[Mono-bugs] [Bug 76733][Wis] New - LayoutKind.Explicit and out-of-order unions

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Changed by ed at catmur.co.uk.

http://bugzilla.ximian.com/show_bug.cgi?id=76733

--- shadow/76733	2005-11-16 22:24:32.000000000 -0500
+++ shadow/76733.tmp.21057	2005-11-16 22:24:32.000000000 -0500
@@ -0,0 +1,53 @@
+Bug#: 76733
+Product: Mono: Runtime
+Version: 1.1
+OS: 
+OS Details: 
+Status: NEW   
+Resolution: 
+Severity: 
+Priority: Wishlist
+Component: misc
+AssignedTo: mono-bugs at ximian.com                            
+ReportedBy: ed at catmur.co.uk               
+QAContact: mono-bugs at ximian.com
+TargetMilestone: ---
+URL: 
+Cc: 
+Summary: LayoutKind.Explicit and out-of-order unions
+
+This is my struct:
+
+	[StructLayout(LayoutKind.Sequential)]
+	internal struct XClientMessageEvent {
+		internal XEventName     type;
+		internal int            serial;
+		internal bool           send_event;
+		internal IntPtr         display;
+		internal IntPtr         window;
+		internal IntPtr         message_type;
+		internal int            format;
+		[StructLayout(LayoutKind.Explicit)]
+		internal struct DataUnion {
+			[FieldOffset(0)] internal IntPtr ptr1;
+			[FieldOffset(4)] internal IntPtr ptr2;
+			[FieldOffset(8)] internal IntPtr ptr3;
+			[FieldOffset(12)] internal IntPtr ptr4;
+			[FieldOffset(16)] internal IntPtr ptr5;
+			[FieldOffset(0)] [MarshalAs(UnmanagedType.ByValTStr, SizeConst=20)]
+				internal string str;
+		}
+		internal DataUnion   data;
+	}
+
+When using it I get:
+
+** ERROR **: Type Egg.XClientMessageEvent/DataUnion which is passed to
+unmanaged code must have a StructLayout attribute
+aborting...
+
+This appears to be because the union is constructed out-of-order; if I
+place the str member at the start of DataUnion it works OK.
+
+It's no problem reordering the union, but the error message is highly
+confusing.