Marshalling: unmanaged strcture containing array into C#

**coolmandar** · Aug 12 '14, 02:28 PM

Hi,
I have some C union which are mapped on C# as below

Code:

[StructLayout(LayoutKind.Explicit, Pack = 4)]
    public struct service_choice_u
    {
        [FieldOffset(0)]
        public RPM_RESPONSE m_stRPM_Response;        
        [FieldOffset(0)]
        public PRIVATE_TRANSFER_REQUEST pt_request;
    }

Code:

[B]//Structure to note[/B]
    [StructLayout(LayoutKind.Explicit, Pack = 4)]
    public struct PRIVATE_TRANSFER_REQUEST
    {
        [FieldOffset(0)]
        public UInt32 m_u32VendorID;
        [FieldOffset(4)]
        public UInt32 m_u32ServiceNumber;
        [FieldOffset(8)]
        public UInt32 m_u32DataLength;       
[B]//See offset[/B]
        [FieldOffset(40)]
        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = StackConstants.MAX_PT_SERVICE_PARAMETER_BYTE_LENGTH)]
        public byte[] m_ai8ServiceParameters;
        [FieldOffset(140)]
        public byte m_bIsConfirmedRequest;
        [FieldOffset(141)]
        public byte m_bIsSerParaPresent;
    }


    [StructLayout(LayoutKind.Explicit, Pack = 4)]
    public struct RPM_RESPONSE
    {
        [FieldOffset(0)]
        public byte m_i8ArrayIndexPresent;
        [FieldOffset(4)]
        public int m_i32ErrorClass;
        [FieldOffset(8)]
        public int m_i32ErrorCode;
        [FieldOffset(12)]
        public int m_i32ObjectType;
        [FieldOffset(16)]
        public int m_i32ObjectInstance;
        [FieldOffset(20)]
        public int m_i32ObjectProperty;
        [FieldOffset(24)]
        public uint m_u32ArrayIndex;
        [FieldOffset(28)]
        public int m_eAccessType;
        [FieldOffset(32)]
        public int m_i32StringLen;
        [FieldOffset(36)]
        public int m_pi8PropValue;
        [FieldOffset(40)]
        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = 96)]
        public byte[] bacnet_property_value_structure;
        [FieldOffset(136)]
        public int m_pstNext;
    }

But when PRIVATE_TRANSFE R_REQUEST structure is change to following, it is generating Type load exception

Code:

//Structure to note
    [StructLayout(LayoutKind.Explicit, Pack = 4)]
    public struct PRIVATE_TRANSFER_REQUEST
    {
        [FieldOffset(0)]
        public UInt32 m_u32VendorID;
        [FieldOffset(4)]
        public UInt32 m_u32ServiceNumber;
        [FieldOffset(8)]
        public UInt32 m_u32DataLength;       
[B]//See offset[/B]
        [FieldOffset(12)]
        [MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U1, SizeConst = StackConstants.MAX_PT_SERVICE_PARAMETER_BYTE_LENGTH)]
        public byte[] m_ai8ServiceParameters;
        [FieldOffset(112)]
        public byte m_bIsConfirmedRequest;
        [FieldOffset(113)]
        public byte m_bIsSerParaPresent;
    }

Exception:
An unhandled exception of type 'System.TypeLoa dException' occurred in Unknown Module.

Additional information: Could not load type 'xxxx.BACnetSta ckIntegration.S tructures.servi ce_choice_u' from assembly 'xxxx.BACnetSta ckIntegration, Version=1.0.11. 0, Culture=neutral , PublicKeyToken= null' because it contains an object field at offset 0 that is incorrectly aligned or overlapped by a non-object field.