Multiple enumeration (ARCH901)

Key dotnet.linq.multiple-enumeration · Severity warning · Category linq

Iterating the same IEnumerable repeatedly re-executes the underlying query each time.

Incorrect

IEnumerable<Order> orders = GetOrders();
if (orders.Any())
{
    var count = orders.Count();   // enumerates again
    var first = orders.First();   // and again
}

Correct

List<Order> orders = GetOrders().ToList();
if (orders.Count > 0)
{
    var count = orders.Count;
    var first = orders[0];
}

Fix

Materialize the source once with ToList() and reuse the list.