  Using string Concat    Using string Join    Using string Format    Using string Append    Using StringBuilder    In an experiment  it has been proved that string Concat   is the best way to approach if the words are less than 1000 approximately  and if the words are more than 1000 then StringBuilder should be used   For more information  check this site       string Join   vs string Concat        The string Concat method here is equivalent to the string Join method invocation with an empty separator  Appending an empty string is fast  but not doing so is even faster  so the string Concat method would be superior here

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For just two strings  you definitely do not want to use StringBuilder   There is some threshold above which the StringBuilder overhead is less than the overhead of allocating multiple strings   So  for more that 2-3 strings  use DannySmurf s code   Otherwise  just use the   operator

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It s also important to point it out that you should use the   operator if you are concatenating string literals      When you concatenate string literals or string constants by using the   operator  the compiler creates a single string  No run time concatenation occurs    How to  Concatenate Multiple Strings  C  Programming Guide

[c#] Most efficient way to concatenate strings?

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