[mono] Force Mono to respect explicit struct size when LayoutKind.Seq…

…uential is used (dotnet#101529)

* respect explicit size with sequential layout

* test for sequential layout with explicit size

src/mono/mono/metadata/class-init.c

@@ -2331,7 +2331,7 @@ mono_class_layout_fields (MonoClass *klass, int base_instance_size, int packing_
 
 			instance_size = MAX (real_size, instance_size);
 
-			if (instance_size & (min_align - 1)) {
+			if (instance_size & (min_align - 1) && !explicit_size) {
 				instance_size += min_align - 1;
 				instance_size &= ~(min_align - 1);
 			}

src/mono/mono/metadata/marshal.c

@@ -5749,7 +5749,7 @@ MonoMarshalType *
 mono_marshal_load_type_info (MonoClass* klass)
 {
 	int j, count = 0;
-	guint32 native_size = 0, min_align = 1, packing;
+	guint32 native_size = 0, min_align = 1, packing, explicit_size = 0;
 	MonoMarshalType *info;
 	MonoClassField* field;
 	gpointer iter;
@@ -5793,7 +5793,7 @@ mono_marshal_load_type_info (MonoClass* klass)
 	info->num_fields = count;
 
 	/* Try to find a size for this type in metadata */
-	mono_metadata_packing_from_typedef (m_class_get_image (klass), m_class_get_type_token (klass), NULL, &native_size);
+	explicit_size = mono_metadata_packing_from_typedef (m_class_get_image (klass), m_class_get_type_token (klass), NULL, &native_size);
 
 	if (m_class_get_parent (klass)) {
 		int parent_size = mono_class_native_size (m_class_get_parent (klass), NULL);
@@ -5879,6 +5879,9 @@ mono_marshal_load_type_info (MonoClass* klass)
 				align_size = FALSE;
 			else
 				min_align = MIN (min_align, packing);
+		} else if (layout == TYPE_ATTRIBUTE_SEQUENTIAL_LAYOUT) {
+			if (explicit_size && native_size == info->native_size)
+				align_size = FALSE;
 		}
 	}
 

src/tests/Interop/ArrayMarshalling/UnalignedStructArray/MarshalUnalignedStructArray.cs

@@ -0,0 +1,75 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+using System;
+using System.Runtime.InteropServices;
+using TestLibrary;
+using Xunit;
+
+public static unsafe class MarshalUnalignedStructArrayTest
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        /*
+         * This test validates that the size and offsets of InnerStruct and OuterStruct are as expected.
+         * It also demonstrates accessing unaligned data in an array.
+         */
+        // Validate that both InnerStruct and OuterStruct have the correct size
+        Assert.Equal(12, sizeof(InnerStruct));
+        Assert.Equal(24, sizeof(OuterStruct));
+
+        // Validate that the fields of InnerStruct are at the expected offsets
+        Assert.Equal(0, Marshal.OffsetOf<InnerStruct>("F0").ToInt32());
+        Assert.Equal(8, Marshal.OffsetOf<InnerStruct>("F1").ToInt32());
+
+        // Validate that the fields of OuterStruct are at the expected offsets
+        Assert.Equal(0, Marshal.OffsetOf<OuterStruct>("F0").ToInt32());
+        Assert.Equal(8, Marshal.OffsetOf<OuterStruct>("F1").ToInt32());
+        Assert.Equal(20, Marshal.OffsetOf<OuterStruct>("F2").ToInt32());
+
+        // Validate that we are able to access unaligned in an array
+        InnerStruct[] arrStructs = new InnerStruct[]
+        {
+            new InnerStruct(1, 2),
+            new InnerStruct(3, 4),
+            new InnerStruct(5, 6),
+        };
+
+        fixed (InnerStruct* pStruct = &arrStructs[0])
+        {
+            byte* ptr = (byte*)pStruct;
+            ptr += 12;
+            Assert.Equal(3, *(long*)ptr);
+            Assert.Equal(4, *(int*)(ptr + 8));
+        }
+
+    }
+}
+
+[StructLayout(LayoutKind.Sequential, Size = 12)]
+struct InnerStruct
+{
+    public long F0;
+    public uint F1;
+
+    public InnerStruct(long f0, uint f1)
+    {
+        F0 = f0;
+        F1 = f1;
+    }
+}
+
+[StructLayout(LayoutKind.Sequential, Size = 24)]
+struct OuterStruct
+{
+    public sbyte F0;
+    public InnerStruct F1;
+    public uint F2;
+
+    public OuterStruct(sbyte f0, InnerStruct f1, uint f2)
+    {
+        F0 = f0;
+        F1 = f1;
+        F2 = f2;
+    }
+}