如何在C#<中模拟C#6空条件6 [英] How to emulate C# 6 null-conditional in C# < 6

问题描述

使用C#6.0，我可以做到

With C# 6.0 I can do this

var isEqual = x.Id == y.Id
              && x.UpdatedAt == y.UpdatedAt
              && x.Name == y.Name                        
              && x.RulesUrl == y.RulesUrl
              && x.OngoingChallenges?.Count == y.OngoingChallenges?.Count
              && x.MembershipIds?.Count == y.MembershipIds?.Count;

有没有很好的解决方案，可以使用C#<6.0?

Is there any nice solution to do this with C# < 6.0?

我的意思是这部分

&& x.OngoingChallenges?.Count == y.OngoingChallenges?.Count
&& x.MembershipIds?.Count == y.MembershipIds?.Count;

由于在旧项目中，我们无法使用C#6.0.如何有效地编写 isEqual ?

Because in old projects we do not have possibility to use C# 6.0. How to write isEqual efficiently?

推荐答案

x.OnGoingChallenges?.Count 等同于 x.OnGoingChallenges！= null?x.OnGoingChallenges.Count:default(int?)(还有其他方法，但是到了最后，空检查的快捷方式称为 null-conditional operator ).

x.OnGoingChallenges?.Count is equivalent to x.OnGoingChallenges != null ? x.OnGoingChallenges.Count : default(int?) (there're other approaches, but at the end of the day is a shortcut to null checking called null-conditional operator).

也就是说，如果没有C#6，就无法使用优雅的语句重写代码，但是您可以使用扩展方法来模仿此新的C#6功能...

That is, your code can't be rewritten with a syntatically elegant statement without C# 6, but you can emulate this new C# 6 feature using extension methods...

public static class StructExtensions
{
    // Check that TProperty is nullable for the return value (this is how C#6's
    // null-conditional operator works with value types
    public static TProperty? GetOrDefault<TObject, TProperty>(this TObject someObject, Func<TObject, TProperty> propertySelectionFunc)
        where TObject : class 
        where TProperty : struct
    {
        Contract.Requires(propertySelectionFunc != null);

        return someObject == null ? default(TProperty?) : propertySelectionFunc(someObject);
    }
}

现在，您在C#5中的代码将如下所示:

And now your code in C#5 would look as follows:

var isEqual = x.Id == y.Id
                          && x.UpdatedAt == y.UpdatedAt
                          && x.Name == y.Name                        
                          && x.RulesUrl == y.RulesUrl
                          && x.OngoingChallenges.GetOrDefault(c => c.Count) == y.OngoingChallenges.GetOrDefault(c => c.Count)
                          && x.MembershipIds.GetOrDefault(m => m.Count) == x.MembershipIds.GetOrDefault(m => m.Count);

整个扩展方法将适用于获取值类型的属性值或其默认值.您可能会或可能不会扩展扩展方法类，使其也支持获取引用类型值或null.

The whole extension method would work for getting a value-typed property value or its default value. You might or might not extend the extension method class to also support getting a reference type value or null.

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