带有连接字符串的 EF GroupBy答案

【问题标题】：EF GroupBy with concatenate string带有连接字符串的 EF GroupBy
【发布时间】：2015-06-04 11:38:49
【问题描述】：

我有一个类似下面的表格条目，

ColumnA       ColumnB      ColumnC      ColumnD

16             651           a          European Union 
16             651           x          Membership
17             651           a          Great Britain 
17             651           x          Economic integration
18             651           a          European Union countries 
18             651           x          Economic integration

结果列表

ColumnA                ColumnB

651                    |a European Union|x Membership
651                    |a European Union countries|x Economic integration
651                    |a Great Britain|x Economic integration

想法是将3列分组，即ColumnA，ColumnB，ColumnC和Concatenate ColumnD。

到目前为止我已经做到了

var sub = (from x in db.Bs
                   where x.BId == BId
                   group x by new { x.BId, x.Sd, x.TNo, x.Tr } into g
                   select new Book
                   {
                       Tagno = g.Key.TNo,
                       value = " |" + g.Key.Sd + " " + g.Key.   // I am not able to get the Value column here.
                   });

【问题讨论】：

因为它是分组的，我们可以使用下面的语法 g.Select(exp=>exp.ValueColumn) 获取该组下的值列表。
抛出异常Values of type 'collection[Edm.String(Nullable=True,DefaultValue=,MaxLength=Max,Unicode=True,FixedLength=False)]' can not be converted to string."
您需要使用 string.join("|",g.Select(exp=>exp.ValueColumn)) 因为分组结果将是一个列表，我们需要使用 string.join 进行连接列表中的值
实体框架不支持String.Join
有什么问题吗？ String.Join 与实体框架无关，我们所做的与我们将如何使用加入字符加入集合的方式相同

标签： c# entity-framework

【解决方案1】：

这是为您准备的完整程序。班级：

public class TestGroup
{
    public Int16 ColumnA { get; set; }
    public Int16 ColumnB { get; set; }
    public String ColumnC { get; set; }
    public String ColumnD { get; set; }
}

方法：

    public static void GroupTest()
    {
        var testGroup = new List<TestGroup>();
        testGroup.Add(new TestGroup { ColumnA = 16, ColumnB = 651, ColumnC = "a", ColumnD = "European Union" });
        testGroup.Add(new TestGroup { ColumnA = 16, ColumnB = 651, ColumnC = "x", ColumnD = "Membership" });
        testGroup.Add(new TestGroup { ColumnA = 17, ColumnB = 651, ColumnC = "a", ColumnD = "Great Britain" });
        testGroup.Add(new TestGroup { ColumnA = 17, ColumnB = 651, ColumnC = "z", ColumnD = "Economic integration" });
        testGroup.Add(new TestGroup { ColumnA = 18, ColumnB = 651, ColumnC = "a", ColumnD = "European Union countries" });
        testGroup.Add(new TestGroup { ColumnA = 18, ColumnB = 651, ColumnC = "x", ColumnD = "Economic integration" });

        var test = (from x in testGroup
            group x by new {x.ColumnA, x.ColumnB}
            into grp select grp).ToList().Select(grp=> new
            {
                TagNo = grp.Key.ColumnB,
                Text = String.Join(" | ", grp.Select(y => y.ColumnC + " " + y.ColumnD))
            });
        foreach (var x in test)
        {
            Console.WriteLine(String.Format("Tag No: {0}\t Text : {1}", x.TagNo, x.Text));
        }
        Console.Read();
    }

【讨论】：

分隔符 | 未正确附加。
该行应该是Text = String.Join("", grp.Select(y => "|" +y.ColumnC + " " + y.ColumnD))