Read array of chars from a byte in unmanaged memory

Question

I am marshaling a native win 32 code to c#, because of mixture of value-type members and reference-type members in structures inside unions, I converted all my reference types to byte and addressed the size of them by using fielOffset..now I want to access the value of one of these members and I do not know how to do it. the below codes are samples.

here is the c++ structure

typedef struct
  {    
    int Port;
    char SubsId[FIELD_SIZE_SUBS_ID+1];
    char Options[FIELD_SIZE_OPTIONS+1];
  } MMTPRcnxReq;

here is the c# equivalent

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct MMTPRcnxReq
{
    [FieldOffset(0)] 
    public Int32 Port;

    [FieldOffset(4)] 
    public byte SubsId;

    [FieldOffset(4 + FIELD_SIZE_SUBS_ID + 1)] 
    public byte Options;
}

now I want to access the value of SubId which is actually an array of chars and for exmaple compare it to a string in my managed code, like below

if(mMTPRcnxReq.SubsId == "12345") // WRONG

how can I do that?

Answer 1

You are probably going to need to use unsafe with fixed size buffers here.

unsafe struct MMTPRcnxReq
{
    public int Port;
    public fixed byte SubsId[FIELD_SIZE_SUBS_ID+1];
    public fixed byte Options[FIELD_SIZE_OPTIONS+1];
}

The fixed size buffer is a value type, and that will allow you to put one of these structs inside a FieldOffset(0) C# union.

Read the strings like this:

string SubsId;
unsafe
{
    fixed (byte* ptr = req.SubsId)
    {
        // I presume that req.SubsId is null-terminated
        SubsId = Marshal.PtrToStringAnsi((IntPtr)ptr);
    }
}

You would probably write helper getter and setter methods of the struct, perhaps as properties, to manage the fixed size buffers.