如何在linq中实现类似sql：从a.joinkey = b.join上的连接b中选择a。*，b.field1为b_field并接收c＃datatable？

Question

我的目标是：linq-join两个c＃DataTables并从第一个表中选择所有列（不枚举列），只从第二个列中选择1列，并获得结果作为DataTable：

  var result = from t in tt.AsEnumerable() join a in aa.AsEnumerable() on t.Field<DateTime>("time") equals a.Field<DateTime>("time")

 select new { t, aa_col1 = a.Field<int>("col1") }    
                ;

是的，该连接的结果无法转换为DataTable。

该问题的基本思想是：在编写linq-join时不要手动枚举tt的所有列（tt列的数量可能很大且是动态的）

我试图解决应用result.CopyToDataTable（）方法的问题，但是编译器产生了错误：类型<anonymous type System.Data.DataRow t，int aa_col1>不能在通用方法“DataTableExtensions”中用作类型T的参数。 CopyToDataTable（IEnumerable的）”

下面显示了测试表tt和aa的代码：

            DataTable tt = new DataTable();
            tt.Columns.Add("time", typeof(DateTime));
            tt.Columns.Add("col1", typeof(int));
            tt.Columns.Add("col2", typeof(int));
            tt.Columns.Add("col3", typeof(int));


            DataRow dr = tt.NewRow();
            dr["time"] = new DateTime(2018,10,5);     dr["col1"] = 24;            dr["col2"] = 14;            dr["col3"] = 15;
            tt.Rows.Add(dr);

            dr = tt.NewRow();
            dr["time"] = new DateTime(2018, 10, 6);  dr["col1"] = 4;            dr["col2"] = 43;            dr["col3"] = 58;
            tt.Rows.Add(dr);

            dr = tt.NewRow();
            dr["time"] = new DateTime(2018, 10, 6); dr["col1"] = 6; dr["col2"] = 3; dr["col3"] = 78;
            tt.Rows.Add(dr);

            dr = tt.NewRow();
            dr["time"] = new DateTime(2018, 10, 7);   dr["col1"] = 1; dr["col2"] = 4; dr["col3"] = 5;
            tt.Rows.Add(dr);

            //    -----
            DataTable aa = new DataTable();
            aa.Columns.Add("time", typeof(DateTime));
            aa.Columns.Add("col1", typeof(int));
            aa.Columns.Add("col2", typeof(int));
            aa.Columns.Add("col3", typeof(int));


            DataRow rr = aa.NewRow();
            rr["time"] = new DateTime(2018, 10, 4);
            rr["col1"] = 6; rr["col2"] = 34; rr["col3"] = 66;
            aa.Rows.Add(rr);

            rr = aa.NewRow();
            rr["time"] = new DateTime(2018, 10, 5);   rr["col1"] = 7; rr["col2"] = 43; rr["col3"] = 98;
            aa.Rows.Add(rr);

            rr = aa.NewRow();
            rr["time"] = new DateTime(2018, 10, 6);  rr["col1"] = 6; rr["col2"] = 3; rr["col3"] = 3;
            aa.Rows.Add(rr);

            rr = aa.NewRow();
            rr["time"] = new DateTime(2018, 10, 7); rr["col1"] = 16; rr["col2"] = 65; rr["col3"] = 12;
            aa.Rows.Add(rr);

linq-join生成类型：{System.Linq.Enumerable.d__38f__AnonymousType0>}

请参阅显示“var result”的图：

https://yadi.sk/i/X8K9HIq5hnavCg

和https://yadi.sk/i/MaViq23zqpRDXw

和https://yadi.sk/i/Re0DFJdVl02RjA

join的目标结果应该是一个表，其中包含来自tt和aa的第一列的所有列 - aa.col1：

  time             col1   col2  col3  aa_col1
   05.10.2018 0:00    24    14    15    7
   06.10.2018 0:00    4     43    58    6
   06.10.2018 0:00    6      3    78    6
   07.10.2018 0:00    1      4     5    16

Answer 1

所以我有一个与DataTables连接的扩展方法，它使用了一个由DataRows组成的匿名对象并创建了一个新的DataTable来保存连接数据，但是能够投影一些连接表是很好的，所以我补充说。它使用反射，所以性能不理想，但除非你这么做，否则应该没问题。

该代码使用了许多扩展方法，使反射更容易，并使用DataTables和IEnumerables。

public static class DataTableJoinExt {
    // ***
    // *** Type Extensions
    // ***
    public static List<MemberInfo> GetPropertiesOrFields(this Type t, BindingFlags bf = BindingFlags.Public | BindingFlags.Instance) =>
        t.GetMembers(bf).Where(mi => mi.MemberType == MemberTypes.Field || mi.MemberType == MemberTypes.Property).ToList();

    // ***
    // *** MemberInfo Extensions
    // ***
    public static Type GetMemberType(this MemberInfo member) {
        switch (member) {
            case FieldInfo mfi:
                return mfi.FieldType;
            case PropertyInfo mpi:
                return mpi.PropertyType;
            case EventInfo mei:
                return mei.EventHandlerType;
            default:
                throw new ArgumentException("MemberInfo must be if type FieldInfo, PropertyInfo or EventInfo", nameof(member));
        }
    }

    public static object GetValue(this MemberInfo member, object srcObject) {
        switch (member) {
            case FieldInfo mfi:
                return mfi.GetValue(srcObject);
            case PropertyInfo mpi:
                return mpi.GetValue(srcObject);
            default:
                throw new ArgumentException("MemberInfo must be of type FieldInfo or PropertyInfo", nameof(member));
        }
    }
    public static T GetValue<T>(this MemberInfo member, object srcObject) => (T)member.GetValue(srcObject);

    // ***
    // *** IEnumerable Extensions
    // ***

    public static IEnumerable<T> AsSingleton<T>(this T first) {
        yield return first;
    }

    // ***
    // *** DataTable Extensions
    // ***
    public static IEnumerable<DataColumn> DataColumns(this DataTable aTable) => aTable.Columns.Cast<DataColumn>();
    public static IEnumerable<string> ColumnNames(this DataTable aTable) => aTable.DataColumns().Select(dc => dc.ColumnName);

    // Create new DataTable from LINQ join results on DataTable
    // Expect T to be anonymous object of form new { [DataRow or other] d1, [DataRow or other] d2, ... }
    public static DataTable FlattenToDataTable<T>(this IEnumerable<T> src) {
        var res = new DataTable();
        if (src.Any()) {
            var firstRow = src.First();
            var memberInfos = typeof(T).GetPropertiesOrFields();
            var allDC = memberInfos.SelectMany(mi => (mi.GetMemberType() == typeof(DataRow)) ? mi.GetValue<DataRow>(firstRow).Table.DataColumns() : new DataColumn(mi.Name, mi.GetMemberType()).AsSingleton());

            foreach (var dc in allDC) {
                var newColumnName = dc.ColumnName;
                if (res.ColumnNames().Contains(newColumnName)) {
                    var suffixNumber = 1;
                    while (res.ColumnNames().Contains($"{newColumnName}.{suffixNumber}"))
                        ++suffixNumber;
                    newColumnName = $"{newColumnName}.{suffixNumber}";
                }
                res.Columns.Add(new DataColumn(newColumnName, dc.DataType));
            }

            foreach (var objRows in src)
                res.Rows.Add(memberInfos.SelectMany(mi => (mi.GetMemberType() == typeof(DataRow)) ? mi.GetValue<DataRow>(objRows).ItemArray : mi.GetValue(objRows).AsSingleton()).ToArray());
        }
        return res;
    }
}

现在定义了此扩展，您可以执行以下操作：

var result = (from t in tt.AsEnumerable()
              join a in aa.AsEnumerable() on t.Field<DateTime>("time") equals a.Field<DateTime>("time")
              select new { t, aa_col1 = a.Field<int>("col1") }).FlattenToDataTable();