What is an IndexOutOfRangeException ArgumentOutOfRangeException and how do I fix it

Question

I have some code and when it executes  it throws a IndexOutOfRangeException  saying       Index was outside the bounds of the array    What does this mean  and what can I do about it   Depending on classes used it can also be ArgumentOutOfRangeException     An exception of type  System ArgumentOutOfRangeException  occurred in mscorlib dll but was not handled in user code Additional information  Index was out of range  Must be non-negative and less than the size of the collection

User · Answer

Simple explanation about what a Index out of bound exception is:

Just think one train is there its compartments are D1,D2,D3. One passenger came to enter the train and he have the ticket for D4. now what will happen. the passenger want to enter a compartment that does not exist so obviously problem will arise.

Same scenario: whenever we try to access an array list, etc. we can only access the existing indexes in the array. array[0] and array[1] are existing. If we try to access array[3], it's not there actually, so an index out of bound exception will arise.

User · Answer

What Is It   This exception means that you re trying to access a collection item by index  using an invalid index  An index is invalid when it s lower than the collection s lower bound or greater than or equal to the number of elements it contains   When It Is Thrown  Given an array declared as   byte   array   new byte 4     You can access this array from 0 to 3  values outside this range will cause IndexOutOfRangeException to be thrown  Remember this when you create and access an array    Array Length In C   usually  arrays are 0-based  It means that first element has index 0 and last element has index Length - 1  where Length is total number of items in the array  so this code doesn t work   array array Length    0    Moreover please note that if you have a multidimensional array then you can t use Array Length for both dimension  you have to use Array GetLength     int    data   new int 10  5   for  int i 0  i  lt  data GetLength 0     i        for  int j 0  j  lt  data GetLength 1     j            data i  j    1            Upper Bound Is Not Inclusive In the following example we create a raw bidimensional array of Color  Each item represents a pixel  indices are from  0  0  to  imageWidth - 1  imageHeight - 1    Color    pixels   new Color imageWidth  imageHeight   for  int x   0  x  lt   imageWidth    x        for  int y   0  y  lt   imageHeight    y            pixels x  y    backgroundColor            This code will then fail because array is 0-based and last  bottom-right  pixel in the image is pixels imageWidth - 1  imageHeight - 1    pixels imageWidth  imageHeight    Color Black    In another scenario you may get ArgumentOutOfRangeException for this code  for example if you re using GetPixel method on a Bitmap class    Arrays Do Not Grow An array is fast  Very fast in linear search compared to every other collection  It is because items are contiguous in memory so memory address can be calculated  and increment is just an addition   No need to follow a node list  simple math  You pay this with a limitation  they can t grow  if you need more elements you need to reallocate that array  this may take a relatively long time if old items must be copied to a new block   You resize them with Array Resize lt T gt     this example adds a new entry to an existing array   Array Resize ref array  array Length   1     Don t forget that valid indices are from 0 to Length - 1  If you simply try to assign an item at Length you ll get IndexOutOfRangeException  this behavior may confuse you if you think they may increase with a syntax similar to Insert method of other collections    Special Arrays With Custom Lower Bound First item in arrays has always index 0  This is not always true because you can create an array with a custom lower bound   var array   Array CreateInstance typeof byte   new int     4    new int     1       In that example  array indices are valid from 1 to 4  Of course  upper bound cannot be changed    Wrong Arguments If you access an array using unvalidated arguments  from user input or from function user  you may get this error   private static string   RomanNumbers       new string      I    II    III    IV    V      public static string Romanize int number        return RomanNumbers number       Unexpected Results This exception may be thrown for another reason too  by convention  many search functions will return -1  nullables has been introduced with  NET 2 0 and anyway it s also a well-known convention in use from many years  if they didn t find anything  Let s imagine you have an array of objects comparable with a string  You may think to write this code      Items comparable with a string Console WriteLine  First item equals to  Debug  is   0          myArray Array IndexOf myArray   Debug          Arbitrary objects Console WriteLine  First item equals to  Debug  is   0          myArray Array FindIndex myArray  x   gt  x Type     Debug        This will fail if no items in myArray will satisfy search condition because Array IndexOf   will return -1 and then array access will throw    Next example is a naive example to calculate occurrences of a given set of numbers  knowing maximum number and returning an array where item at index 0 represents number 0  items at index 1 represents number 1 and so on    static int   CountOccurences int maximum  IEnumerable lt int gt  numbers        int   result   new int maximum   1      Includes 0      foreach  int number in numbers            result number        return result      Of course  it s a pretty terrible implementation but what I want to show is that it ll fail for negative numbers and numbers above maximum   How it applies to List lt T gt    Same cases as array - range of valid indexes - 0  List s indexes always start with 0  to list Count - accessing elements outside of this range will cause the exception   Note that List lt T gt  throws ArgumentOutOfRangeException for the same cases where arrays use IndexOutOfRangeException   Unlike arrays  List lt T gt  starts empty - so trying to access items of just created list lead to this exception   var list   new List lt int gt       Common case is to populate list with indexing  similar to Dictionary lt int  T gt   will cause exception   list 0    42     exception list Add 42      correct   IDataReader and Columns Imagine you re trying to read data from a database with this code   using  var connection   CreateConnection          using  var command   connection CreateCommand              command CommandText    SELECT MyColumn1  MyColumn2 FROM MyTable            using  var reader   command ExecuteReader                  while  reader Read                      ProcessData reader GetString 2       Throws                                    GetString   will throw IndexOutOfRangeException because you re dataset has only two columns but you re trying to get a value from 3rd one  indices are always 0-based    Please note that this behavior is shared with most IDataReader implementations  SqlDataReader  OleDbDataReader and so on       You can get the same exception also if you use the IDataReader overload of the indexer operator that takes a column name and pass an invalid column name  Suppose for example that you have retrieved a column named Column1 but then you try to retrieve the value of that field with   var data   dr  Colum1        Missing the n in Column1    This happens because the indexer operator is implemented trying to retrieve the index of a Colum1 field that doesn t exist  The GetOrdinal method will throw this exception when its internal helper code returns a -1 as the index of  Colum1    Others There is another  documented  case when this exception is thrown  if  in DataView  data column name being supplied to the DataViewSort property is not valid   How to Avoid  In this example  let me assume  for simplicity  that arrays are always monodimensional and 0-based  If you want to be strict  or you re developing a library   you may need to replace 0 with GetLowerBound 0  and  Length with GetUpperBound 0   of course if you have parameters of type System Array  it doesn t apply for T     Please note that in this case  upper bound is inclusive then this code   for  int i 0  i  lt  array Length    i        Should be rewritten like this   for  int i array GetLowerBound 0   i  lt   array GetUpperBound 0     i        Please note that this is not allowed  it ll throw InvalidCastException   that s why if your parameters are T   you re safe about custom lower bound arrays   void foo lt T gt  T   array       void test            This will throw InvalidCastException  cannot convert Int32   to Int32        foo  int Array CreateInstance typeof int   new int     1    new int     1          Validate Parameters If index comes from a parameter you should always validate them  throwing appropriate ArgumentException or ArgumentOutOfRangeException   In the next example  wrong parameters may cause IndexOutOfRangeException  users of this function may expect this because they re passing an array but it s not always so obvious  I d suggest to always validate parameters for public functions   static void SetRange lt T gt  T   array  int from  int length  Func lt i  T gt  function        if  from  lt  0    from gt   array Length          throw new ArgumentOutOfRangeException  from         if  length  lt  0          throw new ArgumentOutOfRangeException  length         if  from   length  gt  array Length          throw new ArgumentException              for  int i from  i  lt  from   length    i          array i    function i       If function is private you may simply replace if logic with Debug Assert     Debug Assert from  gt   0  amp  amp  from  lt  array Length     Check Object State Array index may not come directly from a parameter  It may be part of object state  In general is always a good practice to validate object state  by itself and with function parameters  if needed   You can use Debug Assert    throw a proper exception  more descriptive about the problem  or handle that like in this example   class Table       public int SelectedIndex   get  set        public Row   Rows   get  set         public Row SelectedRow           get               if  Rows    null                  throw new InvalidOperationException                         No or wrong selection  here we just return null for                this case  it may be the reason we use this property                instead of direct access              if  SelectedIndex  lt  0    SelectedIndex  gt   Rows Length                  return null               return Rows SelectedIndex                 Validate Return Values In one of previous examples we directly used Array IndexOf   return value  If we know it may fail then it s better to handle that case   int index   myArray Array IndexOf myArray   Debug    if  index    -1      else       How to Debug  In my opinion  most of the questions  here on SO  about this error can be simply avoided  The time you spend to write a proper question  with a small working example and a small explanation  could easily much more than the time you ll need to debug your code  First of all  read this Eric Lippert s blog post about debugging of small programs  I won t repeat his words here but it s absolutely a must read   You have source code  you have exception message with a stack trace  Go there  pick right line number and you ll see   array index    newValue    You found your error  check how index increases  Is it right  Check how array is allocated  is coherent with how index increases  Is it right according to your specifications  If you answer yes to all these questions  then you ll find good help here on StackOverflow but please first check for that by yourself  You ll save your own time   A good start point is to always use assertions and to validate inputs  You may even want to use code contracts  When something went wrong and you can t figure out what happens with a quick look at your code then you have to resort to an old friend  debugger  Just run your application in debug inside Visual Studio  or your favorite IDE   you ll see exactly which line throws this exception  which array is involved and which index you re trying to use  Really  99  of the times you ll solve it by yourself in a few minutes   If this happens in production then you d better to add assertions in incriminated code  probably we won t see in your code what you can t see by yourself  but you can always bet    The VB NET side of the story  Everything that we have said in the C  answer is valid for VB NET with the obvious syntax differences but there is an important point to consider when you deal with VB NET arrays   In VB NET  arrays are declared setting the maximum valid index value for the array  It is not the count of the elements that we want to store in the array     declares an array with space for 5 integer    4 is the maximum valid index starting from 0 to 4 Dim myArray 4  as Integer   So this loop will fill the array with 5 integers without causing any IndexOutOfRangeException  For i As Integer   0 To 4     myArray i    i Next   The VB NET rule  This exception means that you re trying to access a collection item by index  using an invalid index  An index is invalid when it s lower than the collection s lower bound or greater than equal to the number of elements it contains  the maximum allowed index defined in the array declaration

User · Answer

A side from the very long complete accepted answer there is an important point to make about IndexOutOfRangeException compared with many other exception types  and that is     Often there is complex program state that maybe difficult to have control over at a particular point in code e g a DB connection goes down so data for an input cannot be retrieved etc    This kind of issue often results in an Exception of some kind that has to bubble up to a higher level because where it occurs has no way of dealing with it at that point     IndexOutOfRangeException is generally different in that it in most cases it is pretty trivial to check for at the point where the exception is being raised   Generally this kind of exception get thrown by some code that could very easily deal with the issue at the place it is occurring - just by checking the actual length of the array   You don t want to  fix  this by handling this exception higher up - but instead by ensuring its not thrown in the first instance - which in most cases is easy to do by checking the array length     Another way of putting this is that other exceptions can arise due to genuine lack of control over input or program state BUT IndexOutOfRangeException more often than not is simply just pilot  programmer  error

User · Answer

To easily understand the problem  imagine we wrote this code   static void Main string   args        string   test   new string 3       test 0    hello1       test 1    hello2       test 2    hello3        for  int i   0  i  lt   3  i                  Console WriteLine test i  ToString               Result will be   hello1 hello2 hello3  Unhandled Exception  System IndexOutOfRangeException  Index was outside the bounds of the array    Size of array is 3  indices 0  1 and 2   but the for-loop loops 4 times  0  1  2 and 3   So when it tries to access outside the bounds with  3  it throws the exception

[c#] What is an IndexOutOfRangeException / ArgumentOutOfRangeException and how do I fix it?

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