如何在 vb.net 中使用 lambda

Question

当我在 VB.NET 中使用 AngleSharp 时，C# 中的 lambda 变成了一个问题。 VB.NET 可以用 function 代替 lambda 但似乎只接受一个参数

Dim document = Await context.OpenAsync(Function(Res)
Res.Content("

")
.Address("http://localhost/")
.Header(HeaderNames.SetCookie, my_headers)
End Function
)

错误：

Error BC30518 Overload resolution failed because no accessible 'OpenAsync' can be called with these arguments:
Extension method 'Public Function OpenAsync(response As IResponse, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'IResponse' because 'IResponse' is not a delegate type.
Extension method 'Public Function OpenAsync(request As DocumentRequest, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'DocumentRequest' because 'DocumentRequest' is not a delegate type.
Extension method 'Public Function OpenAsync(url As Url, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'Url' because 'Url' is not a delegate type.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Leading '.' or '!' can only appear inside a 'With' statement.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Leading '.' or '!' can only appear inside a 'With' statement.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': 'HeaderNames' is not declared. It may be inaccessible due to its protection level.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Leading '.' or '!' can only appear inside a 'With' statement.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Leading '.' or '!' can only appear inside a 'With' statement.
Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': 'HeaderNames' is not declared. It may be inaccessible due to its protection level.
Extension method 'Public Function OpenAsync(address As String, [cancellation As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'String' because 'String' is not a delegate type.

我该如何解决？

41686d6564修改后的错误

Error   BC30518 Overload resolution failed because no accessible 'OpenAsync' can be called with these arguments:
    Extension method 'Public Function OpenAsync(response As IResponse, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'IResponse' because 'IResponse' is not a delegate type.
    Extension method 'Public Function OpenAsync(request As DocumentRequest, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'DocumentRequest' because 'DocumentRequest' is not a delegate type.
    Extension method 'Public Function OpenAsync(url As Url, [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'Url' because 'Url' is not a delegate type.
    Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': 'Return' statement in a Sub or a Set cannot return a value.
    Extension method 'Public Function OpenAsync(request As Action(Of VirtualResponse), [cancel As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': 'HeaderNames' is not declared. It may be inaccessible due to its protection level.
    Extension method 'Public Function OpenAsync(address As String, [cancellation As CancellationToken = Nothing]) As Task(Of IDocument)' defined in 'BrowsingContextExtensions': Lambda expression cannot be converted to 'String' because 'String' is not a delegate type.

Answer 1

您的代码有两个问题：

您尝试使用的 overload 需要一个 Action，而不是 Func。因此，您应该使用 Sub 而不是 Function。
在 VB 中，点必须出现在行尾，而不是下一行的开头（在 C# 中，您可以这样做），否则必须使用 line continuation character（即 _）。

将您的代码更改为类似以下内容应该可以编译：

Dim document = Await context.OpenAsync(
    Sub(Res)
        Res.Content("").
        Address("http://localhost/").
        Header(HeaderNames.SetCookie, my_headers)
    End Sub)

或使用明确的行继续：

Dim document = Await context.OpenAsync(
    Sub(Res)
        Res.Content("") _
        .Address("http://localhost/") _
        .Header(HeaderNames.SetCookie, my_headers)
    End Sub)

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