Converting data of datacolumn type Byte[] back to Byte Array

_{Thursday, March 10, 2011 6:09 AM}

Hi, I'm storing a serialized object in a datatable. The column is of type String.Byte[]

Here's how the data is stored:

Dim ms As New System.IO.MemoryStream
Dim bf As New System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
bf.Serialize(ms, myObject)
ms.Position = 0
DataSet.Tables(0).Rows(0)("QualEvaluationData") = ms.ToArray

Which seems to work fine. When I go to deserialize the data with the following:

Dim ms As New System.IO.MemoryStream(CType(DataSet.Tables(0).Rows(0)("QualEvaluationData"), Byte()))
Dim bf As New System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
Dim myObject As MyObjectType = bf.Deserialize(ms)
Return myObject

I get the following conversion error on the first line: Unable to cast object of type 'System.String' to type 'System.Byte[]'.

Any thoughts?

_{Wednesday, March 16, 2011 7:19 AM ✅Answered | 1 vote}

Hi naht,

Thanks for your feedback.

During solving this issue, I found some solutions about  Unable to cast object of type 'System.String' to type 'System.Byte[]' in the codeproject.

The original code as below:

  cmd.CommandText = "select_student";
            cmd.Parameters.AddWithValue("@sno",textBox1.Text);
            da = new SqlDataAdapter(cmd);
            ds = new DataSet();
            da.Fill(ds, "student");
            if (ds.Tables[0].Rows.Count > 0)
            {
                textBox2.Text = ds.Tables[0].Rows[0][0].ToString();
                textBox3.Text = ds.Tables[0].Rows[0][1].ToString();
                if (ds.Tables[0].Rows[0][2] != System.DBNull.Value)
                {
                    byte[] data = (byte[])ds.Tables[0].Rows[0][2];
                    ms = new MemoryStream(data);
                    pictureBox1.Image = Image.FromStream(ms);
                }
                else
                    pictureBox1.Image = null;
            }
            else
                MessageBox.Show("Record does not exixts");

These codes can reflect the error.

It looks like in your database you your image column
(ds.Tables[0].Rows[0][2])
has data type varchar rather than image(or varbinary(max)).
When you try to cast the string(varchar) into array of bytes it returns you the error.
If this is so it will not work.

You have to change the data type for the image column into the proper format and try the code again.

A simple cast is not enough in this case.

Try :

byte[] data = System.Text.UTF8Encoding.GetBytes(ds.Tables[0].Rows[0][2]);

I used UTF8Encoding as an example, substitute it wirh your needed encoding.

If your database is holding the field as a NVCHAR or similar, then it is indeed a string. In which case use

string s = (string)ds.Tables[0].Rows[0][2];
byte[] data = System.Text.Encoding.ASCII.GetBytes(s);
ms = new MemoryStream(data);
pictureBox1.Image = Image.FromStream(ms);

BTW: try not to use "magic numbers" for your code - it makes it hard to read and understand when you come back to maintain it.

Tables[0]

and

Rows[0]

are ok - ish - but

Rows[0][2]

is not as readable as

Rows[0]["DataStream"]

More infomation, see here.

I hoep this can help.

have a nice day,

Jackie Sun [MSFT]
MSDN Community Support | Feedback to us
Get or Request Code Sample from Microsoft
Please remember to mark the replies as answers if they help and unmark them if they provide no help.

_{Sunday, March 13, 2011 7:40 AM}

Hello naht,

Welcome to the MSDN Forum and thanks for posting here.

According to your description, I think a sample from CodeProject can help you. In this sample we are storing a dataset object in a binary file using object serializtion , and after serializtion process we will make deserialization to get all inforamtion from tables without connectiing to database

Here it is:

Imports System.Data
Imports System.Data.SqlClient
Imports System.Runtime.Serialization.Formatters.Binary
Imports System.IO
Module Module1
  Private connStr As String = "Data Source=.\SQLEXPRESS;Database=Northwind;Integrated Security=True"
  Private _file As String = "C:\ds.dat"
  Sub Main()
    Dim obj As StoringDataSetInFile = New StoringDataSetInFile(connStr, _file)
    obj.StoreInFile()
    obj.GetFromFile()
    Dim employees As List(Of String) = obj.GetDataFromDataSet
    For Each employee As String In employees
      Console.WriteLine(employee)
    Next
    Console.Read()
  End Sub
End Module
Public Class StoringDataSetInFile
  Private data As DataSet
  Private connStr As String
  Private _file As String
  Sub New(ByVal connStr As String, ByVal _file As String)
    Me.connStr = connStr
    Me._file = _file
  End Sub
  Sub StoreInFile()
    Dim conn As New SqlConnection
    conn.ConnectionString = connStr
    Try
      conn.Open()
      Dim cmd As New SqlCommand
      cmd.Connection = conn
      cmd.CommandText = "Select * From Employees"
      Dim ada As New SqlDataAdapter
      ada.SelectCommand = cmd
      data = New DataSet
      ada.Fill(data, "Employees")
      conn.Close()
      Dim fs As FileStream = File.Create(Me._file)
      Dim binFormatter As New BinaryFormatter
      binFormatter.Serialize(fs, data)
      fs.Close()
      binFormatter = Nothing
      data = Nothing
    Catch ex As Exception
      Console.WriteLine("Error :" & ex.ToString)
    End Try
  End Sub
  Sub GetFromFile()
    Dim fs As FileStream = File.Open(Me._file, FileMode.Open)
    Dim binFormatter As New BinaryFormatter
    data = CType(binFormatter.Deserialize(fs), DataSet)
    fs.Close()
  End Sub
  Function GetDataFromDataSet() As List(Of String)
    Dim dt As New DataTable
    dt = data.Tables("Employees")
    Dim lstFullNames As New List(Of String)
    For Each _row As DataRow In dt.Rows
      Dim str As String = _row("firstname") & " " & _row("lastname")
      lstFullNames.Add(str)
    Next
    Return lstFullNames
  End Function
End Class

I hope this can help you.

Have a nice weekend!

Jackie Sun [MSFT]
MSDN Community Support | Feedback to us
Get or Request Code Sample from Microsoft
Please remember to mark the replies as answers if they help and unmark them if they provide no help.

_{Monday, March 14, 2011 6:02 AM}

Thanks Jackie, that's kind of close, and i didn't have any problem storing a serialized dataset to a file, but the example I'm working on is serializing an object to a field in the dataset so that it can be stored in the database.  Maybe I'm going about it wrong.  The object is a very expensive object to create (it's actually a list of objects), and I want to be able to store different versions of it (along with the parameters used to create it) in a table in the database so that other users can recall the data.