C# foreach: Not reference to original objects, but copies?

Question

I have some weird behaviour with a foreach-loop:

 IEnumerable<Compound> loadedCompounds;
 ...
 // Loop through the peaks.
 foreach (GCPeak p in peaks)
 {
     // Loop through the compounds.
     foreach (Compound c in loadedCompounds)
     {
         if (c.IsInRange(p) && c.SignalType == p.SignalType)
         {
             c.AddPeak(p);
         }  
     }
 }

So what I'd like to do: Loop through all the GCPeaks (it is a class) and sort them to their corresponding compounds.

AddPeak just adds the GCPeak to a SortedList. Code compiles and runs without exceptions, but the problem is:

After c.AddPeak(p) the SortedList in c contains the GCPeak (checked with Debugger), while the SortedLists in loadedCompounds remains empty.

I am quite confused with this bug I produced:

1. What is the reason for this behavior? Both Compound and GCPeak are classes so I'd expect references and not copies of my objects and my code to work.
2. How to do what I'd like to do properly?

EDIT:

This is how I obtain the IEnumarables (The whole thing is coming from an XML file - LINQ to XML). Compounds are obtained basically the same way.

 IEnumerable<GCPeak> peaksFromSignal = from p in signal.Descendants("IntegrationResults")
                                        select new GCPeak()
                                        {
                                            SignalType = signaltype,
                                            RunInformation = runInformation,
                                            RetentionTime = XmlConvert.ToDouble(p.Element("RetTime").Value),
                                            PeakArea = XmlConvert.ToDouble(p.Element("Area").Value),
                                        };

Thanks!

Answer 1

An IEnumerable won't hold a hard reference to your list. This causes two potential problems for you.

1) What you are enumerating might not be there anymore (for example if you were enumerating a list of facebook posts using a lazy technique like IEnumerable etc, but your connection to facebook for is closed, then it may evaluate to an empty enumerable. The same would occur if you were doing an IEnumerable over a database collection but that DB connection was closed etc.

2) Using an enumerable like that could lead you later to or previously to that to do a multiple enumeration which can have issues. Resharper typically warns against this (to prevent unintended consequences). See here for more info: Handling warning for possible multiple enumeration of IEnumerable

What you can do to debug your situation would be to use the LINQ extension of .toList() to force early evaluation of your IEnumerable. This will let you see what is in the IEnumerable easier and will let you follow this through your code. Do note that doing toList() does have performance implications as compared to a lazy reference like you have currently but it will force a hard reference earlier and help you debug your scenario and will avoid scenarios mentioned above causing challenges for you.

Answer 2

Thanks for your comments.

Indeed converting my loadedCompounds to a List<> worked.

Lesson learned: Be careful with IEnumerable.

EDIT As requested, I am adding the implementation of AddPeak:

public void AddPeak(GCPeak peak)
{
    if (peak != null)
    {
        peaks.Add(peak.RunInformation.InjectionDateTime, peak);
    }
}

RunInformation is a struct.