Marshaling C# struct with array of structs and size param index

Question

I've read several topics about but I still can't understand the real limitation of not being able to convert this structure to byte array easily:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct B {
  public int b_a;
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct A {
  public int sizeB;

  [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)]
  public B[] b;
}

I'm writing a TCP communication program so I want to build my S2C packets in a struct and then send them as byte[] so I'm looking for the cheapest and fastest way to achieve this.

I have already tried Marsheling in many ways but there is always some exception in Marshal.SizeOf().

In this example I get the following error: "[...] cannot be marshaled as an unmanaged structure; no meaningful size or offset can be computed."

Struct initialization eg.:

A a = new A();
B[] b = new B[5];

a.sizeB = 5;
a.b = b;

Marshal.SizeOf(a);

I assume you are trying to call Marshall.SizeOf(a) which is never going to work unless your array is a fixed length. — Jerry
– Jerry, Commented Mar 11, 2019 at 1:00

Jerry · Accepted Answer · 2019-03-11 03:18:54Z

You don't have the same control for low level memory access like you would C or C++. You will need to do some manual work when you have an array of undefined length in C#.

Here are a couple ways of accomplishing that.

struct B
{
    public int b_a;
}

struct A
{
    public int sizeB;

    public B[] b;
}

The first being a BinaryWriter. This can be faster if your structure does not have a lot of fields.

static byte[] ConvertToByte(A a)
{
    using (var ms = new MemoryStream())
    using (var writer = new BinaryWriter(ms))
    {
        writer.Write(a.sizeB);

        foreach (var b in a.b)
            writer.Write(b.b_a);

        return ms.ToArray();
    }
}

The other to use marshalling like you were but explicitly looping through the array.

static byte[] ConvertToByte(A a)
{
    var bStructSize = Marshal.SizeOf<B>();
    var size = bStructSize * a.b.Length;

    var arr = new byte[size + 4];

    var ptr = Marshal.AllocHGlobal(size);

    for (int i = 0; i < a.b.Length; i++)
        Marshal.StructureToPtr(a.b[i], ptr + i * bStructSize, true);

    Marshal.Copy(ptr, arr, 4, size);

    Array.Copy(BitConverter.GetBytes(a.sizeB), arr, 4);

    return arr;
}

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