Question

我已根据一个公共列创建了一个通用extension方法来加入2 tables。代码如下：

public class SomeDTO<T,U>
    {
        public T TableA { get; set; }
        public U TableB { get; set; }
    }
    public static class Helper
    {
        public static IQueryable<SomeDTO<T,U>> JoinExtension<T,U,Key>(this IQueryable<T> tableA, IQueryable<U> tableB, Expression<Func<T,Key>> columnA, Expression<Func<U,Key>> columnB)
        {
            return tableA.Join(tableB, columnA, columnB,(x, y) => new SomeDTO<T, U>{TableA = x,TableB = y});
        }
    }

现在database中的表有2个公共列（Id，Type），我需要编写一个公共扩展方法，根据2个常用列连接这些表，写一下如下：

    public static IQueryable<SomeDTO<T, U>> JoinExtensionTwoColumns<T, U, Key>(this IQueryable<T> tableA, IQueryable<U> tableB, Expression<Func<T, Key>> columnA, Expression<Func<U, Key>> columnB, Expression<Func<T, Key>> columnC, Expression<Func<U, Key>> columnD)
   {
    return tableA.Join(tableB, a => new { columnA, columnB }, b => new { columnC, columnD }, (a, b) => new SomeDTO<T, U> { TableA = a, TableB = b });
   }

编译器在代码行tableA.Join中给出了如下所示的错误....如下所示：

The type arguments for method 'Queryable.Join<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IEnumerable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

它无法正确理解arguments及其性质。

指向我可能出错的地方？

修改

我现在有一个成功编译的方法，但是我得到了一个运行时错误＆＃34; The LINQ expression node type 'Lambda' is not supported in LINQ to Entities.＆＃34;

public static IQueryable<SomeDTO<T, U>> JoinExtensionTwoColumns<T, U, Key>(this IQueryable<T> tableA, IQueryable<U> tableB, Expression<Func<T, Key>> columnA, Expression<Func<U, Key>> columnB, Expression<Func<T, Key>> columnC, Expression<Func<U, Key>> columnD)
        {
return tableA.Join(tableB, a => new object[]{ columnA, columnB }, b => new object []{ columnC, columnD }, (a, b) => new SomeDTO<T, U> { TableA = a, TableB = b });
}

如此调用方法：

var result= (db.table1.JoinExtensionTwoColumns<table1,table2,int>(db.table2, c => c.id.ToString(), d => d.id.ToString(),e => e.type, f => f.type)).Take(10);

还有更多指示。

Answer 1

您已完成所有工作：不需要JoinExtensionTwoColumns方法，您可以使用JoinExtension根据多个列加入表格：

tableA.JoinExtension<TypeA, TypeB, object>(tableB, x => new { x.column1, x.column2 }, 
    x => new { column1 = x.column2, column2 = x.column3  });

但是在这种情况下 - 由于第三个任意 - object类型参数，解决方案不是TypeSafely ，要修复它，您可以通过以下方式重构以前的解决方案：

public static class Helper
{
    public class JoinCondition<TFirst, TSecond>
    {            
        public TFirst column1 { get; set; }
        public TSecond column2 { get; set; }
    }

    public static IQueryable<SomeDTO<T, U>> JoinExtension<T, U, TFirst, TSecond>(this IQueryable<T> tableA, IQueryable<U> tableB, 
        Expression<Func<T, JoinCondition<TFirst, TSecond>>> joinSelectorA, 
        Expression<Func<U, JoinCondition<TFirst, TSecond>>> joinSelectorB)
    {
        return tableA.Join(tableB, joinSelectorA, joinSelectorB, (x, y) => new SomeDTO<T, U> { TableA = x, TableB = y });
    }
}

<强> Implementaion：

var answer = context.TableA.JoinExtension(context.TableB, 
    x => new Helper.JoinCondition<int, string> { column1 = x.prop1, column2 = x.prop2}, 
    x => new Helper.JoinCondition<int, string> { column1 = x.prop3, column2 = x.prop4}
    ).ToList();

P.S。 JoinExtensionTwoColumns方法包含错误：您使用 columnA，columnB等作为列，但它们是谓词，你可以以这种方式修复它（它在EF上下文不起作用）：

public static IQueryable<SomeDTO<T, U>> JoinExtensionTwoColumns<T, U, Key>(this IQueryable<T> tableA, IQueryable<U> tableB, 
    Expression<Func<T, Key>> columnA, Expression<Func<U, Key>> columnB, 
    Expression<Func<T, Key>> columnC, Expression<Func<U, Key>> columnD)
{
    return tableA.Join(tableB, 
        a => new { column1 = columnA.Compile()(a), column2 = columnC.Compile()(a) }, 
        b => new { column1 = columnB.Compile()(b), column2 = columnD.Compile()(b) }, 
        (a, b) => new SomeDTO<T, U> { TableA = a, TableB = b });
}

使用2列的LINQ JOIN的通用扩展方法

1 个答案: