How to write numbers to a file and make them readable between Java and C#

Question

I'm into a "compatibility" issue between two versions of the same program, the first one written in Java, the second it's a port in C#.

My goal is to write some data to a file (for example, in Java), like a sequence of numbers, then to have the ability to read it in C#. Obviously, the operation should work in the reversed order.

For example, I want to write 3 numbers in sequence, represented with the following schema:

first number as one 'byte' (4 bit)
second number as one 'integer' (32 bit)
third number as one 'integer' (32 bit)

So, I can put on a new file the following sequence: 2 (as byte), 120 (as int32), 180 (as int32)

In Java, the writing procedure is more or less this one:

FileOutputStream outputStream;
byte[] byteToWrite;
// ... initialization....

// first byte
outputStream.write(first_byte);

// integers
byteToWrite = ByteBuffer.allocate(4).putInt(first_integer).array();
outputStream.write(byteToWrite);
byteToWrite = ByteBuffer.allocate(4).putInt(second_integer).array();
outputStream.write(byteToWrite);

outputStream.close();

While the reading part it's the following:

FileInputStream inputStream;
ByteBuffer byteToRead;
// ... initialization....

// first byte
first_byte = inputStream.read();

// integers
byteToRead = ByteBuffer.allocate(4);
inputStream.read(byteToRead.array());
first_integer = byteToRead.getInt();

byteToRead = ByteBuffer.allocate(4);
inputStream.read(byteToRead.array());
second_integer = byteToRead.getInt();

inputStream.close();

C# code is the following. Writing:

FileStream fs;
byte[] byteToWrite;
// ... initialization....

// first byte
byteToWrite = new byte[1];
byteToWrite[0] = first_byte;
fs.Write(byteToWrite, 0, byteToWrite.Length);

// integers
byteToWrite = BitConverter.GetBytes(first_integer);
fs.Write(byteToWrite, 0, byteToWrite.Length);
byteToWrite = BitConverter.GetBytes(second_integer);
fs.Write(byteToWrite, 0, byteToWrite.Length);

Reading:

FileStream fs;
byte[] byteToWrite;
// ... initialization....

// first byte
byte[] firstByteBuff = new byte[1];
fs.Read(firstByteBuff, 0, firstByteBuff.Length);
first_byte = firstByteBuff[0];

// integers
byteToRead = new byte[4 * 2];
fs.Read(byteToRead, 0, byteToRead.Length);
first_integer = BitConverter.ToInt32(byteToRead, 0);
second_integer = BitConverter.ToInt32(byteToRead, 4);

Please note that both the procedures works when the same Java/C# version of the program writes and reads the file. The problem is when I try to read a file written by the Java program from the C# version and viceversa. Readed integers are always "strange" numbers (like -1451020...).

There's surely a compatibility issue regarding the way Java stores and reads 32bit integer values (always signed, right?), in contrast to C#. How to handle this?

Jon Skeet · Accepted Answer · 2014-03-21 13:42:39Z

2

It's just an endian-ness issue. You can use my MiscUtil library to read big-endian data from .NET.

However, I would strongly advise a simpler approach to both your Java and your .NET:

In Java, use DataInputStream and DataOutputStream. There's no need to get complicated with ByteBuffer etc.
In .NET, use EndianBinaryReader from MiscUtil, which extends BinaryReader (and likewise EndianBinaryWriter for BinaryWriter)

Alternatively, consider just using text instead.

answered Mar 21, 2014 at 13:42

Jon Skeet

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1 Comment

TheUnexpected Over a year ago

Ok thanks, I'll try but I won't be able to use your personal library MiscUtil. Is there something that's "standard"?

Tim S. · Accepted Answer · 2014-03-21 14:01:14Z

I'd consider using a standard format like XML or JSON to store your data. Then you can use standard serializers in both Java and C# to read/write the file. This sort of approach lets you easily name the data fields, read it from many languages, be easily understandable if someone opens the file in a text editor, and more easily add data to be serialized.

E.g. you can read/write JSON with Gson in Java and Json.NET in C#. The class might look like this in C#:

public class MyData
{
    public byte FirstValue { get; set; }
    public int SecondValue { get; set; }
    public int ThirdValue { get; set; }
}

// serialize to string example
var myData = new MyData { FirstValue = 2, SecondValue = 5, ThirdValue = -1 };
string serialized = JsonConvert.SerializeObject(myData);

It would serialize to

{"FirstValue":2,"SecondValue":5,"ThirdValue":-1}

The Java would, similarly, be quite simple. You can find examples of how to read/write files in each library.

Or if an array would be a better model for your data:

string serialized = JsonConvert.SerializeObject(new[] { 2, 5, -1 }); // [2,5,-1]

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