iSynaptic.Commons和也许单子

Question

我一直在试图找出我怎么能使用在iSynaptic.Commons也许单子的情况下在我的价值猎犬可能抛出一个异常：

例如：

dynamic expando = new Expando(); 
expando.Name = "John Doe"; 

var maybe = Maybe.Defer(()=>(string)expando.NonExistingProperty); 

//In this context I would like the exception which is thrown 
//to result in Maybe<string>.NoValue; 
if(maybe.HasValue) { 
    //Do something 
} 

这可能与执行也许是在那里

Answer 1

有几种方法使用iSynaptic.Commons允许异常。我发现的每种方法都需要使用.Catch（）扩展方法让monad知道默默捕捉异常。另外，访问属性may.Value时要小心。如果此属性为Maybe.NoValue，则会引发InvalidOperationException。

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1）创建一个“OnExceptionNoValue”扩展方法。这将检查Maybe以查看它是否有异常。如果有，则返回NoValue Maybe。否则，原来的Maybe将被退回。

public static class MaybeLocalExtensions 
{ 
    public static Maybe<T> OnExceptionNoValue<T>(this Maybe<T> maybe) 
    { 
     return maybe.Exception != null ? Maybe<T>.NoValue : maybe; 
    } 
} 

// Sample Use Case: 
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch() 
    .OnExceptionNoValue(); 

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2）创建一个 “BindCatch” 扩展方法。这会改变正常绑定的行为，当存在异常时返回Maybe.NoValue而不是抛出异常。

public static class MaybeLocalExtensions 
{ 
    public static Maybe<TResult> BindCatch<T, TResult>(this Maybe<T> @this, Func<T, Maybe<TResult>> selector) 
    { 
     var self = @this; 

     return new Maybe<TResult>(() => { 
      if (self.Exception != null) 
       return Maybe<TResult>.NoValue; 

      return self.HasValue ? selector(self.Value) : Maybe<TResult>.NoValue; 
     }); 
    } 
} 

// Sample Use Case: 
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch() 
    .BindCatch(m => m.ToMaybe()); 

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3）通过这种方式还使用捕捉（）扩展方法，但使用而不是依赖于扩展方法的maybe.HasValue属性。如果Maybe中存在异常，则HasValue属性为false。当这个值为false时，Maybe.NoValue可以替换变量的值，或者在这种情况下需要做的任何事情。

dynamic expando = new ExpandoObject(); 
expando.Name = "John Doe"; 

// This example falls to the else block. 
var maybe = Maybe.Defer(() => (string)expando.NonExistingProperty).Catch(); 

//In this context I would like the exception which is thrown 
//to result in Maybe<string>.NoValue; 
if (maybe.HasValue) { 
    //Do something 
    Console.WriteLine(maybe.Value); 
} else { 
    maybe = Maybe<string>.NoValue; // This line is run 
} 

// This example uses the if block. 
maybe = Maybe.Defer(() => (string)expando.Name).Catch(); 
//to result in Maybe<string>.NoValue; 
if (maybe.HasValue) { 
    //Do something 
    Console.WriteLine(maybe.Value); //This line is run 
} else { 
    maybe = Maybe<string>.NoValue; 
} 

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这些答案都在同一主题的所有变化，但我希望他们是有益的。