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SharePoint: Microsoft.SharePoint.SPListItemCollection' does not contain a definition for 'Cast' and no extension method 'Cast' accepting a first argument of type 'Microsoft.SharePoint.SPListItemCollection

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Here is simple way of getting list of item IDs present in SPListItemCollection using LINQ:

using (SPSite site = new SPSite("http://sp2010:900"))
{
    SPWeb web = site.RootWeb;
    SPList list = web.Lists["ListTitle"];
    int[] itemIds = list.GetItems().Cast<SPListItem>().Select(i => i.ID).ToArray();
}

Remember to include System.Linq namespace, otherwise you will get an error:
Microsoft.SharePoint.SPListItemCollection' does not contain a definition for 'Cast' and no extension method 'Cast' accepting a first argument of type 'Microsoft.SharePoint.SPListItemCollection