C#如何使用表達式樹動態更新類的屬性值

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C#的λ表達式樹是一個好東西，也是別的語言學不來的，熟悉掌握λ表達式就能夠實現各種場景的個性化操作，如動態拼接查詢條件、排序方式等，也能夠實現替代反射的高性能操作，比如我們常用到的IQueryable和IEnumerable，每個擴展方法就全是λ表達式樹。

本文給大家分享C#使用表達式樹(LambdaExpression)動態更新類的屬性值的相關知識，在某些業務中會遇到需要同步兩個類的屬性值的情況，而且有些字段是要過濾掉的。如果手動賦值則需要寫很多重復的代碼：

 public class Teacher
        {
            public Guid Id { get; set; }
            public string Name { get; set; }
            public string Age { get; set; }
        }
        public class Student
        {
            public Guid Id { get; set; }
            public string Name { get; set; }
            public string Age { get; set; }
        }
   /// <summary>
        /// 比如需要把teacher的某些屬性值賦給student，而id不需要賦值
        /// </summary>
        /// <param name="student"></param>
        /// <param name="teacher"></param>
        public static void SetProperty(Student student, Teacher teacher)
        {
            if (student.Name != teacher.Name)
            {
                student.Name = teacher.Name;
            }
            if (student.Age != teacher.Age)
            {
                student.Age = teacher.Age;
            }
        }

使用反射的話性能考慮，嘗試寫一個擴展方法使用lambda表達式樹去構建一個方法

public static class ObjectExtensions
    {
        /// <summary>
        /// 緩存表達式
        /// </summary>
        /// <typeparam name="TSource"></typeparam>
        /// <typeparam name="TTarget"></typeparam>
        public static class MapperAccessor<TSource, TTarget>
        {
            private static Action<TSource, TTarget, string[]> func { get; set; }
            public static TSource Set(TSource source, TTarget target, params string[] properties)
            {
                if (func == null)
                {
                    var sourceType = typeof(TSource);
                    var targetType = typeof(TTarget);
                    BindingFlags bindingFlags = BindingFlags.Instance | BindingFlags.Public;
                    if (properties.Length == 0)
                    {
                        //get all properties
                        if (sourceType == targetType)
                        {
                            //如果是相同類型則獲取所有屬性
                            properties = sourceType.GetProperties(bindingFlags).Select(x => x.Name)
                                .ToArray();
                        }
                        else
                        {
                            //如果沒有傳指定的屬性則默認獲取同名屬性
                            List<PropertyInfo> propertyInfos = new List<PropertyInfo>();
                            foreach (var property in sourceType.GetProperties(bindingFlags))
                            {//不同類型指定同名且類型相同的屬性
                                var targetProperty = targetType.GetProperty(property.Name, bindingFlags);
                                if (targetProperty != null && targetProperty.PropertyType == property.PropertyType)
                                {
                                    propertyInfos.Add(property);
                                }
                            }
                            properties = propertyInfos.Select(x => x.Name).ToArray();
                        }
                    }
                    //定義lambda 3個參數
                    var s = Expression.Parameter(typeof(TSource), "s");
                    var t = Expression.Parameter(typeof(TTarget), "t");
                    var ps = Expression.Parameter(typeof(string[]), "ps");
                    //獲取泛型擴展方法Contains
                    var methodInfo = typeof(Enumerable).GetMethods().FirstOrDefault(e => e.Name == "Contains" && e.GetParameters().Length == 2);
                    if (methodInfo == null)
                    {
                        // properties.Contains()
                        throw new NullReferenceException(nameof(methodInfo));
                    }
                    MethodInfo genericMethod = methodInfo.MakeGenericMethod(typeof(String));//創建泛型方法
                    List<BlockExpression> bs = new List<BlockExpression>();
                    foreach (string field in properties)
                    {
                        //獲取兩個類型里面的屬性
                        var sourceField = Expression.Property(s, field);
                        var targetField = Expression.Property(t, field);
                        //創建一個條件表達式
                        var notEqual = Expression.NotEqual(sourceField, targetField);//sourceField!=targetField
                        var method = Expression.Call(null, genericMethod, ps, Expression.Constant(field));//ps.Contains(f);
                        //構建賦值語句
                        var ifTrue = Expression.Assign(sourceField, targetField);
                        //拼接表達式 sourceField!=targetField&&ps.Contains(f)
                        var condition = Expression.And(notEqual, Expression.IsTrue(method));
                        //判斷是否相同，如果不相同則賦值
                        var expression = Expression.IfThen(condition, ifTrue);
                        bs.Add(Expression.Block(expression));
                    }
                    var lambda = Expression.Lambda<Action<TSource, TTarget, string[]>>(Expression.Block(bs), s, t, ps);
                    func = lambda.Compile();
                }
                func.Invoke(source, target, properties);
                return source;
            }
        }
        /// <summary>
        /// 通過目標類更新源類同名屬性值
        /// </summary>
        /// <typeparam name="TSource">待更新的數據類型</typeparam>
        /// <typeparam name="TTarget">目標數據類型</typeparam>
        /// <param name="source">源數據</param>
        /// <param name="target">目標數據</param>
        /// <param name="properties">要變更的屬性名稱</param>
        /// <returns>返回源數據，更新后的</returns>
        public static TSource SetProperties<TSource, TTarget>(this TSource source, TTarget target, params string[] properties)
        {
            return MapperAccessor<TSource, TTarget>.Set(source, target, properties);
        }
    }

編寫測試方法

 /// <summary>
        /// 比如需要把teacher的某些屬性值賦給student，而id不需要賦值
        /// </summary>
        /// <param name="student"></param>
        /// <param name="teacher"></param>
        public static void SetProperty(Student student, Teacher teacher)
        {
            if (student.Name != teacher.Name)
            {
                student.Name = teacher.Name;
            }
            if (student.Age != teacher.Age)
            {
                student.Age = teacher.Age;
            }
        }
        public static void SetProperty2(Student student, Teacher teacher, params string[] properties)
        {
            var sourceType = student.GetType();
            var targetType = teacher.GetType();
            foreach (var property in properties)
            {
                var aP = sourceType.GetProperty(property);
                var bP = targetType.GetProperty(property);
                var apValue = aP.GetValue(student);
                var bpValue = bP.GetValue(teacher);
                if (apValue != bpValue)
                {
                    aP.SetValue(student, bpValue);
                }
            }
        }

        static (List<Student>, List<Teacher>) CreateData(int length)
        {
            var rd = new Random();
            (List<Student>, List<Teacher>) ret;
            ret.Item1 = new List<Student>();
            ret.Item2 = new List<Teacher>();
            for (int i = 0; i < length; i++)
            {
                Student student = new Student()
                {
                    Id = Guid.NewGuid(),
                    Name = Guid.NewGuid().ToString("N"),
                    Age = rd.Next(1, 100)
                };
                ret.Item1.Add(student);
                Teacher teacher = new Teacher()
                {
                    Id = Guid.NewGuid(),
                    Name = Guid.NewGuid().ToString("N"),
                    Age = rd.Next(1, 100)
                };
                ret.Item2.Add(teacher);
            }
            return ret;
        }
        static void Main(string[] args)
        {

            var length = 1000000;
            var data = CreateData(length);
            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < length; i++)
            {
                SetProperty(data.Item1[i], data.Item2[i]);
            }
            sw.Stop();
            Console.WriteLine($"手寫方法耗時：{sw.ElapsedMilliseconds}ms");
            data.Item1.Clear();
            data.Item2.Clear();
            var data2 = CreateData(length);
            sw.Restart();
            for (int i = 0; i < length; i++)
            {
                data2.Item1[i].SetProperties(data2.Item2[i], nameof(Student.Age), nameof(Student.Name));
            }
            data2.Item1.Clear();
            data2.Item2.Clear();
            sw.Stop();
            Console.WriteLine($"lambda耗時：{sw.ElapsedMilliseconds}ms");
            var data3 = CreateData(length);
            sw.Restart();
            for (int i = 0; i < length; i++)
            {
                SetProperty2(data3.Item1[i], data3.Item2[i], nameof(Student.Age), nameof(Student.Name));
            }
            sw.Stop();
            Console.WriteLine($"反射耗時：{sw.ElapsedMilliseconds}ms");
            data3.Item1.Clear();
            data3.Item2.Clear();
      
            Console.ReadKey();
        }

可以看到性能和手寫方法之間的差距，如果要求比較高還是手寫方法，如果字段多的話寫起來是很痛苦的事。但是日常用這個足夠了，而且是擴展方法，通用性很強。

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