Question

我有一个包含4列的DataTable，如下所示：

    private static DataSet dataSet;
    private const string tableName = "MyTable";
    private const string columnName1 = "Supplier";  //Column names
    private const string columnName2 = "Invoice";
    private const string columnName3 = "Item";
    private const string columnName4 = "Amount";

我按供应商，发票列对表进行分组，并使用以下linq查询计算金额总和：

    private static DataTable GroupQueryA(DataTable dataTable)
    {
        DataTable groupedTable = dataTable.AsEnumerable()
            .GroupBy(r => new { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })
            .Select(g => new GroupSum
            {
                Key1 = g.Key.Key1,
                Key2 = g.Key.Key2,
                Sum = g.Sum(x => x.Field<double>(columnName4))
            }).PropertiesToDataTable<GroupSum>();

        return groupedTable;
    }

GroupSum类型我声明为：

    private class GroupSum
    {
        public string Key1 { get; set; }
        public string Key2 { get; set; }
        public Double Sum { get; set; }
    }

我复制的PropertiesToDataTable（）方法：

Convert Datatable GroupBy Multiple Columns with Sum using Linq

它非常完美，因此对于表行如：

    AddRow(dataTable, "SA", "INVA", "ITA", 10);
    AddRow(dataTable, "SA", "INVA", "ITB", 20);
    AddRow(dataTable, "SB", "INVB", "ITC", 50);

我在结果中收到2行：

    "SA", "INVA", 30
    "SB", "INVB", 50

但是我决定修改我的查询，以便不使用我编写的匿名类型：

    public class GroupKeys
    {
        public string Key1 { get; set; }
        public string Key2 { get; set; }
    }

和查询。

    private static DataTable GroupQueryB(DataTable dataTable)
    {
        DataTable groupedTable = dataTable.AsEnumerable()
            .GroupBy(r => new GroupKeys { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })
            .Select(g => new GroupSum
            {
                Key1 = g.Key.Key1,
                Key2 = g.Key.Key2,
                Sum = g.Sum(x => x.Field<double>(columnName4))
            }).PropertiesToDataTable<GroupSum>();

        return groupedTable;
    }

现在对于相同的源数据，我收到不同的结果：

    "SA", "INVA", 10
    "SA", "INVA", 20
    "SB", "INVB", 50

似乎源数据根本没有分组，查询的唯一区别是一行：

    //QueryA
    .GroupBy(r => new { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })
    //QueryB
    .GroupBy(r => new GroupKeys { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })

有人可以解释一下吗？对于那些想在Visual Studio中运行测试的人，请在下面找到完整的源代码。

internal static class TestForStackOverflow
{
    private static DataSet dataSet;
    private const string tableName = "MyTable";
    private const string columnName1 = "Supplier";  //Column names
    private const string columnName2 = "Invoice";
    private const string columnName3 = "Item";
    private const string columnName4 = "Amount";

    private class GroupKeys
    {
        public string Key1 { get; set; }
        public string Key2 { get; set; }
    }

    private class GroupSum
    {
        public string Key1 { get; set; }
        public string Key2 { get; set; }
        public Double Sum { get; set; }
    }

    public static void Test()
    {
        DataTable dataTable = InitializeDataTable();

        //DataTable groupedTable = GroupQueryA(dataTable);      //Please uncomment to run test A
        DataTable groupedTable = GroupQueryB(dataTable);        //Please uncomment to run test B

        DisplayData(groupedTable);
    }


    private static DataTable GroupQueryA(DataTable dataTable)
    {
        DataTable groupedTable = dataTable.AsEnumerable()
            .GroupBy(r => new { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })
            .Select(g => new GroupSum
            {
                Key1 = g.Key.Key1,
                Key2 = g.Key.Key2,
                Sum = g.Sum(x => x.Field<double>(columnName4))
            }).PropertiesToDataTable<GroupSum>();

        return groupedTable;
    }

    private static DataTable GroupQueryB(DataTable dataTable)
    {
        DataTable groupedTable = dataTable.AsEnumerable()
            .GroupBy(r => new GroupKeys { Key1 = r.Field<string>(columnName1), Key2 = r.Field<string>(columnName2) })
            .Select(g => new GroupSum
            {
                Key1 = g.Key.Key1,
                Key2 = g.Key.Key2,
                Sum = g.Sum(x => x.Field<double>(columnName4))
            }).PropertiesToDataTable<GroupSum>();

        return groupedTable;
    }

    private static System.Data.DataTable PropertiesToDataTable<T>(this System.Collections.Generic.IEnumerable<T> source)
    {
        System.Data.DataTable dt = new System.Data.DataTable();

        //Weź listę właściwości typu <T> i dla każdej właściwości dodaj do tabeli kolumnę tego samego typu co właściwość
        var props = System.ComponentModel.TypeDescriptor.GetProperties(typeof(T));
        foreach (System.ComponentModel.PropertyDescriptor prop in props)
        {
            System.Data.DataColumn dc = dt.Columns.Add(prop.Name, prop.PropertyType);
            dc.Caption = prop.DisplayName;
            dc.ReadOnly = prop.IsReadOnly;
        }
        //Kopiuj rekordy z kwerendy do DataTable
        foreach (T item in source)
        {
            System.Data.DataRow dr = dt.NewRow();
            foreach (System.ComponentModel.PropertyDescriptor prop in props)
            {
                dr[prop.Name] = prop.GetValue(item);
            }
            dt.Rows.Add(dr);
        }
        return dt;
    }

    private static DataTable InitializeDataTable()
    {
        dataSet = new DataSet();
        DataTable dataTable = dataSet.Tables.Add(tableName);

        dataTable.Columns.Add( columnName1, typeof(string));
        dataTable.Columns.Add( columnName2, typeof(string));
        dataTable.Columns.Add( columnName3, typeof(string));
        dataTable.Columns.Add( columnName4, typeof(double));

        AddRow(dataTable, "SA", "INVA", "ITA", 10);
        AddRow(dataTable, "SA", "INVA", "ITB", 20);
        AddRow(dataTable, "SB", "INVB", "ITC", 50);
        return dataTable;
    }
    private static void AddRow( DataTable dataTable, string supplier, string invoice, string item, double amount)
    {
        DataRow row = dataTable.NewRow();
        row[columnName1] = supplier;
        row[columnName2] = invoice;
        row[columnName3] = item;
        row[columnName4] = amount;
        dataTable.Rows.Add(row);
    }
    private static void DisplayData(System.Data.DataTable table)
    {
        foreach (System.Data.DataRow row in table.Rows)
        {
            foreach (System.Data.DataColumn col in table.Columns)
            {
                Console.WriteLine("{0} = {1}", col.ColumnName, row[col]);
            }
            Console.WriteLine("============================");
        }
    }
}

Answer 1

您需要稍微修改一下GroupKeys对象。由于它是一个引用类型，并且GroupBy正在使用Default Equality Comparer，因此它的部分检查将测试引用相等性，它将始终返回false。

您可以通过覆盖Equals和GetHashCode方法来调整GroupKeys类，以测试结构相等性。例如，这是由ReSharper生成的：

private class GroupKeys
{
    public string Key1 { get; set; }
    public string Key2 { get; set; }

    public override int GetHashCode()
    {
        unchecked
        {
            return ((Key1 != null ? Key1.GetHashCode() : 0) * 397) ^ (Key2 != null ? Key2.GetHashCode() : 0);
        }
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj))
            return false;
        if (ReferenceEquals(this, obj))
            return true;
        if (obj.GetType() != this.GetType())
            return false;

        return Equals((GroupKeys)obj);
    }

    public bool Equals(GroupKeys other)
    {
        if (ReferenceEquals(null, other))
            return false;
        if (ReferenceEquals(this, other))
            return true;

        return string.Equals(Key1, other.Key1)
               && string.Equals(Key2, other.Key2);
    }
}

C＃Linq GroupBy方法对于匿名和非匿名类型的工作方式不同

1 个答案: