如何连接数据表

Question

我正在加入数据表以创建一个新的数据表。这是代码：

var row = from r0w1 in dt_vi.AsEnumerable()
                                  join r0w2 in dt_w.AsEnumerable()
                                    on r0w1.Field<int>("wcID") equals r0w2.Field<int>("iD")
                                  join r0w3 in dt_r.AsEnumerable()
                                    on r0w1.Field<int?>("cID") equals r0w3.Field<int?>("r_id")
                                  join r0w4 in dt_d.AsEnumerable()
                                    on r0w1.Field<int?>("dftID") equals r0w4.Field<int?>("dft_id") into ps
                                  from r0w4 in ps.DefaultIfEmpty()
                                  select r0w1.ItemArray.Concat(r0w2.ItemArray.Concat(r0w3.ItemArray.Concat(r0w4 != null ? r0w4.ItemArray : new object[] { }))).ToArray();

这段代码可以工作，但是当我尝试下面的代码时，它不工作并返回 null。

var row = from r0w1 in dt_vi.AsEnumerable()
                                  join r0w2 in dt_w.AsEnumerable()
                                    on r0w1.Field<int>("wcID") equals r0w2.Field<int>("iD")
                                  join r0w3 in dt_r.AsEnumerable()
                                    on r0w1.Field<int?>("cID") equals r0w3.Field<int?>("r_id")
                                  join r0w4 in dt_d.AsEnumerable()
                                    on r0w1.Field<int?>("dftID") equals r0w4.Field<int?>("dft_id") into ps
                                  from r0w4 in ps.DefaultIfEmpty()
                                  select new
                                  {
                                      cID = r0w1.Field<int?>("cID"),
                                      status = r0w1.Field<int?>("status"),
                                      defectID = r0w1.Field<int?>("dftID"),
                                      name = r0w2.Field<string>("name"),
                                      description = r0w3.Field<string>("description"),
                                      dftAID = r0w4.Field<string>("dftAID"),
                                      dftName = r0w4.Field<string>("dftName")
                                  };

                    row.ToList().ForEach(q=>dt.Rows.Add(q.ID,q.status,q.dfID,q.name,q.description,q.dftAID,q.dftName));

我得到的错误是

Value cannot be null.
Parameter name: row

为什么上面的代码返回 null ？

Answer 1

您能否检查它不是来自这些指令以及 Datarow r0w4 是否为空？

dftAID = r0w4.Field<string>("dftAID")
dftName = r0w4.Field<string>("dftName")

在工作代码中，您明确检查 r0w4 的非空大小写。

在这种情况下，您可以像这样添加一些非空检查：

     var row = from r0w1 in dt_vi.AsEnumerable()
        join r0w2 in dt_w.AsEnumerable()
        on r0w1.Field<int>("wcID") equals r0w2.Field<int>("iD")
        join r0w3 in dt_r.AsEnumerable()
        on r0w1.Field<int?>("cID") equals r0w3.Field<int?>("r_id")
        join r0w4 in dt_d.AsEnumerable()
        on r0w1.Field<int?>("dftID") equals r0w4.Field<int?>("dft_id") into ps
        from r0w4 in ps.DefaultIfEmpty()
        select new
        {
            cID = r0w1.Field<int?>("cID"),
            status = r0w1.Field<int?>("status"),
            defectID = r0w1.Field<int?>("dftID"),
            name = r0w2.Field<string>("name"),
            description = r0w3.Field<string>("description"),
            dftAID = r0w4 != null ? r0w4.Field<string>("dftAID") : "",
            dftName = r0w4 != null ? r0w4.Field<string>("dftName") : ""
        };

row.ToList().ForEach(q => dt.Rows.Add(q.cID, q.status, q.defectID, q.name, q.description, q.dftAID, q.dftName));

Answer 2

确保查询中的列名与表匹配。我做了一些修改并让下面的代码工作

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            DataTable dt_vi = new DataTable();
            dt_vi.Columns.Add("wcID", typeof(int));
            dt_vi.Columns.Add("cID", typeof(int));
            dt_vi.Columns["cID"].AllowDBNull = true;
            dt_vi.Columns.Add("dftID", typeof(int));
            dt_vi.Columns["dftID"].AllowDBNull = true;
            dt_vi.Columns.Add("status", typeof(int));
            dt_vi.Columns["status"].AllowDBNull = true;

            DataTable dt_w = new DataTable();
            dt_w.Columns.Add("iD", typeof(int));
            dt_w.Columns.Add("name", typeof(string));

            DataTable dt_r = new DataTable();
            dt_r.Columns.Add("r_id", typeof(int));
            dt_r.Columns["r_id"].AllowDBNull = true;
            dt_r.Columns.Add("description", typeof(string));

            DataTable dt_d = new DataTable();
            dt_d.Columns.Add("dft_id", typeof(int));
            dt_d.Columns["dft_id"].AllowDBNull = true;
            dt_d.Columns.Add("dftAID", typeof(string));
            dt_d.Columns.Add("dftName", typeof(string));

            DataTable dt = new DataTable();
            dt.Columns.Add("qID", typeof(int));
            dt.Columns.Add("status", typeof(int));
            dt.Columns.Add("dfID", typeof(int));
            dt.Columns.Add("name", typeof(string));
            dt.Columns.Add("description", typeof(string));
            dt.Columns.Add("dftAID", typeof(string));
            dt.Columns.Add("dftName", typeof(string));


            dt_vi.Rows.Add(new object[] { 1, 1, 1, 1 });
            dt_vi.Rows.Add(new object[] { 2, 2, 2, 2 });
            dt_vi.Rows.Add(new object[] { 3, 3, 3, 3 });
            dt_vi.Rows.Add(new object[] { 4, 4, 4, 4 });
            dt_vi.Rows.Add(new object[] { 5, 5, 5, 5 });

            dt_w.Rows.Add(new object[] { 1, "a" });
            dt_w.Rows.Add(new object[] { 2, "b" });
            dt_w.Rows.Add(new object[] { 3, "c" });
            dt_w.Rows.Add(new object[] { 4, "d" });
            dt_w.Rows.Add(new object[] { 5, "e" });


            dt_r.Rows.Add(new object[] { 1, "f" });
            dt_r.Rows.Add(new object[] { 2, "g" });
            dt_r.Rows.Add(new object[] { 3, "h" });
            dt_r.Rows.Add(new object[] { 4, "i" });
            dt_r.Rows.Add(new object[] { 5, "j" });

            dt_d.Rows.Add(new object[] { 1, "k", "k" });
            dt_d.Rows.Add(new object[] { 2, "l", "l" });
            dt_d.Rows.Add(new object[] { 3, "m", "m" });
            dt_d.Rows.Add(new object[] { 4, "n", "n" });
            dt_d.Rows.Add(new object[] { 5, "o", "o" });

            var row = from r0w1 in dt_vi.AsEnumerable()
                      join r0w2 in dt_w.AsEnumerable()
                        on r0w1.Field<int>("wcID") equals r0w2.Field<int>("iD")
                      join r0w3 in dt_r.AsEnumerable()
                        on r0w1.Field<int?>("cID") equals r0w3.Field<int?>("r_id")
                      join r0w4 in dt_d.AsEnumerable()
                        on r0w1.Field<int?>("dftID") equals r0w4.Field<int?>("dft_id") into ps
                      from r0w4 in ps.DefaultIfEmpty()
                      select r0w1.ItemArray.Concat(r0w2.ItemArray.Concat(r0w3.ItemArray.Concat(r0w4 != null ? r0w4.ItemArray : new object[] { }))).ToArray();

            var rowAgain = from r0w1 in dt_vi.AsEnumerable()
                      join r0w2 in dt_w.AsEnumerable()
                        on r0w1.Field<int>("wcID") equals r0w2.Field<int>("iD")
                      join r0w3 in dt_r.AsEnumerable()
                        on r0w1.Field<int?>("cID") equals r0w3.Field<int?>("r_id")
                      join r0w4 in dt_d.AsEnumerable()
                        on r0w1.Field<int?>("dftID") equals r0w4.Field<int?>("dft_id") into ps
                      from r0w4 in ps.DefaultIfEmpty()
                      select new { 
                          cID = r0w1.Field<int?>("cID"),
                          status = r0w1.Field<int?>("status"),
                          defectID = r0w1.Field<int?>("dftID"),
                          name = r0w2.Field<string>("name"),
                          description = r0w3.Field<string>("description"),
                          dftAID = r0w4.Field<string>("dftAID"),
                          dftName = r0w4.Field<string>("dftName")
                      };

            foreach(var q in rowAgain.AsEnumerable())
            {
                dt.Rows.Add(q.cID, q.status, q.defectID, q.name, q.description, q.dftAID, q.dftName);
            }

        }
    }
}
​