Collection was modified; enumeration operation may not exec.

using Microsoft.TeamFoundation.Client;

using Microsoft.TeamFoundation.WorkItemTracking.Client;

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace windowsserviceTest

{

class velocityActivity

{

public static Uri collectionUri = new Uri("****---My uri---****);

public static IList> iterationListing = new List>();

public static IList spoint = new List();

public static int task = 0;

public velocityActivity()

{

TfsTeamProjectCollection teamProjectCollection = new TfsTeamProjectCollection(collectionUri);

var tpc = TfsTeamProjectCollectionFactory.GetTeamProjectCollection(collectionUri);

tpc.Authenticate();

teamProjectCollection.EnsureAuthenticated();

var wis = new WorkItemStore(tpc);

ProjectCollection proj = wis.Projects;

foreach (Project projs in proj)

{

if (projs.Name == "AT_ERG")

{

foreach (Node iter in projs.IterationRootNodes)

{

iterationListing.Add(new Tuple(iter.Id, iter.Name, iter.Uri.Segments.ElementAt(3)));

}

WorkItemCollection workItemCollection = wis.Query(@"Select * from WorkItems");

int count = 0;

var workList = new List();

var taskLists = new List();

IList spoints = new List();

foreach (WorkItem item in workItemCollection)

{

workList.Add(item);

}

Parallel.ForEach(workList.ToList(), (i) => activityAction(i));

Console.WriteLine(spoint);

}

public void activityAction(WorkItem item)

{

try

{

task++;

Console.WriteLine("~Started {0}", task);

int storyPoint = 0;

int flag = 0;

// var fel = item.Fields.AsQueryable();

foreach (Field fields in item.Fields)

{

if (fields.Name.ToString() == "Story Points")

{

if (fields.OriginalValue == null)

{

storyPoint = 0;

continue;

}

else

{

storyPoint = Convert.ToInt32(fields.OriginalValue);

continue;

}

if (fields.OriginalValue != null)

{

if (fields.OriginalValue.ToString() == "User Story")

{

flag = 1;

}

if (flag == 1)

{

string iterID = "";

foreach (var lis in iterationListing)

{

if (lis.Item1 == item.IterationId)

{

iterID = lis.Item3;

break;

}

else

{

iterID = "Deleted";

}

string userStoryID = item.Id.ToString();

spoint.Add(new userstoryPoints

{

IterID = iterID,

userStoryID = userStoryID,

storyPoint = storyPoint

});

}

Console.WriteLine("Completed {0}", task);

}

catch(Exception e)

{

Console.WriteLine($"~Failed {task} exception-----{e.ToString()}");

}