Error while adding multiple items to an entity

Question

for (int i = 0; i < skus.Count; i++)
{
    sku item = new sku();
    item = skus[i];
    sku sku = CompanyDbContext.skus.Where(s => s.item_no == item.item_no).FirstOrDefault();

    if (sku == null) // ok to insert [no duplicate item numbers]
    {
        CompanyDbContext.skus.Add(item);                             
    }

}
CompanyDbContext.SaveChanges();

I'm getting

collection was modified enumeration operation may not execute

error. How can I fix this ?

Seems like a duplicate question. See here http://stackoverflow.com/questions/604831/collection-was-modified-enumeration-operation-may-not-execute and here http://stackoverflow.com/questions/2024179/collection-was-modified-enumeration-operation-may-not-execute-in-arraylist — Jakub Rusilko, Nov 30 '16 at 06:42
Well, you are basically modifying the same thing you are iterating. Maybe a solution would be to create a temporary list variable where you would add all items that are ok to insert, and then when you loop finishes add them all in one go? — Jakub Rusilko, Nov 30 '16 at 06:45
I tried that thing too.. using CompanyDbContext.skus.AddRange(skuListAdd); but this gave me the same result — Ravi Rajindu, Nov 30 '16 at 06:48

score 0 · Answer 1 · edited Dec 03 '16 at 08:35

The problem is that you are modify the same thing that you are iterating. As best practice you should update your method something like this:

//get search predicat from List<sku> skus
var item_nos = skus.Select(s=>s.item_no).ToList();
//items already in repo
var addedItems = CompanyDbContext.skus.Where(s => item_nos.Contains(s.item_no)).ToList();
var newItems = skus.Except(addedItems).ToList();
foreach(var sku in newItems){
    CompanyDbContext.skus.Add(item);  
}
CompanyDbContext.SaveChanges();

ColinM · Accepted Answer · 2016-12-04T23:30:56.823

As mentioned in the comments, this happens because you are modifying the collection which you are looping through as you're performing your work.

One option you have is to create a temporary collection and add your sku items to that, and finally add the contents of the temporary List<sku> to your CompanyDbContext

// Create a new temporary list
List<sku> tempSkus = new List<sku>();
for (int i = 0; i < skus.Count; i++)
{
    // Let's assign item to skus[i] immediately, we don't need a new instance here when we're later re-pointing to an existing instance
    sku item = skus[i];

    // Use LINQ Any function to determine whether there are any existing SKU's already
    bool existingSku = CompanyDbContext.skus.Any(s => s.item_no == item.item_no);

    // There are no duplicates, let's add this sku item to our temporary List
    if(!existingSku) 
    {
        tempSkus.Add(item);
    }
}
// Add the Range of tempSkus List to the CompanyDbContext
CompanyDbContext.skus.AddRange(tempSkus);
CompanyDbContext.SaveChanges();

Or if you prefer LINQ

// Create a new temporary list
List<sku> tempSkus = skus.Where(p => CompanyDbContext.skus.Any(s => s.item_no != p.item_no)).ToList();
// Add the Range of tempSkus List to the CompanyDbContext
CompanyDbContext.skus.AddRange(tempSkus);
CompanyDbContext.SaveChanges();

Error while adding multiple items to an entity

2 Answers2