如何将lambda传递给Razor助手方法?

问题描述：

我有一个需要使用Func<>的剃刀助手方法，该方法将返回一些HTML内容以进行打印.这就是我最初的经历:

I have a razor helper method that needs to take in a Func<> that will return some HTML content to print out. This is what I originally had:

@helper node(string title, Func<HelperResult> descriptions)
{
    ....
    <div>@descriptions()</div>
    ....
}

@node("title", 
              new Func<HelperResult>(() => 
              {
                 return new HelperResult(
                     @<text>
                     <span>"desc1"</span>
                     <span>"desc2"</span>
                     </text>);
              }))

不幸的是，我的文字从未被打印出来.也没有错误.

Unfortunately with this my text never gets printed out. No error either.

因此，我了解了内联帮助器，并将调用方法更改为:

So I learned about inline helpers, and changed the calling method to this:

@node("title",                     
              @<text>
              <span>"desc1"</span>
              <span>"desc2"</span>
              </text>)

但是现在我收到一个编译错误，提示

However now I get a compilation error saying

"Delegate'System.Func'不接受1个参数".

"Delegate 'System.Func' does not take 1 arguments".

但是我没有传递任何参数.

But I'm not passing in any arguments.

因此，如果将其更改为Func<object,HelperResult>，然后使用@descriptions(null)进行调用，则会出现以下错误:

So if I change it to Func<object,HelperResult> and then call it using @descriptions(null) I get the following error:

不能将lambda表达式用作动态调度的操作，而无需先将其强制委托给委托人或表达式树类型"

"Cannot use a lambda expression as an argument to a dynamically dispatched operation without first casting it to a delegate or expression tree type"

我确定某个地方出了点问题，但是我不确定那是什么.

I'm sure I have something wrong somewhere, but I'm not sure what it is.

编辑:我想我可能已经解决了这个问题，但是它引入了其他一些问题.

Edit: I think I may have solved that problem but it introduces some other issues.

我要做的是在传递给动态方法之前先转换lambda.我想这就是错误要说的:

What I did was to cast the lambda before passing into a dynamic method. I guess that's what the error was trying to say:

@node("title",                     
              ((Func<dynamic, HelperResult>)(@<text>
              <span>"desc1"</span>
              <span>"desc2"</span>
              </text>))

这有效，并且可以正确打印出span标签.不幸的是，调用此Func时，我必须传递一个无用的参数.

That works and it prints out the span tags correctly. Unfortunately I have to pass in a useless parameter when calling this Func.

现在我遇到的问题是，我的实际功能不仅仅是编写一些跨度.像这样:

Now the issue I have is that my real function does a bit more than just write some spans. It's more like this:

@node("title",                     
              ((Func<dynamic, HelperResult>)(@<text>
              <span>@Helpers.Format(resource.Description,"item")</span>
              </text>))

其中@Helpers.Format是另一个帮助器，而资源是页面模型中的(动态)变量.

Where @Helpers.Format is another helper and resource is a (dynamic) variable from the page model.

当然，现在代码可以运行，但是什么都没有打印出来(在<span>标记内).我在Format辅助函数中放置了一个断点，命中了断点，并且所有参数都已正确设置，所以我不确定为什么它不能正确输出.同样，如果我将其更改为资源.描述那么什么也没得到输出.

Of course now the code runs but nothing is printed out (inside the <span> tag). I put a breakpoint inside my Format helper function, and it hits it and all the parameters are correctly set, so I'm not sure why it wouldn't output correctly. Similarly if I just change it to resource.Description then nothing still gets output.

由于它在这种情况下效果很好，所以我想知道Razor的内联助手不会捕获外部变量吗?

Since it works well outside of this context, I wonder does Razor's inline helpers not capture the outer variables?

答

实际上HelperResult是Microsoft希望您不使用的东西，如文档所示:

Actually HelperResult is something Microsoft would rather you didn't use, as evidenced by documentation:

公共类HelperResult:名称空间中的IHtmlString System.Web.WebPages

public class HelperResult : IHtmlString in namespace System.Web.WebPages

摘要:此类型/成员支持.NET Framework基础结构不能直接在您的代码中使用.

Summary: This type/member supports the .NET Framework infrastructure and is not intended to be used directly from your code.

可能的解决方案是将描述函数包装在另一个帮助程序中，然后将该帮助程序作为

A possible solution to your problem might be to wrap your description function in another helper and then pass that helper as a method group to your node helper, like this:

@helper Node(string title, Func<HelperResult> descriptions)
{
    <div>@descriptions()</div>
}

@helper Description() {
    <span>desc1</span>
    <span>desc2</span>
}

@Node("title", Description)

在任何情况下，您的第一个想法都不可行，因为Func类型的参数实际上等于无参数函数，在这种情况下，您需要这样编写lambda表达式:

In any case, your first idea shouldn't work because a parameter of type Func is in fact equal to a parameterless function, in which case you need to write the lambda expression like this:

myFunction( () => doSomething)

所以您的函数调用应该是:

So your function call would have been:

@node("title", () =>                    
              @<text>
              <span>"desc1"</span>
              <span>"desc2"</span>
              </text>)

由于这些助手的未来是有点可疑，因此，我会考虑改用 Partials (用于更大的块).

Since the future of these helpers is a bit dubious though, I would consider switching to either HtmlHelpers for small snippets of html or Partials for larger chunks.

如何将lambda传递给Razor助手方法?

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