Question

我有三张桌子。表A

ID_A  data_A
1        a
2        b

表B

ID_B  data_B ID_A
1      2      1
2      3      2

表c

ID_C   data_C   ID_A   ID_B
1      1         1        1

我的linq查询

var data = (from a in _db.Table_A

join b in db.Table_B
on a.ID_A equals b.ID_A

join c in _db.Table_C
on new { ID_A = a.ID_A, ID_B = b.ID_B} equals new { ID_A = c.ID_A, ID_B = c.ID_B }

select a)

在连接table_C时，连接显示错误（连接子句中某个表达式的类型不正确。在要加入的cal中类型推断失败）。
我想在table_C join的连接条件中同时使用table_A和Table_B id

Answer 1

您的查询应该像

var data = (from a in _db.Table_A    
            join b in _db.Table_B on a.ID_A equals b.ID_A    
            join c in _db.Table_C on new { a.ID_A,b.ID_B} equals new { c.ID_A,c.ID_B }    
            select a)

如果表C中没有nullable值，

这里有demo

如果table_C中的某个字段为nullable，则此处为解决问题的解决方法

var data = (from a in _db.Table_A()    
            join b in _db.Table_B() on a.ID_A equals b.ID_A    
            join c in _db.Table_C() on new { m=a.ID_A,n=(int?)b.ID_B} equals new { m=c.ID_A,n=c.ID_B }    
        select a);

工作demo

希望它会帮助你

Linq连接条件与来自两个表

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