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POConvert C Union to C# (Incorrectly Aligned)
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  1. POConvert C Union to C# (Incorrectly Aligned)
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    copied!<p>I want to call the <a href="http://msdn.microsoft.com/en-us/library/windows/desktop/aa363286%28v=vs.85%29.aspx" rel="nofollow">DhcpGetClientInfo</a> API from C# but I have a question on conversion of this C struct to C#:</p> <pre><code>typedef struct _DHCP_CLIENT_SEARCH_INFO { DHCP_SEARCH_INFO_TYPE SearchType; union { DHCP_IP_ADDRESS ClientIpAddress; DHCP_CLIENT_UID ClientHardwareAddress; LPWSTR ClientName; } SearchInfo; } DHCP_SEARCH_INFO, *LPDHCP_SEARCH_INFO; </code></pre> <p>I think the Correct conversion is this:</p> <pre><code>[StructLayout(LayoutKind.Explicit, Size=12)] public struct DHCP_SEARCH_INFO { [FieldOffset(0)] public DHCP_SEARCH_INFO_TYPE SearchType; [FieldOffset(4)] public DHCP_IP_ADDRESS ClientIpAddress; [FieldOffset(4)] public DHCP_BINARY_DATA ClientHardwareAddress; [FieldOffset(4), MarshalAs(UnmanagedType.LPWStr)] public string ClientName; }; </code></pre> <p>But that gives an System.TypeLoadException: Additional information: Could not load type 'Dhcpsapi.DHCP_SEARCH_INFO' from assembly 'ConsoleApplication3, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' because it contains an object field at offset 4 that is incorrectly aligned or overlapped by a non-object field.</p> <p>This is the conversion of the other types in case you want to compile:</p> <pre><code>public enum DHCP_SEARCH_INFO_TYPE : uint { DhcpClientIpAddress = 0, DhcpClientHardwareAddress = 1, DhcpClientName = 2 }; [StructLayout(LayoutKind.Sequential)] public struct DHCP_BINARY_DATA { public uint DataLength; public IntPtr Data; }; [StructLayout(LayoutKind.Sequential)] public struct DHCP_IP_ADDRESS { public UInt32 IPAddress; } </code></pre> <p><strong>EDIT:</strong></p> <p>I verified sizeof and offsets in C:</p> <pre><code>#pragma comment(lib,"Dhcpsapi.lib") int _tmain(int argc, _TCHAR* argv[]) { DHCP_SEARCH_INFO si; printf("sizeof(DHCP_SEARCH_INFO)=%d\n", sizeof(DHCP_SEARCH_INFO)); printf("ClientIpAddress offset=%d\n", (PBYTE)&amp;si.SearchInfo.ClientIpAddress - (PBYTE)&amp;si); printf("ClientHardwareAddress offset=%d\n", (PBYTE)&amp;si.SearchInfo.ClientHardwareAddress - (PBYTE)&amp;si); printf("ClientName offset=%d\n", (PBYTE)&amp;si.SearchInfo.ClientName - (PBYTE)&amp;si); return 0; } </code></pre> <p>Output is:</p> <pre><code>sizeof(DHCP_SEARCH_INFO)=12 ClientIpAddress offset=4 ClientHardwareAddress offset=4 ClientName offset=4 </code></pre> <p><strong>EDIT:</strong> Based on Camford's answer I declared the struct as below. Using sizeof should make it correct for x64 as well.</p> <pre><code>[StructLayout(LayoutKind.Explicit, Size=12)] public struct DHCP_SEARCH_INFO { [FieldOffset(0)] public DHCP_SEARCH_INFO_TYPE SearchType; [FieldOffset(sizeof(DHCP_SEARCH_INFO_TYPE))] public DHCP_IP_ADDRESS ClientIpAddress; [FieldOffset(sizeof(DHCP_SEARCH_INFO_TYPE))] public IntPtr ClientName; [FieldOffset(sizeof(DHCP_SEARCH_INFO_TYPE))] public DHCP_BINARY_DATA ClientHardwareAddress; }; </code></pre>
 

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