diff --git a/src/libraries/System.Drawing.Common/src/System/Drawing/BufferedGraphicsContext.Windows.cs b/src/libraries/System.Drawing.Common/src/System/Drawing/BufferedGraphicsContext.Windows.cs
index 97896f67a1a73..1eaaba5447c9a 100644
--- a/src/libraries/System.Drawing.Common/src/System/Drawing/BufferedGraphicsContext.Windows.cs
+++ b/src/libraries/System.Drawing.Common/src/System/Drawing/BufferedGraphicsContext.Windows.cs
@@ -89,7 +89,7 @@ private BufferedGraphics AllocBuffer(Graphics? targetGraphics, IntPtr targetDC,
         /// table or bitmasks, as appropriate.
         /// </summary>
         /// <returns>True if successful, false otherwise.</returns>
-        private bool FillBitmapInfo(IntPtr hdc, IntPtr hpal, ref NativeMethods.BITMAPINFO_FLAT pbmi)
+        private unsafe bool FillBitmapInfo(IntPtr hdc, IntPtr hpal, ref NativeMethods.BITMAPINFO_FLAT pbmi)
         {
             IntPtr hbm = IntPtr.Zero;
             bool bRet = false;
@@ -104,8 +104,7 @@ private bool FillBitmapInfo(IntPtr hdc, IntPtr hpal, ref NativeMethods.BITMAPINF
                     throw new OutOfMemoryException(SR.GraphicsBufferQueryFail);
                 }
 
-                pbmi.bmiHeader_biSize = Marshal.SizeOf(typeof(NativeMethods.BITMAPINFOHEADER));
-                pbmi.bmiColors = new byte[NativeMethods.BITMAPINFO_MAX_COLORSIZE * 4];
+                pbmi.bmiHeader_biSize = sizeof(NativeMethods.BITMAPINFOHEADER);
 
                 // Call first time to fill in BITMAPINFO header.
                 SafeNativeMethods.GetDIBits(new HandleRef(null, hdc),
diff --git a/src/libraries/System.Drawing.Common/src/System/Drawing/Gdiplus.cs b/src/libraries/System.Drawing.Common/src/System/Drawing/Gdiplus.cs
index 6fee50f351aa3..e42459df59d1b 100644
--- a/src/libraries/System.Drawing.Common/src/System/Drawing/Gdiplus.cs
+++ b/src/libraries/System.Drawing.Common/src/System/Drawing/Gdiplus.cs
@@ -562,22 +562,6 @@ public struct BITMAP
             public IntPtr bmBits;
         }
 
-        [StructLayout(LayoutKind.Sequential)]
-        public class BITMAPINFOHEADER
-        {
-            public int biSize = 40;
-            public int biWidth;
-            public int biHeight;
-            public short biPlanes;
-            public short biBitCount;
-            public int biCompression;
-            public int biSizeImage;
-            public int biXPelsPerMeter;
-            public int biYPelsPerMeter;
-            public int biClrUsed;
-            public int biClrImportant;
-        }
-
         // https://devblogs.microsoft.com/oldnewthing/20101018-00/?p=12513
         // https://devblogs.microsoft.com/oldnewthing/20120720-00/?p=7083
 
diff --git a/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Unix.cs b/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Unix.cs
index 62e345e1caca6..96c81dab0eded 100644
--- a/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Unix.cs
+++ b/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Unix.cs
@@ -449,7 +449,7 @@ private void SaveBestSingleIcon(BinaryWriter writer, int width, int height)
             SaveIconImage(writer, (IconImage)imageData![best]);
         }
 
-        private void SaveBitmapAsIcon(BinaryWriter writer)
+        private unsafe void SaveBitmapAsIcon(BinaryWriter writer)
         {
             writer.Write((ushort)0);    // idReserved must be 0
             writer.Write((ushort)1);    // idType must be 1
@@ -466,7 +466,7 @@ private void SaveBitmapAsIcon(BinaryWriter writer)
             ide.imageOffset = 22;   // 22 is the first icon position (for single icon files)
 
             BitmapInfoHeader bih = default;
-            bih.biSize = (uint)Marshal.SizeOf(typeof(BitmapInfoHeader));
+            bih.biSize = (uint)sizeof(BitmapInfoHeader);
             bih.biWidth = bitmap.Width;
             bih.biHeight = 2 * bitmap.Height; // include both XOR and AND images
             bih.biPlanes = 1;
diff --git a/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Windows.cs b/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Windows.cs
index ad699974c90f5..02021231f9f29 100644
--- a/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Windows.cs
+++ b/src/libraries/System.Drawing.Common/src/System/Drawing/Icon.Windows.cs
@@ -738,7 +738,7 @@ private unsafe Bitmap BmpFrame()
                         uint* pixelPtr = (uint*)bmpdata.Scan0.ToPointer();
 
                         // jumping the image header
-                        int newOffset = (int)(_bestImageOffset + Marshal.SizeOf(typeof(SafeNativeMethods.BITMAPINFOHEADER)));
+                        int newOffset = (int)(_bestImageOffset + sizeof(NativeMethods.BITMAPINFOHEADER));
                         // there is no color table that we need to skip since we're 32bpp
 
                         int lineLength = Size.Width * 4;
diff --git a/src/libraries/System.Drawing.Common/src/System/Drawing/NativeMethods.cs b/src/libraries/System.Drawing.Common/src/System/Drawing/NativeMethods.cs
index 5b12a5bdc5828..b317e652b968e 100644
--- a/src/libraries/System.Drawing.Common/src/System/Drawing/NativeMethods.cs
+++ b/src/libraries/System.Drawing.Common/src/System/Drawing/NativeMethods.cs
@@ -9,7 +9,7 @@ namespace System.Drawing
 {
     internal class NativeMethods
     {
-        internal static HandleRef NullHandleRef = new HandleRef(null, IntPtr.Zero);
+        internal static HandleRef NullHandleRef => new HandleRef(null, IntPtr.Zero);
 
         public const int MAX_PATH = 260;
         internal const int SM_REMOTESESSION = 0x1000;
@@ -20,9 +20,9 @@ internal class NativeMethods
         internal const int BITMAPINFO_MAX_COLORSIZE = 256;
 
         [StructLayout(LayoutKind.Sequential)]
-        internal struct BITMAPINFO_FLAT
+        internal unsafe struct BITMAPINFO_FLAT
         {
-            public int bmiHeader_biSize; // = Marshal.SizeOf(typeof(BITMAPINFOHEADER));
+            public int bmiHeader_biSize; // = sizeof(BITMAPINFOHEADER)
             public int bmiHeader_biWidth;
             public int bmiHeader_biHeight;
             public short bmiHeader_biPlanes;
@@ -34,14 +34,13 @@ internal struct BITMAPINFO_FLAT
             public int bmiHeader_biClrUsed;
             public int bmiHeader_biClrImportant;
 
-            [MarshalAs(UnmanagedType.ByValArray, SizeConst = BITMAPINFO_MAX_COLORSIZE * 4)]
-            public byte[] bmiColors; // RGBQUAD structs... Blue-Green-Red-Reserved, repeat...
+            public fixed byte bmiColors[BITMAPINFO_MAX_COLORSIZE * 4]; // RGBQUAD structs... Blue-Green-Red-Reserved, repeat...
         }
 
         [StructLayout(LayoutKind.Sequential)]
-        internal class BITMAPINFOHEADER
+        internal struct BITMAPINFOHEADER
         {
-            public int biSize = 40;
+            public int biSize;
             public int biWidth;
             public int biHeight;
             public short biPlanes;