Marshal.StructureToPtr causes GC alloc calls in Unity

Hi everyone,
I'm trying to use some commonly found code for converting a struct to a byte[] in c#. It uses the System.Runtime.InteropServices.Marshal class. However, it seems in unity that this generates garbage.

Below is an example script. I am running Unity 2018.3.2f1 with ScriptingRuntimeEnvironment = .NET 4.x equivalent.

Code (CSharp):

1. public class MarshallTest : MonoBehaviour
2. {
3.     public int testCount = 300;
4.     byte[] bytes = new byte[1024];
5.        
6.     void FixedUpdate()
7.     {
8.         for (int i = 0; i < testCount; i++)
9.         {
10.             MarshallTestStruct str = new MarshallTestStruct()
11.             {
12.                 a = 123456,
13.                 b = 30.02
14.             };
15.  
16.             GCHandle h = default(GCHandle);
17.             try
18.             {
19.                 h = GCHandle.Alloc(bytes, GCHandleType.Pinned);
20.  
21.                 Profiler.BeginSample("Marshal.StructureToPtr");
22.                 Marshal.StructureToPtr(str, h.AddrOfPinnedObject(), false);
23.                 Profiler.EndSample();
24.             }
25.             finally
26.             {
27.                 if (h.IsAllocated) h.Free();
28.             }
29.         }
30.     }
31. }
32.  
33. struct MarshallTestStruct
34. {
35.     public long a;
36.     public double b;
37. }

I have tested in development build with the profiler attached and the result is the same. Does anyone know why this is happening? Is there a better way to convert struct to byte[] without any GC alloc?

 

Looks like Marshal.StructureToPtr takes an object, so your structure will be boxed and unboxed. Could also be some additional stuff going on under the hood as well. This is just a guess looking at the docs, I didn't actually test it. Maybe try the generic version?

 

There is a generic version available if you target .net framework 4.x:
https://docs.microsoft.com/en-us/do...ureToPtr__1___0_System_IntPtr_System_Boolean_

That one shouldn't have boxing garbage.

Just tested though, and it too creates garbage.

[edit]
LOL

I just looked at the implementation from the .net source:
https://referencesource.microsoft.com/#mscorlib/system/runtime/interopservices/marshal.cs,945

The generic method literally just calls the object version... dafuq.

Though I guess this makes sense since the object version is an external call and generics won't work for those. But what's the point of even having a generic version then?

 

I don't know if anything changed in the mean time, but I just tried the same: switching to the generic version of Marshal.PtrToStructure and my GC Alloc went straight to zero! So, thanks for that. This is on Unity 2019.3.11f1 on Linux, if that matters at all. And I just tested in the Editor, not even a build.

 

@meken generic version of Marshal.PtrToStructure work perfectly without calling GC if you try return a struct instance. But generic version of Marshal.StructureToPtr will call GC if you pass a struct instance.

 

Interesting, so this is what causes massive frame hitching due to GC taking up all the time on recently released Oddworld Soulstorm:

 

Wait what???