Marshaling in C# - passing pointer to reference of structure ("double ref" ?)

Question

I'm using native functions and have small problem with marshaling structs in c#. I have pointer to struct in another struct - e.g. C# declarations:

 [StructLayout(LayoutKind.Sequential, Pack=1, CharSet = CharSet.Auto)] public struct PARENT { [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)] public string Name; [MarshalAs(UnmanagedType.Struct, SizeConst=8)] public CHILD pChild; } [StructLayout(LayoutKind.Sequential, Pack=1, CharSet = CharSet.Auto)] public struct CHILD { public UInt32 val1; public UInt32 val2; }

In PARENT struct I should have a pointer to CHILD struct. I need to pass a 'pointer to reference' (of PARENT struct) as argument of API function.

There's no problem with single reference ("ref PARENT" as argument of imported dll function) but how to pass "ref ref" ? Is it possible without using unsafe code (with C pointer) ?

greetings Arthur

Declare CHILD as a class instead and remove the attribute, you'll get the pointer for free. — Hans Passant
– Hans Passant, Commented Jun 28, 2012 at 11:04

weismat · Accepted Answer · 2012-06-28 10:57:19Z

If you do not want to use unsafe code, then you need to define Child as IntPtr and add a property which access then the values from the Child IntPtr.

 [StructLayout(LayoutKind.Sequential, Pack=1, CharSet = CharSet.Auto)] public struct PARENT { [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)] public string Name; public IntPtr pChild; public CHILD Child{ get { return (CHILD)Marshal.PtrToStructure(pChild, typeof(CHILD)); } } } [StructLayout(LayoutKind.Sequential, Pack=1, CharSet = CharSet.Auto)] public struct CHILD { public UInt32 val1; public UInt32 val2; }

I think it is easier and cleaner with unsafe code/pointers.

YoryeNathan · Accepted Answer · 2012-06-28 08:53:08Z

This is a combination of safe and unsafe, as I believe is reasonable here.

fixed (void* pt = new byte[Marshal.SizeOf(myStructInstance)]) { var intPtr = new IntPtr(pt); Marshal.StructureToPtr(myStructInstance, intPtr, true); // now "pt" is a pointer to your struct instance // and "intPtr" is the same, but wrapped with managed code }

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