.Net Framework 4.0 - Opcodes.Box present in Dictionary with int key

Question

I'm trying to investigate whether dictionaries with enum keys still generate garbage in newer versions of .Net (say >= 4)

See Shawn Hargreaves blog post here for details on why I'm even fretting about this... (http://blogs.msdn.com/b/shawnhar/archive/2007/07/02/twin-paths-to-garbage-collector-nirvana.aspx) Very specific I know but garbage on the xbox is / can be a very really problem.

I created a little .Net v4 console application comparing the IL generated for Dictionary and Dicationary and noticed a 'box' opcode in both sets of code which really confused me.

.method private hidebysig 
    instance int32 FindEntry (
        !TKey key
    ) cil managed 
{
    // Method begins at RVA 0x61030
    // Code size 138 (0x8a)
    .maxstack 3
    .locals init (
        [0] int32,
        [1] int32
    )

    IL_0000: ldarg.1
    IL_0001: box !TKey   <----Hmmmm!
    IL_0006: brtrue.s IL_000e

    IL_0008: ldc.i4.5
    IL_0009: call void System.ThrowHelper::ThrowArgumentNullException(valuetype System.ExceptionArgument)

    IL_000e: ldarg.0
    IL_000f: ldfld int32[] class System.Collections.Generic.Dictionary`2<!TKey, !TValue>::buckets
    IL_0014: brfalse.s IL_0088

https://msdn.microsoft.com/en-us/library/system.reflection.emit.opcodes.box%28v=vs.110%29.aspx

Convert a value type (of the type specified in valTypeToken) to a true object reference.

Is the box here not a heap allocation? If not, then how can I tell when there's heap allocations that might cause the Xbox to struggle?(from looking at the IL) Does it depend on some other context? Would a memory profiler (CLR Profiler for example) be the only way to tell for sure?

Answer 1

Yes it is a box, but no it shouldn't matter in this case - at least, not for regular .NET; this is a != null check; the JIT knows how to recognise these for value-types, and can remove that check from the machine code.

Allegedly.

To tell for sure, you'd need to look at the post-JIT machine code, not the IL.

It will also matter which JIT you are using, which makes it harder.

Worst case: you could use the CoreCLR code to roll your own value-type dictionary.