blocks|key|1100287|text|据我所知，F#没有任何与C#+as等价的内置运算符，因此您需要编写一些更复杂的表达式。对于使用match的代码，您也可以使用if，因为运算符:?的使用方式与C#中的is相同：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1100288|let+res+=+if+(inputValue+:?+Type1)+then+inputValue+:?>+Type1+else+null|code-block|syntax|javascript|1100289|当然，您可以编写一个函数来封装此行为(通过编写一个简单的泛型函数，该函数接受一个Object并将其强制转换为指定的泛型类型参数)：|1100290|let+castAs<'T+when+'T+:+null>+(o:obj)+=+
++match+o+with
++%7C+:?+'T+as+res+->+res
++%7C+_+->+null|1100291|该实现返回null，因此它要求类型参数将null作为适当的值(或者，您可以使用Unchecked.defaultof<'T>，它相当于C#中的default(T)+)。现在，您只需编写：|1100292|let+res+=+castAs<Type1>(inputValue)|1100293|entityMap^0|F|2|1B|5|1Q|2|1Y|2|2A|2|0|0|14|6|0|0|5|4|K|4|13|N|1Z|A|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1A|8|@$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1J|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]]|T|$]]

As far as I know, F# doesn't have any built-in operator equivalent to C# <code>as</code> so you need to write some more complicated expression. Alternatively to your code using <code>match</code>, you could also use <code>if</code>, because the operator <code>:?</code> can be use in the same way as <code>is</code> in C#:

<pre><code>let res = if (inputValue :? Type1) then inputValue :?&gt; Type1 else null
</code></pre>

You can of course write a function to encapsulate this behavior (by writing a simple generic function that takes an <code>Object</code> and casts it to the specified generic type parameter):

<pre><code>let castAs&lt;'T when 'T : null&gt; (o:obj) = 
 match o with
 | :? 'T as res -&gt; res
 | _ -&gt; null
</code></pre>

This implementation returns <code>null</code>, so it requires that the type parameter has <code>null</code> as a proper value (alternatively, you could use <code>Unchecked.defaultof&lt;'T&gt;</code>, which is equivalent to <code>default(T)</code> in C#). Now you can write just:

<pre><code>let res = castAs&lt;Type1&gt;(inputValue)
</code></pre>

blocks|key|1100397|text|您可以创建自己的运算符来执行此操作。这实际上与Tomas的示例相同，但显示了一种略有不同的调用方式。下面是一个例子：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1100398|let+(~~)+(x:obj)+=+
++match+x+with
++%7C+:?+'t+as+t+->+t+//'
++%7C+_+->+null

let+o1+=+"test"
let+o2+=+2
let+s1+=+(~~o1+:+string)++//+s1+=+"test"
let+s2+=+(~~o2+:+string)+//+s2+=+null|code-block|syntax|javascript|1100399|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can create your own operator to do this. This is virtually identical to Tomas's example, but shows a slightly different way to call it. Here's an example:

<pre><code>let (~~) (x:obj) = 
 match x with
 | :? 't as t -&gt; t //'
 | _ -&gt; null

let o1 = "test"
let o2 = 2
let s1 = (~~o1 : string) // s1 = "test"
let s2 = (~~o2 : string) // s2 = null
</code></pre>

blocks|key|1100474|text|我会使用主动模式。下面是我使用的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1100475|let+(%7CAs%7C_%7C)+(p:'T)+:+'U+option+=
++++let+p+=+p+:>+obj
++++if+p+:?+'U+then+Some+(p+:?>+'U)+else+None|code-block|syntax|javascript|1100476|下面是As的用法示例|offset|length|style|CODE|1100477|let+handleType+x+=+
++++match+x+with
++++%7C+As+(x:int)+->+printfn+"is+integer:+%25d"+x
++++%7C+As+(s:string)+->+printfn+"is+string:+%25s"+s
++++%7C+_+->+printfn+"Is+neither+integer+nor+string"

//+test+'handleType'
handleType+1
handleType+"tahir"
handleType+2.
let+stringAsObj+=+"tahir"+:>+obj
handleType+stringAsObj|1100478|entityMap^0|0|0|3|2|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

I would use an active pattern. Here is the one I use:

<pre><code>let (|As|_|) (p:'T) : 'U option =
 let p = p :&gt; obj
 if p :? 'U then Some (p :?&gt; 'U) else None
</code></pre>

Here is a sample usage of <code>As</code>:

<pre><code>let handleType x = 
 match x with
 | As (x:int) -&gt; printfn "is integer: %d" x
 | As (s:string) -&gt; printfn "is string: %s" s
 | _ -&gt; printfn "Is neither integer nor string"

// test 'handleType'
handleType 1
handleType "tahir"
handleType 2.
let stringAsObj = "tahir" :&gt; obj
handleType stringAsObj
</code></pre>

In C# I can do:

<pre><code>var castValue = inputValue as Type1
</code></pre>

In F#, I can do:

<pre><code>let staticValue = inputValue :&gt; Type1
let dynamicValue = inputValue :?&gt; Type1
</code></pre>

But neither of them is the equivalent of the C#'s keyword <code>as</code>.

I guess I need to do a match expression for the equivalent in F#

<pre><code>match inputValue with
| :? Type1 as type1Value -&gt; type1Value
| _ -&gt; null
</code></pre>

Is this correct?

C# and F# casting - specifically the 'as' keyword

在C#中，我可以这样做：var castValue = inputValue as Type1在F#中，我可以执行以下操作：let staticValue = inputValue :> Type1let dynamicValue = inputValue :?> Type1但是它们都不能等同于C#的关键字as。我想我...

问C#和F#类型转换-特别是'as‘关键字
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问C#和F#类型转换-特别是'as‘关键字EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问C#和F#类型转换-特别是'as‘关键字
EN