c# - 在c#中向数据表添加行

Question

我正在做一个项目，我想向数据表中添加行，如果我从数据库中获取两行，则只有其中之一被添加到数据表中，但我希望两者都被添加到数据表，即数据表无法保存第一行的数据，数据正在被第二行的数据覆盖。该方法的代码是。这是我的电话声明。

    DataTable dttable = new DataTable();
    dttable = gettable(dtgreater, dtcurrentdate);
    public DataTable gettable(List<DateTime> objct1, DateTime objct2)
    {

    DataTable data=null;
    for (int j = 0; j < dtgreater.Count; j++)
    {
        sql = "select library_issue.STUDENTCODE,library_issue.studentname,library_book.bookname,library_issue.issuedate,library_issue.returndate from library_issue             join library_book on library_book.book_id = library_issue.book_id where library_issue.returndate ='" + objct1[j].ToString("dd/MM/yyyy") + "'";
        ds = obj.openDataset(sql, Session["SCHOOLCODE"].ToString());
        Label1.Text = (ds.Tables[0].Rows.Count).ToString();
        data = new DataTable();
        data.Columns.Add("STUDENTCODE", typeof(int));
        data.Columns.Add("Studentname", typeof(string));
        data.Columns.Add("Bookname", typeof(string));
        data.Columns.Add("Issuedate", typeof(string));
        data.Columns.Add("Returndate", typeof(string));
        data.Columns.Add("NO of Days Exceeded", typeof(string));
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {

            TimeSpan ts = objct1[j] - objct2;
            Label1.Text = ts.ToString("dd");
            data.Rows.Add(ds.Tables[0].Rows[i]["STUDENTCODE"], ds.Tables[0].Rows[i]["studentname"], ds.Tables[0].Rows[i]["bookname"], ds.Tables[0].Rows[i]["issuedate"], ds.Tables[0].Rows[i]["returndate"], ts.ToString("dd"));

        }

    }

     return data;
} 

更新:

   public DataTable gettable(List<DateTime> objct1, DateTime objct2)
    {

        DataTable data = new DataTable();

        data.Columns.Add("STUDENTCODE", typeof(int));
        data.Columns.Add("Studentname", typeof(string));
        data.Columns.Add("Bookname", typeof(string));
        data.Columns.Add("Issuedate", typeof(string));
        data.Columns.Add("Returndate", typeof(string));
        data.Columns.Add("NO of Days Exceeded", typeof(string));
    for (int j = 0; j < dtgreater.Count; j++)
    {
        sql = "select library_issue.STUDENTCODE,library_issue.studentname,library_book.bookname,library_issue.issuedate,library_issue.returndate from library_issue             join library_book on library_book.book_id = library_issue.book_id where library_issue.returndate ='" + objct1[j].ToString("dd/MM/yyyy") + "'";
        ds = obj.openDataset(sql, Session["SCHOOLCODE"].ToString());
        Label1.Text = (ds.Tables[0].Rows.Count).ToString();

        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {

            TimeSpan ts = objct1[j] - objct2;
            Label1.Text = ts.ToString("dd");
            data.Rows.Add(ds.Tables[0].Rows[i]["STUDENTCODE"], ds.Tables[0].Rows[i]["studentname"], ds.Tables[0].Rows[i]["bookname"], ds.Tables[0].Rows[i]["issuedate"], ds.Tables[0].Rows[i]["returndate"], ts.ToString("dd"));

        }

    }

     return data;
} 

描述:当我在 for 循环中初始化数据表时，导致每次都创建新的数据表实例。 结果我每次只得到一行。

Answer 1

您正在循环内创建数据表，因此在第二次迭代中，它将丢弃第一个数据表中的第一项，并为第二项创建一个新的空表。

创建数据表并在循环之前向其中添加列:

DataTable dttable = new DataTable();
dttable = gettable(dtgreater, dtcurrentdate);

public DataTable gettable(List<DateTime> objct1, DateTime objct2)
{

  DataTable data = new DataTable();
  data.Columns.Add("STUDENTCODE", typeof(int));
  data.Columns.Add("Studentname", typeof(string));
  data.Columns.Add("Bookname", typeof(string));
  data.Columns.Add("Issuedate", typeof(string));
  data.Columns.Add("Returndate", typeof(string));
  data.Columns.Add("NO of Days Exceeded", typeof(string));
  for (int j = 0; j < dtgreater.Count; j++) {
    sql = "select library_issue.STUDENTCODE,library_issue.studentname,library_book.bookname,library_issue.issuedate,library_issue.returndate from library_issue             join library_book on library_book.book_id = library_issue.book_id where library_issue.returndate ='" + objct1[j].ToString("dd/MM/yyyy") + "'";
    ds = obj.openDataset(sql, Session["SCHOOLCODE"].ToString());
    Label1.Text = (ds.Tables[0].Rows.Count).ToString();
    for (int i = 0; i < ds.Tables[0].Rows.Count; i++) {

        TimeSpan ts = objct1[j] - objct2;
        Label1.Text = ts.ToString("dd");
        data.Rows.Add(ds.Tables[0].Rows[i]["STUDENTCODE"], ds.Tables[0].Rows[i]["studentname"], ds.Tables[0].Rows[i]["bookname"], ds.Tables[0].Rows[i]["issuedate"], ds.Tables[0].Rows[i]["returndate"], ts.ToString("dd"));

    }

  }

  return data;
}