Handling warning for possible multiple enumeration of IEnumerable

If your data is always going to be repeatable, perhaps don't worry about it. However, you can unroll it too - this is especially useful if the incoming data could be large (for example, reading from disk/network):

if(objects == null) throw new ArgumentException();
using(var iter = objects.GetEnumerator()) {
    if(!iter.MoveNext()) throw new ArgumentException();

    var firstObject = iter.Current;
    var list = DoSomeThing(firstObject);  

    while(iter.MoveNext()) {
        list.Add(DoSomeThingElse(iter.Current));
    }
    return list;
}

Note I changed the semantic of DoSomethingElse a bit, but this is mainly to show unrolled usage. You could re-wrap the iterator, for example. You could make it an iterator block too, which could be nice; then there is no list - and you would yield return the items as you get them, rather than add to a list to be returned.

The problem with taking IEnumerable as a parameter is that it tells callers "I wish to enumerate this". It doesn't tell them how many times you wish to enumerate.

I can change the objects parameter to be List and then avoid the possible multiple enumeration but then I don't get the highest object that I can handle.

The goal of taking the highest object is noble, but it leaves room for too many assumptions. Do you really want someone to pass a LINQ to SQL query to this method, only for you to enumerate it twice (getting potentially different results each time?)

The semantic missing here is that a caller, who perhaps doesn't take time to read the details of the method, may assume you only iterate once - so they pass you an expensive object. Your method signature doesn't indicate either way.

By changing the method signature to IList/ICollection, you will at least make it clearer to the caller what your expectations are, and they can avoid costly mistakes.

Otherwise, most developers looking at the method might assume you only iterate once. If taking an IEnumerable is so important, you should consider doing the .ToList() at the start of the method.

It's a shame .NET doesn't have an interface that is IEnumerable + Count + Indexer, without Add/Remove etc. methods, which is what I suspect would solve this problem.