diff --git a/CHD/CompressedMapEntryV5.cs b/CHD/CompressedMapEntryV5.cs
index d5437b8..0b21665 100644
--- a/CHD/CompressedMapEntryV5.cs
+++ b/CHD/CompressedMapEntryV5.cs
@@ -1,28 +1,31 @@
+using System.Runtime.InteropServices;
+
 namespace SabreTools.Models.CHD
 {
     /// <see href="https://github.com/mamedev/mame/blob/master/src/lib/util/chd.h"/> 
-    public class CompressedMapEntryV5
+    [StructLayout(LayoutKind.Sequential)]
+    public sealed class CompressedMapEntryV5
     {
         /// <summary>
         /// Compression type
         /// </summary>
-        public byte Compression { get; set; }
+        public byte Compression;
 
         /// <summary>
         /// Compressed length
         /// </summary>
         /// <remarks>Actually UInt24</remarks>
-        public uint CompLength { get; set; }
+        public uint CompLength;
 
         /// <summary>
         /// Offset
         /// </summary>
         /// <remarks>Actually UInt48</remarks>
-        public ulong Offset { get; set; }
+        public ulong Offset;
 
         /// <summary>
         /// CRC-16 of the data
         /// </summary>
-        public ushort CRC { get; set; }
+        public ushort CRC;
     }
 }
diff --git a/CHD/CompressedMapHeaderV5.cs b/CHD/CompressedMapHeaderV5.cs
index 19f4447..1ec0e35 100644
--- a/CHD/CompressedMapHeaderV5.cs
+++ b/CHD/CompressedMapHeaderV5.cs
@@ -1,42 +1,45 @@
+using System.Runtime.InteropServices;
+
 namespace SabreTools.Models.CHD
 {
     /// <see href="https://github.com/mamedev/mame/blob/master/src/lib/util/chd.h"/> 
-    public class CompressedMapHeaderV5
+    [StructLayout(LayoutKind.Sequential)]
+    public sealed class CompressedMapHeaderV5
     {
         /// <summary>
         /// Length of compressed map
         /// </summary>
-        public uint Length { get; set; }
+        public uint Length;
 
         /// <summary>
         /// Offset of first block
         /// </summary>
         /// <remarks>Actually UInt48</remarks>
-        public ulong DataStart { get; set; }
+        public ulong DataStart;
 
         /// <summary>
         /// CRC-16 of the map
         /// </summary>
-        public ushort CRC { get; set; }
+        public ushort CRC;
 
         /// <summary>
         /// Bits used to encode complength
         /// </summary>
-        public byte LengthBits { get; set; }
+        public byte LengthBits;
 
         /// <summary>
         /// Bits used to encode self-refs
         /// </summary>
-        public byte HunkBits { get; set; }
+        public byte HunkBits;
 
         /// <summary>
         /// Bits used to encode parent unit refs
         /// </summary>
-        public byte ParentUnitBits { get; set; }
+        public byte ParentUnitBits;
 
         /// <summary>
         /// Future use
         /// </summary>
-        public byte Reserved { get; set; }
+        public byte Reserved;
     }
 }
diff --git a/CHD/MapV1.cs b/CHD/MapV1.cs
index bdb8451..e7efc23 100644
--- a/CHD/MapV1.cs
+++ b/CHD/MapV1.cs
@@ -1,16 +1,19 @@
+using System.Runtime.InteropServices;
+
 namespace SabreTools.Models.CHD
 {
     /// <see href="https://github.com/mamedev/mame/blob/master/src/lib/util/chd.h"/> 
-    public class MapV1
+    [StructLayout(LayoutKind.Sequential)]
+    public sealed class MapV1
     {
         /// <summary>
         /// Starting offset within the file
         /// </summary>
-        public ulong StartingOffset { get; set; }
+        public ulong StartingOffset;
 
         /// <summary>
         /// Length of data; If == hunksize, data is uncompressed
         /// </summary>
-        public ulong Length { get; set; }
+        public ulong Length;
     }
 }
diff --git a/CHD/MapV3.cs b/CHD/MapV3.cs
index 9953ac6..1ed611d 100644
--- a/CHD/MapV3.cs
+++ b/CHD/MapV3.cs
@@ -1,31 +1,34 @@
+using System.Runtime.InteropServices;
+
 namespace SabreTools.Models.CHD
 {
     /// <see href="https://github.com/mamedev/mame/blob/master/src/lib/util/chd.h"/> 
-    public class MapV3
+    [StructLayout(LayoutKind.Sequential)]
+    public sealed class MapV3
     {
         /// <summary>
         /// Starting offset within the file
         /// </summary>
-        public ulong StartingOffset { get; set; }
+        public ulong StartingOffset;
 
         /// <summary>
         /// 32-bit CRC of the uncompressed data
         /// </summary>
-        public uint CRC32 { get; set; }
+        public uint CRC32;
 
         /// <summary>
         /// Lower 16 bits of length
         /// </summary>
-        public ushort LengthLo { get; set; }
+        public ushort LengthLo;
 
         /// <summary>
         /// Upper 8 bits of length
         /// </summary>
-        public byte LengthHi { get; set; }
+        public byte LengthHi;
 
         /// <summary>
         /// Flags, indicating compression info
         /// </summary>
-        public byte Flags { get; set; }
+        public byte Flags;
     }
 }
diff --git a/CHD/UncompressedMapV5.cs b/CHD/UncompressedMapV5.cs
index 4fcb346..e9b20b2 100644
--- a/CHD/UncompressedMapV5.cs
+++ b/CHD/UncompressedMapV5.cs
@@ -1,11 +1,14 @@
+using System.Runtime.InteropServices;
+
 namespace SabreTools.Models.CHD
 {
     /// <see href="https://github.com/mamedev/mame/blob/master/src/lib/util/chd.h"/> 
+    [StructLayout(LayoutKind.Sequential)]
     public class UncompressedMapV5
     {
         /// <summary>
         /// Starting offset / hunk size
         /// </summary>
-        public uint StartingOffset { get; set; }
+        public uint StartingOffset;
     }
 }