Question

Somebody explain to me this:

I am trying to delete items from a list with matching ids contained in another list of strings.

Step 1 is as below:

I’m trying to delete Items from myListingSyncIDs where the ListingNumber matches ListingNumbers in lstListingsUpdatedIn24Hrs.

The item at [0] Equals a value from lstListingsUpdatedIn24Hrs, as shown in Step 2:

But as shown in Step3: The Remove fails:

Then After doing a RemoveAll(func) Step4: The Remove works

Somebody explain why the Remove(item) doesn’t work, Please …

Code:

        myListingSyncIDs.AddRange(myListingSync.Listings);

        #region Remove Listing References Fetched In The Last 24Hrs
        // Listing References Fetched In The Last 24Hrs
        // These will be excluded to optimise the running of the App.
        // Basically meaning that a complete sync of all listings 
        // will only be done once every 24hrs
        // So that if this is run every hr, it will not slow down the most recent additions
        List<String> lstListingsUpdatedIn24Hrs = DAL.PropertyPortalDAL.GetSahtWebserviceUpdatesIn24Hrs();
        List<P24SyncService.ListingSyncItem> myListingsUpdatedIn24Hrs =
            lstListingsUpdatedIn24Hrs.Select(p => new P24SyncService.ListingSyncItem()
            {
                ListingNumber = p,
                Status = P24SyncService.ListingState.AddedModified
            }).ToList();
        foreach (P24SyncService.ListingSyncItem myLSI in myListingsUpdatedIn24Hrs)
        {
            myListingSyncIDs.Remove(myLSI);
        }

        myListingSyncIDs.RemoveAll(p => lstListingsUpdatedIn24Hrs.Contains(p.ListingNumber));

        #endregion

ListingSyncItem is:

public partial class ListingSyncItem {

    private string listingNumberField;

    private ListingState statusField;

    /// <remarks/>
    public string ListingNumber {
        get {
            return this.listingNumberField;
        }
        set {
            this.listingNumberField = value;
        }
    }

    /// <remarks/>
    public ListingState Status {
        get {
            return this.statusField;
        }
        set {
            this.statusField = value;
        }
    }
}

Answer 1

At a guess, your ListingSyncItem type doesn’t override Equals, so List<T>.Remove doesn’t know that the item to remove is “equal” to the item in your list.

Simply overriding Equals and GetHashCode appropriately (to check for the equality of ListNumber and Status, presumably, and build a hash code based on those) should fix the problem.

For RemoveAll, you’re providing a predicate. That doesn’t use ListingSyncItem.Equals, which is why it’s working.