Marshalling a structure to byte array only works for single bytes?

Question

I am trying to serialize a structure to disk as raw bytes. This is a (simplified)version of it.

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public class TestData :BaseStructure
{
    public byte[] bytes = new byte[]{65,66,67}; // this doesn't write ABC as expected
}

A write function uses ConvertStructureToBytes method to convert this to a byte array and a binary writer then writes it.

    public void Write(BaseStructure baseStructure)
    {
        binaryWriter.Write(ConvertStructureToBytes(baseStructure));
    }

The ConvertStructureToBytes section

 public byte[] ConvertStructureToBytes(BaseStructure baseStructure)
    {
        int len = Marshal.SizeOf(baseStructure);
        byte[] arr = new byte[len];

        IntPtr ptr = Marshal.AllocHGlobal(len);
        Marshal.StructureToPtr(baseStructure, ptr,false);
        Marshal.Copy(ptr, arr, 0, len);

        Marshal.FreeHGlobal(ptr);
        return arr;
    }

If I replace the bytes line to

public byte byte = 65; // This now writes an A , as expected

I have tried

public byte[] bytes = Encoding.ASCII.GetBytes("ABC"); //doesn't work either

This probably has something to do with the ConvertStructureToBytes function , it isn't treating the byte array as it should.

What do I need to do to be able to write 'ABC' successfully?

I have edited your title. Please see, "Should questions include “tags” in their titles?", where the consensus is "no, they should not". — John Saunders
– John Saunders, Commented Sep 12, 2014 at 21:04
What is BaseStructure? Your TestData class inherits from it, but you are not serialising a TestData instance but a BaseStructure instance, which is not aware of the data declared in the TestData class. — Guffa
– Guffa, Commented Sep 12, 2014 at 21:10

Hans Passant · Accepted Answer · 2014-09-12 21:15:07Z

3

Several problems. First your structure declaration is not correct, you have to in-line the array so it no longer a pointer:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public class TestData {
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
    public byte[] bytes = new byte[] { 65, 66, 67 };
}

Then your ConvertStructureToBytes() method is not correct, it is only ever going to marshal BaseStructure. You need to make it generic:

public static byte[] ConvertStructureToBytes<T>(T baseStructure) {
    // rest the same...
}

Do note the kind of trouble you can get into with this approach, it most certainly is not a universal way to marshal data. Only very specific classes can be serialized this way. That [MarshalAs] attribute is of course very painful to maintain. You might as well use binary serialization.

answered Sep 12, 2014 at 21:15

Hans Passant

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2 Comments

iAteABug_And_iLiked_it Over a year ago

Thank you, this WORKS! Yay... I am actually writing raw bytes for ISO9660 structure and the classes involved are directories,files and path tables, all predefined and the fields, such as bytes above wont' change. So I think this solution works fine for me :)

Hans Passant Over a year ago

Okay, a fixed set of structure declarations is fine. Please close your question.

Matt · Accepted Answer · 2014-09-12 21:08:57Z

1

Change the class definition to this and try it:

StructLayout(LayoutKind.Sequential, Pack = 1)]
public class TestData :BaseStructure
{
     [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]
    public byte[] bytes = new byte[]{65,66,67}; 
}

answered Sep 12, 2014 at 21:08

Matt

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Comments

TechneWare · Accepted Answer · 2014-09-12 21:12:35Z

0

Try this:

byte[] bytes = Encoding.ASCII.GetBytes("ABC").ToArray();

Or, after I tested that. Maybe you mean this:

bytes = new byte[] { 65, 66, 67 };
string test = Encoding.UTF8.GetString(bytes).ToString();

edited Sep 12, 2014 at 21:12

answered Sep 12, 2014 at 21:05

TechneWare

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