Question

我可以在c＃中没有问题地执行此操作：

public class Whatever
{
    public string State { get; set; }
    public string City { get; set; }
}

var whatevers = new[] {
    new Whatever { State = "SomeState", City = "Eastville" },
    new Whatever { State = "SomeState", City = "Southville" }
};

var group = whatevers.GroupBy(
    x => x.State, x => x.City, 
    (k, v) => new { State = k, Cities = v.ToArray() }
).ToArray();

但VB.NET似乎有推理问题：

Option Infer On

Public Class Whatever
    Property State As String
    Property City As String
End Class

Dim whatevers = New Whatever() {
    New Whatever With {.State = "SomeState", .City = "Eastville"},
    New Whatever With {.State = "SomeState", .City = "Southville"}
}

Dim group = whatevers.GroupBy(
    Function(x) x.State,
    Function(x) x, 'this is the problem; see below
    Function(k, v) New With {.State = k, .Cities = v.ToArray()}
).ToArray()

对于我的元素选择器，我想返回Function(x) x.City。如果我未指定.City，则x会正确推断为Whatever。但是，当我输入.City时，x无法再被推断出来。

我花了一个多小时才到这里，所以是时候来SE了。

Answer 1

小修复可以发挥魔力。

Public Class Whatever
  Property State As String
  Property City As String
End Class

Dim whatevers = New Whatever() {
    New Whatever With {.State = "SomeState", .City = "Eastville"},
    New Whatever With {.State = "SomeState", .City = "Southville"}
}

Dim group = whatevers.GroupBy(
    Function(x As Whatever) x.State,
    Function(x As Whatever) x.City, 'this is not the problem;
    Function(k, v) New With {.State = k, .Cities = v.ToArray()}
).ToArray()

VB.NET GroupBy元素选择器参数类型无法推断

1 个答案: