Convert Big file to bytes in c# win. form

OpenFileDialog f = new OpenFileDialog();
f.ShowDialog();
if (f.FileName != null && f.FileName != "")
{
        string filePath = f.FileName;
        string connectionString = Program.connectionString;
        FileStream stream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
        BinaryReader reader = new BinaryReader(stream);
        byte[] file = reader.ReadBytes((int)stream.Length);//This line gives error
        reader.Close();
        stream.Close();
}