ar4ebaldello

var dateFromString = txtSelectByDate1.Text;
var dateToString = txtSelectByDate2.Text;

if (!string.IsNullOrEmpty(dateFromString) && !string.IsNullOrEmpty(dateToString))                
{
    var dateFrom = Convert.ToDateTime(dateFromString).ToString("yyyy-MM-dd");
    var dateTo = Convert.ToDateTime(dateToString).ToString("yyyy-MM-dd");
    
    var query = string.Format("Select * from Purchase where P_Date >= '{0}' and P_Date <= '{1}'", dateFrom, dateTo);
    
    var command = new SqlCommand(query, cn);
    var adapter = new SqlDataAdapter(command);
    var table = new DataTable();
    adapter.Fill(table);
    
    gvPurchase.DataSourceID = null;
    gvPurchase.DataSource = table;
}

using (var client = new System.Net.WebClient() { Encoding = Encoding.UTF8 })
{
    // Скачиваем код страницы
    var page = client.DownloadString("http://PageAddress");
    // Находим ссылку
    var source = System.Text.RegularExpressions.Regex.Match(page, $"(?<=href=\").*?\\\.mp3(?=\")").Value;
    // Скачиваем файл
    client.DownloadFile(source, "audio.mp3");
}

var stream1 = new BinaryReader(File.OpenRead("someFile1.bin"));
var stream2 = new BinaryReader(File.OpenRead("someFile2.bin"));
var output = new BinaryWriter(File.OpenWrite("output.bin"));

if (stream1.PeekChar() != -1 && stream2.PeekChar() != -1)
{
    var val1 = stream1.ReadInt32();
    var val2 = stream2.ReadInt32();

    while (true)
    {
        if (val1 == val2)
        {
            output.Write(val1);

            if (stream1.PeekChar() == -1 || stream2.PeekChar() == -1) break;
            val1 = stream1.ReadInt32();
            val2 = stream2.ReadInt32();
        }
        else if (val1 < val2)
        {
            if (stream1.PeekChar() == -1) break;
            val1 = stream1.ReadInt32();
        }
        else
        {
            if (stream2.PeekChar() == -1) break;
            val2 = stream2.ReadInt32();
        }
    }
}

stream1.Close();
stream2.Close();
output.Close();