C#: Convert List of simple structs to byte[]

Question

In C# 4.0, say I have

List<HSZPAIR> myList

with three elements where the HSZPAIR struct is defined by:

[StructLayout(LayoutKind.Sequential)]
public struct HSZPAIR
{
    public IntPtr hszSvc;
    public IntPtr hszTopic;
}

How do I create a byte array for the entire myList? In C++, you could just cast as array of structs down to a byte array. I'm not sure how to do that in C#.

I'm using an old Windows API function in the DDEML library that requires a byte array and the number of elements in the array as arguments. If you are interested in more background, the API function is:

[DllImport("user32.dll", EntryPoint="DdeCreateDataHandle", CharSet=CharSet.Ansi)]
    public static extern IntPtr DdeCreateDataHandle(int idInst, byte[] pSrc, int cb, int cbOff, IntPtr hszItem, int wFmt, int afCmd);

Here is it's documentation on MSDN. The pSrc argument is the byte array of HSZPAIR structs. The size of the array is the cb argument.

I've managed to convert a single struct into a byte array given the tutorial at developerfusion.com/article/84519/mastering-structs-in-c. Converting an entire list of structs into a byte array is what's throwing me for a loop. — dmjones
– dmjones, Commented Nov 5, 2010 at 15:58

Jim Mischel · Accepted Answer · 2010-11-05 15:37:59Z

2

Converting a struct to an array of bytes is kind of painful. You have to serialize it yourself. But it might not be necessary.

Given your list:

List<HSZPAIR> myList;

You can get an array by calling ToArray:

HSZPAIR[] myArray = myList.ToArray();

Now, change your managed prototype so that it takes an HSZPAIR[] rather than a byte[]:

public static extern IntPtr DdeCreateDataHandle(
    int idInst, HSZPAIR[] pSrc, int cb, int cbOff, IntPtr hszItem, int wFmt, int afCmd);

That should work. After all, as you pointed out, an array of HSZPAIR really is just an array of bytes.

answered Nov 5, 2010 at 15:37

Jim Mischel

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6 Comments

dmjones Over a year ago

I like the idea, but the DdeCreateDataHandle function is sort of a general purpose function that is meant to convert arrays of data of many types to a pointer for other DDE functions to make use of. If I change the declaration, then it might break those other usages. The function that takes the IntPtr that DdeCreateDataHandle creates is supposed to know what type of struct the data is.

dmjones Over a year ago

Also, I don't have enough reputation to upvote you yet, but will come back and give you an upvote once I do.

Jim Mischel Over a year ago

@dmjones: You might find what you're looking for in the array marshaling samples at msdn.microsoft.com/en-us/library/aa719546(v=VS.71).aspx. It looks like the page is broken, as the C# examples are intermixed with the VB examples. But there they show how to marshal arrays in several different ways.

Jim Mischel Over a year ago

Also, you can create multiple prototpyes for DdeCreateDataHandle, just name them differently. For example DdeCreateDataHandleHSZPAIR, etc. I wouldn't necessarily recommend it as a general solution, but it should work to let you know if you're able to call the method.

dmjones Over a year ago

@Jim I'll take a look at the web link you provided for marshalling arrays, but the idea of overloading the extern prototype looks like it works! Thanks a lot for taking the time to answer!

|

Giawa · Accepted Answer · 2010-11-05 16:16:14Z

2

I think you can use Marshal.StructureToPtr.

static byte[] StructureToByteArray(object obj)
{
   int length = Marshal.SizeOf(obj);
   byte[] data = new byte[length];
   IntPtr ptr = Marshal.AllocHGlobal(length);
   Marshal.StructureToPtr(obj, ptr, true);
   Marshal.Copy(ptr, data, 0, length);
   Marshal.FreeHGlobal(ptr);
   return data;
}

As for the list itself, it will have to be serialized separately.

answered Nov 5, 2010 at 16:16

Giawa

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1 Comment

dmjones Over a year ago

That's very similar to what I used from the Mastering Structs in C# link in my original post for marshaling a single struct. I'm sure others will appreciate the info, though.

Paul Sonier · Accepted Answer · 2010-11-05 15:37:13Z

1

I believe you need to define the type of the pSrc in the extern declaration as IntPtr, instead of byte[].

answered Nov 5, 2010 at 15:37

Paul Sonier

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2 Comments

dmjones Over a year ago

After looking at your comment, I did see via google that others have defined the extern definition with an IntPtr or a byte[]. But I'm not sure how using an IntPtr instead of a byte[] gets me closer to passing this array of structs. (Also, I don't have enough reputation to upvote you yet, but will come back and give you an upvote once I do).

Paul Sonier Over a year ago

@dmjones: IntPtr has a bit of "priveleged status" within C#; basically, it "just works".

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