编组具有结构数组的C结构

Question

I referred the similar questions in this forum but didn't get the solution for my problem.

I have been struggling with marshaling problem for a while. I have a structure that contains an array of another structure, The platform is Win CE . I am using Visual Studio 2008 and .NET CF 3.5.

The code:

C Structures:

 struct dot11Rate
 {
    unsigned int rate;
    unsigned char mode; 
 };

 typedef struct my_supported_rates
 {
    unsigned short n_rates;
    struct dot11Rate srates[36];
    unsigned char  isSet;
    unsigned char no_of_HTStreams;
 }MY_SUPPORTED_DATA_RATES;

Size of the struct MY_SUPPORTED_DATA_RATES is 296 bytes in C

This is my attempt to convert it into a C# struct:

C# Converted:

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct dot11Rate
    {
        public uint rate;
        public byte mode; /* HT=1, non-HT=0*////
    };

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct my_supported_rates
    {       
        public ushort n_rates;
        [MarshalAs(UnmanagedType.ByValArray,SizeConst = 36)]
        public dot11Rate[] srates;
        public byte isSet;
        public byte no_of_HTStreams;
    };

Here I am getting the size as 304 bytes using Marshal.SizeOf(my_supported_rates);

I have tried the following things without any success:

• Adding and removing various attrubute elements in MarshalAs attribute in my_supported_rates struct such as ArraySubType = UnmanagedType.Struct
• I have Intptr with required data and I tried to convert ptr to the struct using the code my_supported_rates = (my_supported_rates) Marshal.PtrToStructure(ptr,my_supported_rates.GetType()); . But proper conversion is not happend.
• Some other suggestions on blogs and StackOverflow which didn't prove useful to me

Answer 1

Your translations look good to me. Running on desktop rather than CE I find that, for these types

[StructLayout(LayoutKind.Sequential)]
public struct dot11Rate
{
    public uint rate;
    public byte mode;
};

[StructLayout(LayoutKind.Sequential)]
public struct my_supported_rates
{       
    public ushort n_rates;
    [MarshalAs(UnmanagedType.ByValArray,SizeConst = 36)]
    public dot11Rate[] srates;
    public byte isSet;
    public byte no_of_HTStreams;
};

that

Marshal.SizeOf(typeof(my_supported_rates)) == 296

So it would seem to be something odd in the CE pinvoke marshaller. You might need to force the hand of the marshaller by doing this:

[StructLayout(LayoutKind.Explicit, Size=296)]
public struct my_supported_rates
{       
    [FieldOffset(0)]
    public ushort n_rates;
    [FieldOffset(4)]
    [MarshalAs(UnmanagedType.ByValArray,SizeConst = 36)]
    public dot11Rate[] srates;
    [FieldOffset(292)]
    public byte isSet;
    [FieldOffset(293)]
    public byte no_of_HTStreams;
};

That is, if LayoutKind.Explicit and FieldOffset are supported on CE.

If they are not supported then you'll need to marshal by hand. You are looking for Marshal.AllocHGlobal and then Marshal.ReadByte , Marshal.ReadInt16 and so on.