C Collection was modified enumeration operation may not execute

Question

Possible Duplicate    Collection was modified  enumeration operation may not execute       HI there   I am create an project estimation program and am getting the following error  C  Collection was modified  enumeration operation may not execute   It is related to using this   I initally declare the diciontary globally with this   Dictionary lt int  int gt  rankings   new Dictionary lt int  int gt       The Next Method  containing this dictionary does the following   private void getFirstEstimation         List lt int gt  array   new List lt int gt          string strConnection   ConfigurationSettings AppSettings  ConnectionString        MySqlConnection connection   new MySqlConnection strConnection       MySqlCommand command   connection CreateCommand        MySqlDataReader reader      command CommandText    SELECT idprojects FROM  test   projects  WHERE application layers       applicationTiers      connection Open         reader   command ExecuteReader        while  reader Read                  array Add Convert ToInt32 reader  idprojects   ToString                foreach  int i in array                rankings i    15            connection Close        I call it for a second time here   private void getSecondEstimation         Dictionary lt int  string gt  sqltext   new Dictionary lt int  string gt         Dictionary lt int  int gt  valueForSql   new Dictionary lt int  int gt         Dictionary lt int  int gt  weightings   new Dictionary lt int  int gt         sqltext Add 1   project type        valueForSql Add 1  projectType       weightings Add 1  10       sqltext Add 2   application domain        valueForSql Add 2  applicationDomain       weightings Add 2  8       sqltext Add 3   organisation size        valueForSql Add 3  organizationSize       weightings Add 3  8       sqltext Add 4   no of locations        valueForSql Add 4  noOfLocations       weightings Add 4  7       sqltext Add 5   development process        valueForSql Add 5  developmentProcess       weightings Add 5  6       sqltext Add 6   rules engine        valueForSql Add 6  rulesEngine       weightings Add 6  5       sqltext Add 7   middleware        valueForSql Add 7  middleware       weightings Add 7  4       sqltext Add 8   location of development        valueForSql Add 8  locationOfDevelopment       weightings Add 8  3       sqltext Add 9   programming language        valueForSql Add 9  programmingLanguage       weightings Add 9  3       sqltext Add 10   development environment        valueForSql Add 10  developmentEnvironment       weightings Add 10  3       sqltext Add 11   backend        valueForSql Add 11  backend       weightings Add 11  3       sqltext Add 12   webserver        valueForSql Add 12  webServer       weightings Add 12  3        List lt int gt  array   new List lt int gt          string strConnection   ConfigurationSettings AppSettings  ConnectionString        MySqlConnection connection   new MySqlConnection strConnection       MySqlCommand command   connection CreateCommand        MySqlDataReader reader       for  int i   1  i  lt   12  i                  command CommandText    SELECT idprojects FROM  test   projects  WHERE     sqltext i            valueForSql i           connection Open              int testInt          reader   command ExecuteReader            while  reader Read                          array Add Convert ToInt32 reader  idprojects   ToString                        foreach  int a in array                        if   rankings ContainsKey a                                 rankings a    0                            rankings a    rankings a    weightings i                     connection Close                     The problem arises at this area of the code   private void getThirdEstimation         ArrayList tempModuleHolder       string strConnection   ConfigurationSettings AppSettings  ConnectionString        MySqlConnection connection   new MySqlConnection strConnection       MySqlCommand command   connection CreateCommand        MySqlDataReader reader      int similarModules       foreach  KeyValuePair lt int  int gt  kvp in rankings                similarModules   0          tempModuleHolder   new ArrayList            command CommandText    SELECT id modules FROM  test   modules in project  WHERE id project       kvp Key          connection Open             reader   command ExecuteReader            while  reader Read                          tempModuleHolder Add Convert ToInt32 reader  id modules   ToString                         foreach  int i in tempModuleHolder                        if modules Contains i                                 similarModules                                    if  double  similarModules modules Count  gt 0 6                          kvp Value   kvp Value   4              rankings kvp Key    rankings kvp Key    4                    connection Close              Any help with where the problem lies will be much appreciated

User · Answer

The error tells you EXACTLY what the problem is (and running in the debugger or reading the stack trace will tell you exactly where the problem is):

C# Collection was modified; enumeration operation may not execute.

Your problem is the loop

foreach (KeyValuePair<int, int> kvp in rankings) {
    //
}

wherein you modify the collection rankings. In particular, the offensive line is

rankings[kvp.Key] = rankings[kvp.Key] + 4;

Before you enter the loop, add the following line:

var listOfRankingsToModify = new List<int>();

Replace the offending line with

listOfRankingsToModify.Add(kvp.Key);

and after you exit the loop

foreach(var key in listOfRankingsToModify) {
    rankings[key] = rankings[key] + 4;
}

That is, record what changes you need to make, and make them without iterating over the collection that you need to modify.

User · Answer

The problem is where you are executing   rankings kvp Key    rankings kvp Key    4    You cannot modify the collection you are iterating through in a foreach loop   A foreach loop requires the loop to be immutable during iteration   Instead  use a standard  for  loop or create a new loop that is a copy and iterate through that while updating your original

User · Answer

As others have pointed out  you are modifying a collection that you are iterating over and that s what s causing the error   The offending code is below   foreach  KeyValuePair lt int  int gt  kvp in rankings                   if  double  similarModules modules Count  gt 0 6                rankings kvp Key    rankings kvp Key    4       lt --- This line is the problem                   What may not be obvious from the code above is where the Enumerator comes from   In a blog post from a few years back about Eric Lippert provides an example of what a foreach loop gets expanded to by the compiler   The generated code will look something like         IEnumerator lt int gt  e     IEnumerable lt int gt  values  GetEnumerator        lt -- This                                                           is where the Enumerator                                                           comes from      try                int m     OUTSIDE THE ACTUAL LOOP in C  4 and before  inside the loop in 5         while e MoveNext                             loop code goes here                     finally              if  e    null    IDisposable e  Dispose              If you look up the MSDN documentation for IEnumerable  which is what GetEnumerator   returns  you will see      Enumerators can be used to read the data in the collection  but they cannot be used to modify the underlying collection    Which brings us back to what the error message states and the other answers re-state  you re modifying the underlying collection

User · Answer

I suspect the error is caused by this   foreach  KeyValuePair lt int  int gt  kvp in rankings    rankings is a dictionary  which is IEnumerable  By using it in a foreach loop  you re specifying that you want each KeyValuePair from the dictionary in a deferred manner  That is  the next KeyValuePair is not returned until your loop iterates again   But you re modifying the dictionary inside your loop   rankings kvp Key    rankings kvp Key    4    which isn t allowed   so you get the exception   You could simply do this  foreach  KeyValuePair lt int  int gt  kvp in rankings ToArray

User · Answer

Any collection that you iterate over with foreach may not be modified during iteration   So while you re running a foreach over rankings  you cannot modify its elements  add new ones or delete any

[c#] C# Collection was modified; enumeration operation may not execute

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