c# code seems to get optimized in an invalid way such that an object value becomes null

Question

I have the following code that exhibits a strange problem:

var all = new FeatureService().FindAll();
System.Diagnostics.Debug.Assert(all != null, \"FindAll must          


        

Answer 1

After looking over the code over TeamViewer, and finally downloading, compiling and running the code on my own machine, I believe this is a case of compiler error in C# 4.0.

---

I've posted a question with request for verification, after managing to reduce the problem to a few simple projects and files. It is available here: Possible C# 4.0 compiler error, can others verify?

---

The likely culprit is not this method:

protected override List GetModels() {
    var fs = new FeatureService();
    var all = fs.FindAll();
    var wr = new WeakReference(all);
    System.Diagnostics.Debug.Assert(all != null, "FindAll must not return null");
    System.Diagnostics.Debug.WriteLine(wr.IsAlive);
    System.Diagnostics.Debug.WriteLine(all.ToString()); // throws NullReferenceException
    return all;
}

But the method it calls, FeatureService.FindAll:

public List FindAll() {
    string key = Cache.GetQueryKey("FindAll");
    var value = Cache.Load>(key);
    if (value == null) {
        var query = Context.Features;
        value = query.ToList().Select(x => Map(x)).ToList();
        var policy = Cache.GetDefaultCacheItemPolicy(value.Select(x => Cache.GetObjectKey(x.Id.ToString())), true);
        Cache.Store(key, value, policy);
    }
    value = new List();
    return value;
}

If I changed the call in GetModels from this:

var all = fs.FindAll();

to this:

var all = fs.FindAll().ToList(); // remember, it already returned a list

then the program crashes with a ExecutionEngineException.

---

After doing a clean, a build, and then looking at the compiled code through Reflector, here's how the output looks (scroll to the bottom of the code for the important part):

public List FindAll()
{
    List value;
    Func CS$<>9__CachedAnonymousMethodDelegate6 = null;
    List CS$<>9__CachedAnonymousMethodDelegate7 = null;
    string key = base.Cache.GetQueryKey("FindAll");
    if (base.Cache.Load>(key) == null)
    {
        if (CS$<>9__CachedAnonymousMethodDelegate6 == null)
        {
            CS$<>9__CachedAnonymousMethodDelegate6 = (Func) delegate (Feature x) {
                return this.Map(x);
            };
        }
        value = base.Context.Features.ToList().Select(((Func) CS$<>9__CachedAnonymousMethodDelegate6)).ToList();
        if (CS$<>9__CachedAnonymousMethodDelegate7 == null)
        {
            CS$<>9__CachedAnonymousMethodDelegate7 = (List) delegate (FeatureModel x) {
                return base.Cache.GetObjectKey(x.Id.ToString());
            };
        }
        Func policy = (Func) base.Cache.GetDefaultCacheItemPolicy(value.Select((Func) CS$<>9__CachedAnonymousMethodDelegate7), true);
        base.Cache.Store>(key, value, (CacheItemPolicy) policy);
    }
    value = new List();
    bool CS$1$0000 = (bool) value;
    return (List) CS$1$0000;
}

Notice the 3 last lines of the method, here's what they look like in the code:

value = new List();
return value;

here's what Reflector says:

value = new List();
bool CS$1$0000 = (bool) value;
return (List) CS$1$0000;

It creates the list, then casts it to a boolean, then casts it back to a list and returns it. Most likely this causes a stack problem.

Here's the same method, in IL (still through Reflector), I've stripped away most of the code:

.method public hidebysig instance class [mscorlib]System.Collections.Generic.List`1 FindAll() cil managed
{
    .maxstack 5
    .locals init (
        [0] string key,
        [1] class [mscorlib]System.Collections.Generic.List`1 'value',
        [2] class [System.Data.Entity]System.Data.Objects.ObjectSet`1 query,
        [3] class [mscorlib]System.Func`2 policy,
        [4] class [mscorlib]System.Func`2 CS$<>9__CachedAnonymousMethodDelegate6,
        [5] class [mscorlib]System.Collections.Generic.List`1 CS$<>9__CachedAnonymousMethodDelegate7,
        [6] bool CS$1$0000,
        [7] char CS$4$0001)
    ...
    L_009f: newobj instance void [mscorlib]System.Collections.Generic.List`1::.ctor()
    L_00a4: stloc.1 
    L_00a5: ldloc.1 
    L_00a6: stloc.s CS$1$0000
    L_00a8: br.s L_00aa
    L_00aa: ldloc.s CS$1$0000
    L_00ac: ret 
}

Here's a screencast showing the debug session, if you just want the Reflector output, skip to about 2:50.