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

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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<int, int> rankings = new Dictionary<int, int>(); 

The Next Method containing this dictionary does the following:

private void getFirstEstimation() {     List<int> array = new List<int>();      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<int, string> sqltext = new Dictionary<int, string>();     Dictionary<int, int> valueForSql = new Dictionary<int, int>();     Dictionary<int, int> weightings = new Dictionary<int, int>();     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<int> array = new List<int>();      string strConnection = ConfigurationSettings.AppSettings["ConnectionString"];     MySqlConnection connection = new MySqlConnection(strConnection);     MySqlCommand command = connection.CreateCommand();     MySqlDataReader reader;      for (int i = 1; i <= 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<int, int> 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)>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

Answer 1

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.

Answer 2

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.