public static class PHVExtensions
    public static IEnumerable<T> SetValue<T>(this IEnumerable<T> items, Action<T>
        foreach (T item in items)
        return items;

With that method in place, I can write my statement like this:
customers.Where(c => c.IsValid).SetValue(c => c.CreditLimit = 1000).ToList();

Or like this:
var newCustomers = customers.Where(c => c.IsValid).SetValue(c => c.CreditLimit = 1000);

Or like this for Entity Framework:
db.Customers.Where(c => c.IsValid).ToList().SetValue(c => c.CreditLimit = 1000);
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