用于selectmany + join + let的LINQ查询语法

Question

我想比较一个命名数据字典列表与从数据提供者读取的实际数据.结果应该是以下形式的平面列表:[表]:[Key]从[OldValue]更改为[NewValue].

我想使用linq的查询语法,不需要性能.具有相同名称的词典总是具有相同的键,不需要检查.

我提出了以下查询(您可以在LINQ-Pad中使用它)但我在第一次连接中无法访问table2.错误:"名称'table2'在当前上下文中不可用"(第8行).

有任何想法吗？

void Main()
{
    var db1 = new[] { new Table { Name = "TableA", Data = new Dictionary<string, string> { { "KeyA", "000" } } } };

    var changes = from table1 in db1
                let table2 = ReadTable(table1.Name)
                from row1 in table1.Data
                join row2 in table2.Data
                    on row1.Key equals row2.Key
                where !row1.Value.Equals(row2.Value)
                select new { Table = table1.Name, Key = row1.Key, From = row1.Value, To = row2.Value };

    changes.Dump(); 
}

Table ReadTable(string Name)
{
    return new Table { Name = "TableA", Data = new Dictionary<string, string> { { "KeyA", "111" } } };
}

class Table
{
    public string Name { get; set; }
    public Dictionary<string, string> Data { get; set; }
}

Answer 1

连接点是在两个独立的数据源中查找匹配元素.换句话说,连接右侧的元素不能依赖于连接的"当前"左侧的元素.因此,当你扁平化时,你无法加入.您只需要有效地隔离"给定两个表,找到差异"部分,然后将这些结果展平.我相信这会做你想要的:

var changes = from table1 in db1
              let table2 = ReadTable(table1.Name)
              from change in
                  (from row1 in table1.Data
                   join row2 in table2.Data
                     on row1.Key equals row2.Key
                   where !row1.Value.Equals(row2.Value)
                   select new 
                          { 
                             Table = table1.Name, 
                             Key = row1.Key, 
                             From = row1.Value, 
                             To = row2.Value 
                          })
              select change;