[小技巧]讓C#的空值處理變得更優雅

參考

http://www.codePRoject.com/Articles/739772/Dynamically-Check-Nested-Values-for-IsNull-Values?msg=4895299#xx4895299xx

http://devtalk.net/csharp/chained-null-checks-and-the-maybe-monad/

介紹

C#中的空值處理，一直是一件比較讓人不爽的工作。假設現在有如下類：

public class Union { public string Name { get; set; } }  public class Dep {        public Union Union { get; set; }        public string Name { get; set; } }  public  class Person    {        public  Dep Dep { get;set;}        public string Name { get; set; }    }

如果Person.Dep.Union.Name.Length>5,則寫入日志，代碼該怎么寫呢？很可能是這樣：

 if (person != null && person.Dep != null && person.Dep.Union != null && person.Dep.Union.Name != null && person.Dep.Union.Name.Length > 5)            {                Console.WriteLine(person.Dep.Union.Name);            }

邏輯語句寫得多的各位，對于這樣寫的繁瑣應該深有體會。

對策

1擴展方法在訪問對象前，會先進入其對應的靜態方法。可以很方便的對this參數進行判斷和處理，而不引起nullreference異常；

2委托可以很好的進行擴展方法的后續動作。

我們可以這樣寫上面的語句

             person                .GoTo(p => p.Dep.Union.Name)                .If(n => n.Length >= 5)                .Do(Console.WriteLine);

很簡潔，是不是？怎么實現呢？只需加入下列擴展類：

using System;using System.Linq;using System.Linq.Expressions;using System.Reflection;namespace LinqTesting{    public static class Helper    {        class IsNullVisitor : ExpressionVisitor        {            public bool IsNull { get; private set; }            public object CurrentObject { get; set; }            protected override Expression VisitMember(MemberExpression node)            {                //it will call this overrided method with higher level node                base.VisitMember(node);                if (CheckNull())                    return node;                var member = (PropertyInfo)node.Member;                CurrentObject = member.GetValue(CurrentObject, null);                CheckNull();                return node;            }            private bool CheckNull()            {                if (CurrentObject == null)                    IsNull = true;                return IsNull;            }        }        public static TReturn GoTo<T, TReturn>(this T root, Expression<Func<T, TReturn>> funcGetValue)        {            var visitor = new IsNullVisitor();            visitor.CurrentObject = root;            visitor.Visit(funcGetValue);            if (visitor.IsNull)                return default(TReturn);            return (TReturn)visitor.CurrentObject;        }        public static TInput If<TInput>(this TInput o, Func<TInput, bool> evaluator)          where TInput : class        {            if (o == null) return null;            return evaluator(o) ? o : null;        }        public static TInput Unless<TInput>(this TInput o, Func<TInput, bool> evaluator)          where TInput : class        {            if (o == null) return null;            return evaluator(o) ? null : o;        }        public static TInput Do<TInput>(this TInput o, Action<TInput> action)        where TInput : class        {            if (o == null) return null;            action(o);            return o;        }    }}