Error while adding multiple items to an entity - asp.net-mvc

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 ?

Answer 1

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();

Answer 2

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();