add bag allocation test

src/cs/Tests/MonoGame.Extended.Tests/Collections/BagTests.cs

@@ -0,0 +1,42 @@
+using MonoGame.Extended.Collections;
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Xunit;
+
+namespace MonoGame.Extended.Tests.Collections
+{
+    public class BagTests
+    {
+        [Fact]
+        public void Bag_Enumeration_Does_Not_Allocate()
+        {
+            var bag = new Bag<int>();
+            for (int i = 0; i < 100; i++) bag.Add(i);
+            // ensure we have plenty of memory and that the heap only increases for the duration of this test
+            Assert.True(GC.TryStartNoGCRegion(Unsafe.SizeOf<Bag<int>.BagEnumerator>() * 1000));
+            var heapSize = GC.GetAllocatedBytesForCurrentThread();
+
+            // this should NOT allocate
+            foreach (int i in bag)
+            {
+                // assert methods cause the NoGCRegion to fail, so do this manually
+                if (GC.GetAllocatedBytesForCurrentThread() != heapSize)
+                    Assert.True(false);
+            }
+
+            // sanity check: this SHOULD allocate
+            foreach (int _ in (IEnumerable<int>)bag)
+            {
+                // assert methods cause the NoGCRegion to fail, so do this manually
+                if (GC.GetAllocatedBytesForCurrentThread() == heapSize)
+                    Assert.True(false);
+            }
+
+            GC.EndNoGCRegion();
+        }
+
+    }
+}