In this article, we will learn how to convert a file into byte array and get back the original file from the byte array via FileUpload control.

Step 1: Create a page having control FileUpload and Button

<div>
   <asp:FileUpload ID="FileUpload1" runat="server"  />
   <asp:Button ID="btnGetFile" runat="server" Text="GetFile" OnClick="btnGetFile_Click"/>
</div>

Step 2: On button click, call function FileToByteArray and GetDocument

 protected void btnGetFile_Click(object sender, EventArgs e)
    {
        string fileName = FileUpload1.PostedFile.FileName;

        //To convert file into byte array
        byte[] byteArray = FileToByteArray(FileUpload1.PostedFile.InputStream);

        //To get back file from byte array
        GetDocument(fileName, byteArray);
    }

Step 3: Write FileToByteArray function to convert file into byte array

public byte[] FileToByteArray(Stream fileStream)
    {
        byte[] buffer = new byte[fileStream.Length];
        using (MemoryStream ms = new MemoryStream())
        {
            int read;
            while ((read = fileStream.Read(buffer, 0, buffer.Length)) > 0)
            {
                ms.Write(buffer, 0, read);
            }
            return ms.ToArray();
        }
    }

Step 4: Write GetDocument fumction to  get back the original file from byte array

 private void GetDocument(string fileName, byte[] fileContent)
    {
        try
        {
            //Split the string by character . to get file extension type
            string[] stringParts = fileName.Split(new char[] { '.' });
            string strType = stringParts[1];
            Response.Clear();
            Response.ClearContent();
            Response.ClearHeaders();
            Response.AddHeader("content-disposition", "attachment; filename=" + fileName.Replace(" ",""));
            //Set the content type as file extension type
            Response.ContentType = strType;
            //Write the file content
            this.Response.BinaryWrite(fileContent);
            this.Response.End();
        }
        catch (Exception ex)
        {

        }
    }

Hope, this will help you!