How to emulate C# 6 null-conditional in C# < 6
本文关键字:lt in null-conditional to How emulate | 更新日期: 2023-09-27 18:28:07
使用C#6.0,我可以完成
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;
有什么好的解决方案可以用C#<6.0?
我是说这部分
&& x.OngoingChallenges?.Count == y.OngoingChallenges?.Count
&& x.MembershipIds?.Count == y.MembershipIds?.Count;
因为在旧项目中,我们不可能使用C#6.0。如何有效地编写isEqual
?
x.OnGoingChallenges?.Count
等效于x.OnGoingChallenges != null ? x.OnGoingChallenges.Count : default(int?)
(还有其他方法,但最终有一种空检查的快捷方式,称为空条件运算符)。
也就是说,如果没有C#,您的代码就无法用语法优雅的语句重写,但您可以使用扩展方法来模拟这个新的C#特性。。。
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中的代码如下所示:
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);
整个扩展方法将用于获取值类型的属性值或其默认值。您可能会也可能不会扩展扩展方法类以支持获取引用类型值或null。
在C#版本中<6.0您将使用三元表达式
var isEqual = x.Id == y.Id
&& x.UpdatedAt == y.UpdatedAt
&& x.Name == y.Name
&& x.RulesUrl == y.RulesUrl
&& (x.OngoingChallenges == null ? 0 : x.OngoingChallenges.Count) ==
(y.OngoingChallenges == null ? 0 : y.OngoingChallenges.Count)
&& (x.MembershipIds == null : 0 ? x.MembershipIds.Count) ==
(y.MembershipIds == null : 0 : y.MembershipIds.Count);
正如@Hamlet Hakobyan所指出的,这在语义上并不完全等同于使用?.
的原始C#6.0解决方案,但您可以将其更改为(根据@hvd):
int? count = x.MembershipIds == null : default(int?) ? x.MembershipIds.Count;
这取决于您是否希望将丢失的集合和空集合视为相等。
您也可以使用null合并运算符??
并提供一个替换对象。假设你的对象是某种列表:
var empty = new List<int>();
var isEqual = x.Id == y.Id
&& x.UpdatedAt == y.UpdatedAt
&& x.Name == y.Name
&& x.RulesUrl == y.RulesUrl
&& (x.OngoingChallenges ?? empty).Count == (y.OngoingChallenges ?? empty).Count
&& (x.MembershipIds ?? empty).Count == (y.MembershipIds ?? empty).Count;
在C#6之前,我使用了类似于的东西
public static class CommonExtensions
{
public static TValue TryGet<TObject, TValue>(this TObject obj, Func<TObject, TValue> getter, TValue defaultValue = default(TValue))
where TObject : class
{
return obj == null ? defaultValue : getter(obj);
}
//If objects types are equals
public static bool KeyEquals<TObject, TValue>(this TObject a, TObject b, Func<TObject, TValue> keyGetter)
where TObject : class
{
return a != null
&& b != null
&& EqualityComparer<TValue>.Default.Equals(keyGetter(a), keyGetter(b));
}
}
var isEqual = x.Id == y.Id
&& x.UpdatedAt == y.UpdatedAt
&& x.Name == y.Name
&& x.RulesUrl == y.RulesUrl
//v1
&& x.OngoingChallenges.TryGet(v => v.Count) == y.OngoingChallenges.TryGet(v => v.Count)
//v2
&& x.MembershipIds.KeyEquals(y.MembershipIds, v => v.Count);