如何在c＃中将数据表转换为json字符串

Question

当数据表中的数据很小，即10列和300行时，它的工作非常完美。但是当数据表中的数据量很大时，它就无法工作。 这是我的代码

[WebMethod]

public static string getData()
{
    string _cs = "";
    string _query = "";
    OleDbConnection _conn = new OleDbConnection();
    OleDbCommand _cmd = new OleDbCommand();
    OleDbDataAdapter _da = new OleDbDataAdapter();
    DataTable _dt = new DataTable();

    try
    {
        _cs = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + _path + ";Extended Properties=  \"Excel 8.0;HDR=Yes;IMEX=1\";";
        //_query = "SELECT * FROM [3123121288$A1:L290]";  work perfect
        _query = "SELECT * FROM [3123121288$]";         //did not work
        _conn.ConnectionString = _cs;
        _cmd.Connection = _conn;
        _cmd.CommandText = _query;
        _da.SelectCommand = _cmd;
        _da.Fill(_dt);
    }
    catch (Exception ex)
    { }
    return JsonConvert.SerializeObject(_dt);
}