CODEWITHSUNDEEP
c#struct
Duncan Drennan
Duncan Drennan

Reputation: 920

How do I convert a C# byte array into structured data?

I am passing 64 byte data packets over USB to a microcontroller. In the microcontroller C code the packets have the structure,

typedef union
{
    unsigned char data[CMD_SIZE];
    cmd_get_t get;
    // plus more union options
} cmd_t;

with

typedef struct
{
    unsigned char cmd;          //!< Command ID
    unsigned char id;           //!< Packet ID
    unsigned char get_id;       //!< Get identifier
    unsigned char rfu[3];       //!< Reserved for future use
    union
    {
        unsigned char data[58];     //!< Generic data
        cmd_get_adc_t adc;          //!< ADC data
        // plus more union options
    } data;                     //!< Response data
} cmd_get_t;

and

typedef struct
{
    int16_t supply;
    int16_t current[4];
} cmd_get_adc_t;

On the PC side in C# I have been provided with a function which returns the 64 byte packet as a Byte[]. The function uses Marshal.Copy to copy the received data into the Byte[] array. I then used a C# struct of the form

[StructLayout(LayoutKind.Sequential, Pack=1)]
    public struct COMMAND_GET_ADC
    {
        public byte CommandID;
        public byte PacketID;
        public byte GetID;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst=3)]
            public byte[] RFU;
        public short Supply;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst=4)]
            public short[] Current;
    }

and again used Marshal.Copy to copy the byte array into the struct so that I could work with is as structured data, e.g.

COMMAND_GET_ADC cmd = (COMMAND_GET_ADC)RawDeserialize(INBuffer, 1, typeof(COMMAND_GET_ADC));
short supply = cmd.Supply;

with

public static object RawDeserialize(Byte[] rawData, int position, Type anyType)
{
    int rawsize = Marshal.SizeOf(anyType);
    if(rawsize > rawData.Length)
    {
        return null;
    }
    IntPtr buffer = Marshal.AllocHGlobal(rawsize);
    Marshal.Copy(rawData, position, buffer, rawsize);
    object retobj = Marshal.PtrToStructure(buffer, anyType);
    Marshal.FreeHGlobal(buffer);
    return retobj;
}

This just feels like I am making lots of copies of the data and like it might not be the most productive way of achieving what I want to. I also need to convert the structured data back to a byte array for commands to the device. I have a method which uses the same process (i.e. use a struct and then serialise it to a byte array and pass the byte array to the write function).

Are there better alternatives?

Upvotes: 7

Views: 5887

Answers (2)

Mihailo
Mihailo

Reputation: 764

If you are calling native dll yourself you could define your DllImport-s so they return and accept COMMAND_GET_ADC directly - provided that you have structs represented correctly. The framework itself should take care of it.

If you have to use byte array mandated by usage of methods provided to you - then I don't know, I never had such a constraint. I always tried to represent my interoperability data in the same way as in native dlls and I don't remember I had major issues with that.

EDIT:

[StructLayout(LayoutKind.Explicit)]
public struct COMMAND_GET
{
    [FieldOffset(0)]
    public byte CommandID;
    [FieldOffset(1)]
    public byte PacketID;
    [FieldOffset(2)]
    public byte GetID;
    [FieldOffset(3)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=3)]
    public byte[] RFU;
    [FieldOffset(6)]
    public ADC_Data Adc_data;
    [FieldOffset(6)]
    public SomeOther_Data other_data;
    [FieldOffset(6)]
    ....
}


[StructLayout(LayoutKind.Sequential, Pack=1)]
public struct ADC_Data
{
    public short Supply;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=4)]
    public short[] Current;
}

Basically where you have FieldOffset(6) you're creating union like data union in cmd_get_t

Upvotes: 2

logicnp
logicnp

Reputation: 5836

If you can use unsafe code, you can cast the byte array to a pointer to your structure using the 'fixed' keyword.

Upvotes: 2

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