Writing bytes from a struct into a file with c#

Question

Good morning everyone!

I'm with a new problem here. I have to write down a data that comes from a struct that I declared in my sistem.

The struct I created has only two fields and I use to to later conevrt it into bytes.

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
public struct MyStructData
{
    public short Id;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
    public string Name;
}

I convert this struct in bytes with the following code:

private byte[] getBytes(MyStructData aux)
{
    int length = Marshal.SizeOf(aux);
    IntPtr ptr = Marshal.AllocHGlobal(length);
    byte[] myBuffer = new byte[length];

    Marshal.StructureToPtr(aux, ptr, true);
    Marshal.Copy(ptr, myBuffer, 0, length);
    Marshal.FreeHGlobal(ptr);

    return myBuffer;
}

This function is made to transform each element inside in a List structure of MyStructData type elements where I have all the registers I want to send to another machine, and I do it with the code pasted below:

string saveToFilePath = "D:\\" + filename;
Stream myStream = myFtpRequest.GetRequestStream();

using (FileStream myFileStream = new FileStream(saveToFilePath, FileMode.Create))
{
    foreach (MyStructData myData in myStructDataList)
    {
        int length = 2048;
        byte[] newBuffer = new byte[length];
        newBuffer = getBytes(myCust);

        int len = 0;
        int bytesRead = 0;

        myFileStream.Write(newBuffer, 0, len);
        bytesRead += len;
    }

    myFileStream.Close();
}

My problem comes that I see that my new file is empty and I can't see why it doesn't gets the information of my bytes. I already checked if the List comes with data or not and I also checked that my byte convertion function works also perfectly, but I can't get to the point to see what's causing that my file is empty.

If someone knows the light at the end of the tunnel I would appreciate a lot your help!

EDIT Now I have another method to write the data into the file and it works good:

using (Stream stream = new FileStream(saveToFilePath, FileMode.Create, FileAccess.Write, FileShare.ReadWrite))
{
    using (BinaryWriter writer = new BinaryWriter(stream, Encoding.Default))
    {
        // get all data
        foreach (MyStructData myData in myStructDataList)
        {
            byte[] newBuffer = getBytes(cd);
            writer.Write(newBuffer);
        }
    }
}

Answer 1

The third parameter to FileStream.Write is the count of bytes to write . It should not be 0.

string saveToFilePath = "D:\\" + filename;
Stream myStream = myFtpRequest.GetRequestStream();

int bytesWritten = 0;
using (FileStream myFileStream = new FileStream(saveToFilePath, FileMode.Create))
{
    foreach (MyStructData myData in myStructDataList)
    {
        newBuffer = getBytes(myData);
        myFileStream.Write(newBuffer, 0, newBuffer.Length);
        bytesWritten += newBuffer.Length;
    }
}

Answer 2

I don't think you should be writing the data to a file like this. For a simple struct like that, you would better off using a BinaryFormatter .

Here's an example. First you need to make MyStructData serializable by adding the [Serializable] attribute:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
[Serializable]
public struct MyStructData
{
    public short Id;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 20)]
    public string Name;
}

Then you can write a list of those structs to a file like so:

List<MyStructData> data = new List<MyStructData>();

for (short i = 0; i < 100; ++i)
    data.Add(new MyStructData{Id = i, Name = i.ToString()});

string filename = "C:\\test\\test.data";

using (var file = File.OpenWrite(filename))
{
    var writer = new BinaryFormatter();
    writer.Serialize(file, data); // Writes the entire list.
}

And you can read them back like so:

using (var file = File.OpenRead(filename))
{
    var reader = new BinaryFormatter();
    data = (List<MyStructData>) reader.Deserialize(file); // Reads the entire list.
}

foreach (var item in data)
    Console.WriteLine("Id = {0}, Name = {1}", item.Id, item.Name);

If the only reason you added the marshalling to the struct was so you can write and read it to and from a file, you could then remove it so your struct would look like this:

[Serializable]
public struct MyStructData
{
    public short Id;
    public string Name;
}

---

[EDIT]

I come from the future to note that BinaryFormatter is now considered insecure :

The BinaryFormatter type is dangerous and is not recommended for data processing. Applications should stop using BinaryFormatter as soon as possible, even if they believe the data they're processing to be trustworthy. BinaryFormatter is insecure and can't be made secure.

A much better modern binary serialization format to use is MessagePack-CSharp available via NuGet.