.NET decimal cross-platform standard

Question

If I save C# decimal array to a binary blob with a binary writer, will I be able to seamlessly read this blob in Java or Python or R? Do these language have a corresponding data

Answer 1

The documentation for the System.Decimal type does not state what internal representation it uses (unlike System.Single and System.Double which use IEEE-754). Given this, you cannot assume that the raw binary state representation of any decimal number will be consistent between .NET Framework versions or even between physical machines running the same version.

I assume you'd be using the BinaryWriter.Write(Decimal value) method. The documentation for this method does not make any statement about the actual format being used.

Looking at the .NET Reference source code, we see that BinaryWriter.Write(Decimal) uses an internal method: Decimal.GetBytes(Byte[] buffer) which uses an undocumented format which might change in future or may differ per-machine.

However! The Decimal class does provide a public method GetBits which does make guarantees about the format of data it returns, so I suggest you use this instead:

// Writing
Int32[] bits = myDecimal.GetBits();

binaryWriter.Write( (byte)bits.Length );
foreach(Int32 component in bits) binaryWriter.Write( component );

// Reading
byte count = binaryReader.ReadByte();
Int32[] bits = new Int32[ count ];
for(Int32 i = 0; i < bits.Count; i++ ) {
    bits[i] = binaryReader.ReadInt32();
}

Decimal value = new Decimal( bits );

This approach uses 17 bytes to store a Decimal instance in a way that is documented and guaranteed to work regardless of the internal design of the Decimal type. 16 bytes store the actual value, and 1 byte stores the number of 32-bit integers needed to represent the decimal (in case the number ever changes from 4).