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c# - 执行表连接后在 C# 中处理 lambda 查询结果

问题描述

所以我有一个将两个数据表连接在一起的查询：

var results = (
   from t1 in table1.AsEnumerable()
   join t2 in table2.AsEnumerable() on
        new { a = t1["col1"], b= t1["col2"], c = t1["col3"] } equals
        new { a= t2["col1"], b= t2["col2"], c= t2["col3"] }
   into joinedComboTable
   select joinedComboTable);

它产生一个类型为IEnumerable<IEnumerable<datarow>>"

我如何将其转换为DataTable？表 1 和表 2 是 C# DataTable。我确实看到了 304 个结果，我可以通过调试器和我拥有的 results.inner (Non-Public) 参数看到这些结果，DataColumn并且我可以看到 304 行。但我无法弄清楚如何获得实际结果并将其保存到DataTable.

更新：2020.01.23 @ 9:04pm

所以，我检查了下面的几个选项，当我执行 results.ToList() 时，我基本上得到了一个包含 304 个条目的列表，但每一行的值都是 System.Data.DataRow[0]。我肯定错过了什么....

Visual Studio QuickWatch 数据

对此进行迭代不会产生预期的结果。

标签： c#linqlambdadatatable

解决方案

尝试这个

    static void Main(string[] args)
    {
        var table1 = new DataTable();
        table1.Columns.Add("col1", typeof(string));
        table1.Columns.Add("col2", typeof(string));
        table1.Columns.Add("col3", typeof(string));
        table1.Columns.Add("col4", typeof(string));

        var row = table1.NewRow();
        row["col1"] = "1";
        row["col2"] = "1";
        row["col3"] = "1";
        row["col4"] = "something different";
        table1.Rows.Add(row);

        row = table1.NewRow();
        row["col1"] = "2";
        row["col2"] = "2";
        row["col3"] = "2";
        row["col4"] = "something different";
        table1.Rows.Add(row);

        var table2 = new DataTable();
        table2.Columns.Add("col1", typeof(string));
        table2.Columns.Add("col2", typeof(string));
        table2.Columns.Add("col3", typeof(string));
        table2.Columns.Add("col4", typeof(string));

        row = table2.NewRow();
        row["col1"] = "1";
        row["col2"] = "1";
        row["col3"] = "1";
        row["col4"] = "Another different thing";
        table2.Rows.Add(row);

        var results = (
            from t1 in table1.AsEnumerable()
            join t2 in table2.AsEnumerable() on
                new { a = t1["col1"], b = t1["col2"], c = t1["col3"] } equals
                new { a = t2["col1"], b = t2["col2"], c = t2["col3"] }
                into joinedComboTable
            select joinedComboTable).ToList();

        //Result
        var newTable = results.FirstOrDefault()?.CopyToDataTable();

        //However to get col4 form table 2 you need to do this
        var result2 = (
            from t1 in table1.AsEnumerable()
            join t2 in table2.AsEnumerable() on
                new { a = t1["col1"], b = t1["col2"], c = t1["col3"] } equals
                new { a = t2["col1"], b = t2["col2"], c = t2["col3"] }
            select new { a = t1["col1"], b = t1["col2"], c = t1["col3"], d = t1["col4"], e = t2["col4"] });

        //Result
        var newTable2 = table1.Clone();
        newTable2.Columns.Add("col4FromTable2", typeof(string));

        foreach (var x1 in result2)
        {
            var r = newTable2.NewRow();
            r["col1"] = x1.a;
            r["col2"] = x1.b;
            r["col3"] = x1.c;
            r["col4"] = x1.d;
            r["col4FromTable2"] = x1.e;
            newTable2.Rows.Add(r);
        }
    }

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