如何通过 IDataReader 读取图像

Question

我在 C# vs05 窗口上工作。我想通过 IDataReader....在 oledb 中读取图像....假设我想从数据库列名学生 id 中读取 int 值然后我用下面的方式编写代码

private IDataReader _reader;

public int GetInt32(String sFieldName)
        {
            return (_reader[sFieldName]==DBNull.Value)? (Int32) 0 :Convert.ToInt32(_reader[sFieldName]);
        }

为了保存图像，我使用下面的代码

     DataTable oTable = new DataTable();
                   oTable.Columns.Add("BoardImage");
                   DataRow oRow = oTable.NewRow();
                   oRow["BoardImage"] = oItem.BoardImage;
               SQL= Insert into table oRow["BoardImage"];



Now i want to read image BY Idatareader ......to do that i write bellow code but i get error message "Parameter is not valid."



    public Image GetImage(String sFieldName)
        {
            try
            {
                //MemoryStream stream = new MemoryStream();
                //stream.Write(image, 0, image.Length);
                //Bitmap bitmap = new Bitmap(stream);

                byte[] imageData = null;
                long byteSize = 0;
                byteSize = _reader.GetBytes(_reader.GetOrdinal(sFieldName), 0, null, 0, 0);  **//May be error occur here**

                imageData = new byte[byteSize];
                long bytesread = 0;
                int curpos = 0, chunkSize = 500;
                while (bytesread < byteSize)
                {
                    // chunkSize is an arbitrary application defined value 
                    bytesread += _reader.GetBytes(_reader.GetOrdinal(sFieldName), curpos, imageData, curpos, chunkSize);
                    curpos += chunkSize;
                }

                byte[] imgData = imageData;

                MemoryStream ms = new MemoryStream(imgData); **//May be error occur here**
                Image oImage = Image.FromStream((Stream)ms);
                return oImage;    



            }
            catch(Exception ex)
            {
                throw new Exception("Failed to get Image: " + ex.Message);
            }
        }

任何人都可以帮助我吗？我的错误在哪里.....如何解决这个问题

Answer 1

尝试类似（确保阅读器处于顺序模式）：

using(MemoryStream ms = new MemoryStream()) {
    const int BUFFER_SIZE = 1024;
    byte[] buffer = new byte[BUFFER_SIZE];
    int col = reader.GetOrdinal(fieldName), bytesRead, offset = 0;
    while((bytesRead = (int)reader.GetBytes(col,offset,buffer,0,BUFFER_SIZE)) > 0) {
        ms.Write(buffer, 0, bytesRead);
        offset += bytesRead;
    }
    ms.Position = 0;
    return Image.FromStream(ms);
}

我在原始代码中看到的最大问题是没有过多关注每次调用 GetBytes 时返回的字节数（即假设你有一个完整的块）。