C#如何使用表达式树动态更新类的属性值

本篇内容介绍了“C#如何使用表达式树动态更新类的属性值”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！

C#的λ表达式树是一个好东西，也是别的语言学不来的，熟悉掌握λ表达式就能够实现各种场景的个性化操作，如动态拼接查询条件、排序方式等，也能够实现替代反射的高性能操作，比如我们常用到的IQueryable和IEnumerable，每个扩展方法就全是λ表达式树。

本文给大家分享C#使用表达式树(LambdaExpression)动态更新类的属性值的相关知识，在某些业务中会遇到需要同步两个类的属性值的情况，而且有些字段是要过滤掉的。如果手动赋值则需要写很多重复的代码：

 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的某些属性值赋给student，而id不需要赋值        /// </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;            }        }

使用反射的话性能考虑，尝试写一个扩展方法使用lambda表达式树去构建一个方法

public static class ObjectExtensions    {        /// <summary>        /// 缓存表达式        /// </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)                        {                            //如果是相同类型则获取所有属性                            properties = sourceType.GetProperties(bindingFlags).Select(x => x.Name)                                .ToArray();                        }                        else                        {                            //如果没有传指定的属性则默认获取同名属性                            List<PropertyInfo> propertyInfos = new List<PropertyInfo>();                            foreach (var property in sourceType.GetProperties(bindingFlags))                            {//不同类型指定同名且类型相同的属性                                var targetProperty = targetType.GetProperty(property.Name, bindingFlags);                                if (targetProperty != null && targetProperty.PropertyType == property.PropertyType)                                {                                    propertyInfos.Add(property);                                }                            }                            properties = propertyInfos.Select(x => x.Name).ToArray();                        }                    }                    //定义lambda 3个参数                    var s = Expression.Parameter(typeof(TSource), "s");                    var t = Expression.Parameter(typeof(TTarget), "t");                    var ps = Expression.Parameter(typeof(string[]), "ps");                    //获取泛型扩展方法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));//创建泛型方法                    List<BlockExpression> bs = new List<BlockExpression>();                    foreach (string field in properties)                    {                        //获取两个类型里面的属性                        var sourceField = Expression.Property(s, field);                        var targetField = Expression.Property(t, field);                        //创建一个条件表达式                        var notEqual = Expression.NotEqual(sourceField, targetField);//sourceField!=targetField                        var method = Expression.Call(null, genericMethod, ps, Expression.Constant(field));//ps.Contains(f);                        //构建赋值语句                        var ifTrue = Expression.Assign(sourceField, targetField);                        //拼接表达式 sourceField!=targetField&&ps.Contains(f)                        var condition = Expression.And(notEqual, Expression.IsTrue(method));                        //判断是否相同，如果不相同则赋值                        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>        /// 通过目标类更新源类同名属性值        /// </summary>        /// <typeparam name="TSource">待更新的数据类型</typeparam>        /// <typeparam name="TTarget">目标数据类型</typeparam>        /// <param name="source">源数据</param>        /// <param name="target">目标数据</param>        /// <param name="properties">要变更的属性名称</param>        /// <returns>返回源数据，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);        }    }

编写测试方法

 /// <summary>        /// 比如需要把teacher的某些属性值赋给student，而id不需要赋值        /// </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($"手写方法耗时：{sw.ElapsedMilliseconds}ms");            data.Item1.Clear();            data.Item2.Clear();            var data2 = CreateData(length);            sw.Restart();            for (int i = 0; i < length; i++)            {                data2.Item1[i].SetProperties(data2.Item2[i], nameof(Student.Age), nameof(Student.Name));            }            data2.Item1.Clear();            data2.Item2.Clear();            sw.Stop();            Console.WriteLine($"lambda耗时：{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($"反射耗时：{sw.ElapsedMilliseconds}ms");            data3.Item1.Clear();            data3.Item2.Clear();                  Console.ReadKey();        }

可以看到性能和手写方法之间的差距，如果要求比较高还是手写方法，如果字段多的话写起来是很痛苦的事。但是日常用这个足够了，而且是扩展方法，通用性很强。

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