Converting a byte() array to a double in VB.Net

Question

Hi, I have this situation. I have a real stored in a varbinary field in a sql 2005 database. As I can't convert a varbinary to a real in sql 2005, I'm trying to do that in vb.net.

That field gets stored as a byte() array in a DataTable.

Now I would like to read that byte() into a double, or decimal variable. But I don't have much of a clue on how to do that...

Answer 1

How is it stored in the varbinary field? String representation or actual binary representation?

Answer 2

It really depends on how it's stored, but BitConverter.ToDouble may be your friend. That's assuming it's in IEE754 format. Where are you getting the data from in the first place?

Answer 3

I don't know VB.net well, but know the .NET libraries.

Wrap the byte[] in a MemoryStream and wrap that in a BinaryReader. Then use the BinaryReader.ReadDouble() method. See here and here for MSDN pages.

Edit in response to this

You are looking for a piece of code looking like this:

//declare a test array
Dim testArray As Byte() = {0, 0, 0, 0}
//wrap it into a memory stream
Dim memStream As MemoryStream = new MemoryStream(testArray)
//wrap the stream in a binary reader
Dim bReader As BinaryReader = new BinaryReader(memStream)
//read a 32bit integer from the stream using the reader
Dim count As Integer = bReader.ReadInt32();

Answer 4

Jon Skeet, it doesn't work...

biozinc, I'm trying to do as you say but I'm getting a "Unable to read beyond the end of the stream" exception.

This is my code:

Dim con As SqlConnection = New SqlConnection("Server=OMNILOGADMIN-PC;Database=master;Trusted_Connection=True;")
Dim cmd As SqlCommand = New SqlCommand()
Dim da As SqlDataAdapter = New SqlDataAdapter()
Dim dt As DataTable = New DataTable()
Dim ba As Byte()
Dim val As Decimal
Dim memStream As MemoryStream = New MemoryStream()
Dim bReader As BinaryReader

cmd.CommandText = "select convert(varbinary(max), cast(500 as real)) val"
cmd.Connection = con

da.SelectCommand = cmd

Try
    con.Open()
    da.Fill(dt)
    ba = dt.Rows(0).Item(0)
    'val = BitConverter.ToDouble(ba, 0)
    memStream.Write(ba, 0, ba.Length)

    MessageBox.Show( _
     memStream.Capacity.ToString() & vbCrLf & _
     memStream.Length.ToString() & vbCrLf & _
     memStream.Position.ToString())

    bReader = New BinaryReader(memStream)
    memStream.Seek(0, SeekOrigin.Begin)
    Dim testArray As Byte() = {0, 0, 0, 0}
    Dim count As Integer = bReader.Read(testArray, 0, 3)

    MessageBox.Show(count & " - " & bReader.ReadDecimal())

Catch ex As Exception
    MessageBox.Show("Erro: " & vbCrLf & ex.Message)
Finally
    con.Close()
End Try

Any clues/sugestions? thanks a lot!

Answer 5

Public Function GetDateFromBytes(ByRef value() As Byte, _
                                    ByRef startindex As Int32) As Date
    'create a aray of Ints
    Dim IntValues() As Int32 = {BitConverter.ToInt32(value, startindex), _
                                    BitConverter.ToInt32(value, (startindex + 7)), _
                                    BitConverter.ToInt32(value, startindex + 15), _
                                     BitConverter.ToInt32(value, startindex + 31)}

    Return Date.FromBinary(New Decimal(IntValues))

End Function