相同的(？)C#和VB.NET LINQ查询返回不同的结果

Question

可能真的很容易,但我看不出来......

我在LINQ中复制MS Access查询.我先用C#编写它来测试它,因为我更喜欢C#,然后我把它翻译成VB.Net语法.据我所知,两个查询应该是相同的,但是当C#查询返回正确的结果时,VB.NET会返回零结果.

谁能看出差异可能在哪里？

C#查询:

var table1 = dc.MainTable.Where(o => o.Year == 423).ToList().Select(o => new
{
    Key_ID = o.Key_ID.Value,
    CropID = o.CropID.Value,
    GroupID = o.GroupID.Value,
    Surface1 = o.Surface1.Value,
    Surface2 = o.Surface2.Value
});

var table2 = dc.OtherTable.Where(o => o.Year == 423).ToList().Select(o => new
{
    Key_ID = o.Key_ID.Value,
    CropID = int.Parse(o.SAKU_CD),
    GroupID = int.Parse(o.SAN_DAN_NO),
    Surface1 = Convert.ToDouble(o.KEIHAN_MEN.Value),
    Surface2 = Convert.ToDouble(o.SAKU_MEN.Value)
});

var output = table1.Join(table2, t1 => new 
{ 
    t1.Key_ID, 
    t1.CropID, 
    t1.GroupID, 
    t1.Surface1, 
    t1.Surface2 
}, 
t2 => new 
{ 
    t2.Key_ID, 
    t2.CropID, 
    t2.GroupID, 
    t2.Surface1, 
    t2.Surface2 
}, (t1, t2) => new OutputDataType() 
{ 
    Key_ID = t1.Key_ID, 
    Year = 423 
}).ToList();

VB.NET查询:

Dim table1 = MainTable.Where(Function(o) o.Year.Value = 423).ToList().Select(Function(o) New With
{
    .Key_ID = o.Key_ID.Value,
    .CropID = o.CropID.Value,
    .GroupID = o.GroupID.Value,
    .Surface1 = o.Surface1.Value,
    .Surface2 = o.Surface2.Value
}).ToList()

Dim table2 = OtherTable.Where(Function(o) o.Year.Value = 423).ToList().Select(Function(o) New With
{
    .Key_ID = o.Key_ID.Value,
    .CropID = Convert.ToInt32(o.SAKU_CD),
    .GroupID = Convert.ToInt32(o.SAN_DAN_NO),
    .Surface1 = Convert.ToDouble(o.KEIHAN_MEN.Value),
    .Surface2 = Convert.ToDouble(o.SAKU_MEN.Value)
}).ToList()

Dim output = table1.Join(table2, Function(t1) New With
{
    t1.Key_ID,
    t1.CropID,
    t1.GroupID,
    t1.Surface1,
    t1.Surface2
}, Function(t2) New With
{
    t2.Key_ID,
    t2.CropID,
    t2.GroupID,
    t2.Surface1,
    t2.Surface2
}, Function(t1, t2) New OutputDataType With {.Key_ID = t1.Key_ID, .Year = 423}).ToList()

在C#和VB.Net table1和table2是相同的,所以它必须是Join其失败.

编辑

我刚刚将JoinVB.Net改为查询语法,如下所示:

Dim output = From t1 In MainTable
                 Join t2 In OtherTable
                 On t1.Key_ID Equals t2.Key_ID And t1.GroupID Equals t2.GroupID And t1.CropID Equals t2.CropID And t1.Surface1 Equals t2.Surface1 And t1.Surface2 Equals t2.Surface2
                 Select New OutputDataTypeData With {.Key_ID = t1.Key_ID, .Year = 423}

这给出了正确的结果.但我真的不明白这与扩展方法Join语法有何不同？

Answer 1

使用Join扩展方法时,使用该方法比较您提供的键outerKeySelector和innerKeySelector参数Equals.

但是C#和VB.Net 在这里处理匿名类型的方式不同:

C#

var a = new {Foo = 1, Bar = 2 };
var b = new {Foo = 1, Bar = 2 };
bool result = a.Equals(b); // true

VB.Net

Dim a = new with {.Foo = 1, .Bar = 2}
Dim b = new with {.Foo = 1, .Bar = 2}
Dim result = a.Equals(b) ' False '

这里发生了什么事？

C#使用值相等来比较两个对象,比较属性的值.

VB.Net使用引用相等来比较两个对象,因此结果是False.

要使代码有效,您必须明确告诉VB.Net使用Key关键字比较属性:

关键属性在几个基本方面与非关键属性不同:

• 仅比较关键属性的值以确定两个实例是否相等.
• 键属性的值是只读的,不能更改.
• 对于匿名类型,编译器生成的哈希码算法中仅包含键属性值.

Dim a = new with {Key .Foo = 1, Key .Bar = 2}
Dim b = new with {Key .Foo = 1, Key .Bar = 2}
Dim result = a.Equals(b) # True

查询语法有效,因为在这种情况下,您不是比较匿名类型/对象,而只是ints和doubles.