Theres a Do method in System.Reactive to a execute a mutator for each item in the sequence.
Is there any equivalent method for IEnumerable either in standard library or third parties like morelinq?
LINQ query operators are generally meant to be free from side-effects. You could just use a foreach loop to "do" your thing.
Or do it in a Select or Where method if you don't care about side effects:
enumerable.Select(x => { /*do something here */ return x; })
For a List<T>, there is a ForEach extension method that can be used to execute an Action<T> for each element.