C# 表达式树 Expression

表达式树是定义代码的数据结构。 它们基于编译器用于分析代码和生成已编译输出的相同结构。

几种常见的表达式

BinaryExpression 包含二元运算符的表达式

                BinaryExpression binaryExpression = Expression.MakeBinary(ExpressionType.Add,Expression.Constant(1),Expression.Constant(2));
                Console.WriteLine(binaryExpression.ToString());//(1+2) 不进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.AddChecked, Expression.Constant(3), Expression.Constant(4));
                Console.WriteLine(binaryExpression.ToString());//(3+4) 进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.Subtract, Expression.Constant(5), Expression.Constant(6));
                Console.WriteLine(binaryExpression.ToString());//(5-6) 不进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.SubtractChecked, Expression.Constant(7), Expression.Constant(8));
                Console.WriteLine(binaryExpression.ToString());//(7-8) 进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.Multiply, Expression.Constant(9), Expression.Constant(10));
                Console.WriteLine(binaryExpression.ToString());//(9*10) 不进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.MultiplyChecked, Expression.Constant(11), Expression.Constant(12));
                Console.WriteLine(binaryExpression.ToString());//(11*12) 进行溢出检查
                binaryExpression = Expression.MakeBinary(ExpressionType.Divide, Expression.Constant(13), Expression.Constant(14));
                Console.WriteLine(binaryExpression.ToString());//(13/14) 
                binaryExpression = Expression.MakeBinary(ExpressionType.Modulo, Expression.Constant(15), Expression.Constant(16));
                Console.WriteLine(binaryExpression.ToString());//(15%16) 

BlockExpression 包含一个表达式序列的块，表达式中可定义变量

                BlockExpression blockExpr = Expression.Block(
                     Expression.Call(null, typeof(Console).GetMethod("Write", new Type[] { typeof(String) }), Expression.Constant("Hello ")),
                     Expression.Call(null, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }), Expression.Constant("World!")),
                     Expression.Constant(42)
                );
                var result = Expression.Lambda<Func<int>>(blockExpr).Compile()();
                Console.WriteLine("**************************");
                foreach (var expr in blockExpr.Expressions)
                    Console.WriteLine(expr.ToString());
                Console.WriteLine("**************************");
                Console.WriteLine(result);

程序执行结果

ConditionalExpression 具有条件运算符的表达式

Expression conditionExpr = Expression.Condition(Expression.Constant(num > 10),Expression.Constant("num is greater than 10"),Expression.Constant("num is smaller than 10"));

ConstantExpression 具有常数值的表达式

Expression.Constant(5.5);
Expression.Constant("Hello World！");

DefaultExpression 类型或空表达式的默认值

                Expression defaultExpr = Expression.Default(typeof(byte));

                // Print out the expression.
                Console.WriteLine(defaultExpr.ToString());//  等价于 default(byte)

                // The following statement first creates an expression tree,
                // then compiles it, and then executes it.
                Console.WriteLine(Expression.Lambda<Func<byte>>(defaultExpr).Compile()());//0

ParameterExpression 命名的参数表达式

ParameterExpression param = Expression.Parameter(typeof(int));

IndexExpression 编制属性或数组的索引

                ParameterExpression arrayExpr = Expression.Parameter(typeof(int[]), "Array");
                ParameterExpression indexExpr = Expression.Parameter(typeof(int), "Index");
                ParameterExpression valueExpr = Expression.Parameter(typeof(int), "Value");
                Expression arrayAccessExpr = Expression.ArrayAccess(
                    arrayExpr,
                    indexExpr
                );//Array[Index]

InvocationExpression 将委托或 lambda 表达式应用于参数表达式列表的表达式

                Expression<Func<int, int, bool>> largeSumTest =(num1, num2) => (num1 + num2) > 1000;
                InvocationExpression invocationExpression =Expression.Invoke(largeSumTest,Expression.Constant(539),Expression.Constant(281));
                Console.WriteLine(invocationExpression.ToString());//Invoke((num1, num2) => ((num1 + num2) > 1000), 539, 281)

LambdaExpression 描述一个 lambda 表达式。 这将捕获与 .NET 方法体类似的代码块

                ParameterExpression paramExpr = Expression.Parameter(typeof(int), "arg");
                LambdaExpression lambdaExpr = Expression.Lambda(Expression.Add(paramExpr,Expression.Constant(1)),new List<ParameterExpression>() { paramExpr });
                Console.WriteLine(lambdaExpr);// arg => (arg +1)

ElementInit   表示 IEnumerable 集合的单个元素的初始值设定项

ListInitExpression 表示包含集合初始值设定项的构造函数调用

NewExpression 构造函数调用

                string tree1 = "maple";
                string tree2 = "oak";

                MethodInfo addMethod = typeof(Dictionary<int, string>).GetMethod("Add");

                // Create two ElementInit objects that represent the
                // two key-value pairs to add to the Dictionary.
                ElementInit elementInit1 =Expression.ElementInit(addMethod,Expression.Constant(tree1.Length),Expression.Constant(tree1));
                ElementInit elementInit2 =Expression.ElementInit(addMethod,Expression.Constant(tree2.Length),Expression.Constant(tree2));

                // Create a NewExpression that represents constructing
                // a new instance of Dictionary<int, string>.
                NewExpression newDictionaryExpression = Expression.New(typeof(Dictionary<int, string>));//等价 new Dictionary<int, string>();
                
                // Create a ListInitExpression that represents initializing
                // a new Dictionary<> instance with two key-value pairs.
                ListInitExpression listInitExpression = Expression.ListInit(newDictionaryExpression, elementInit1, elementInit2);//等价 var dic= new Dictionary<int, string>{}; dic.Add(5,"maple");dic.Add(3,"oak");
                
                Console.WriteLine(listInitExpression.ToString());

LoopExpression 无限循环。 可以使用“break”退出它

LabelTarget  表示 GotoExpression 的目标

                ParameterExpression value = Expression.Parameter(typeof(int), "value");
                ParameterExpression result = Expression.Parameter(typeof(int), "result");
                LabelTarget label = Expression.Label(typeof(int));
                BlockExpression block = Expression.Block(
                    new[] { result },
                    Expression.Assign(result, Expression.Constant(1)),
                        Expression.Loop(
                           Expression.IfThenElse(
                               Expression.GreaterThan(value, Expression.Constant(1)),
                               Expression.MultiplyAssign(result,
                                   Expression.PostDecrementAssign(value)),
                               Expression.Break(label, result)
                           ),
                       label
                    )
                );
                //var s =value=>
                //{
                //    var result = 1;
                //    for (int i = value; i >1; i--)
                //    {
                //        result *= i;
                //    }
                //    return result;
                //};

MemberAssignment 针对对象的字段或属性的赋值运算

MemberBinding 提供一种基类，该基类派生表示绑定的类，这些绑定用于初始化新创建对象的成员

MemberExpression 访问字段或属性

MemberInitExpression 调用构造函数并初始化新对象的一个或多个成员

MemberListBinding 初始化新创建对象的集合成员的元素

MemberMemberBinding 初始化新创建对象的成员的成员

    public class BaseEntity
    {
        /// <summary>
        /// 创建人账号
        /// </summary>    
        [DataMember]
        [Display(Name = "创建人账号")]
        [Column]
        public string CreateMan { get; set; }
        /// <summary>
        /// 创建时间
        /// </summary>    
        [DataMember]
        [Display(Name = "创建时间")]
        public DateTime CreateDateTime { get; set; }
        /// <summary>
        /// 异动人账号
        /// </summary>    
        [DataMember]
        [Display(Name = "异动人账号")]
        public string TrMan { get; set; }
        /// <summary>
        /// 异动时间
        /// </summary>    
        [DataMember]
        [Display(Name = "异动时间")]
        public DateTime TrDateTime { get; set; }
        /// <summary>
        /// 时间戳
        /// </summary>    
        [DataMember]
        [Display(Name = "时间戳")]
        public DateTime? TrVersion { get; set; }
    }

                BaseEntity entity = new BaseEntity();
                NewExpression newExp = Expression.New(typeof(BaseEntity));

                MemberInfo createMan = typeof(BaseEntity).GetMember("CreateMan")[0];
                MemberInfo trMan = typeof(BaseEntity).GetMember("TrMan")[0];
                MemberBinding createManMemberBinding = Expression.Bind(createMan, Expression.Constant("horse"));
                MemberBinding trManMemberBinding = Expression.Bind(trMan, Expression.Constant("admin"));
                MemberInitExpression memberInitExpression = Expression.MemberInit(newExp, createManMemberBinding, trManMemberBinding);

                Console.WriteLine(memberInitExpression.ToString());

NewArrayExpression 创建新数组并可能初始化该新数组的元素

                List<Expression> trees =new List<Expression>()
                { 
                    Expression.Constant("oak"),
                    Expression.Constant("fir"),
                    Expression.Constant("spruce"),
                    Expression.Constant("alder") 
                };
                NewArrayExpression newArrayExpression =Expression.NewArrayInit(typeof(string), trees);
                
                // new [] {"oak", "fir", "spruce", "alder"}
                Console.WriteLine(newArrayExpression.ToString());

SwitchCase  SwitchExpression 的一个事例

SwitchExpression 一个控制表达式，该表达式通过将控制传递到 SwitchCase 来处理多重选择

                ConstantExpression switchValue = Expression.Constant(2);
                SwitchExpression switchExpr =Expression.Switch(switchValue,new SwitchCase[] {
                    Expression.SwitchCase(Expression.Call(null,
                    typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                    Expression.Constant("First")),Expression.Constant(1)),
                    Expression.SwitchCase(Expression.Call(null,
                    typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                    Expression.Constant("Second")),Expression.Constant(2))});
                Expression.Lambda<Action>(switchExpr).Compile()();

TryExpression  try/catch/finally/fault 块

CatchBlock  try 块中的 catch 语句

                    TryExpression tryCatchExpr =Expression.TryCatch(
                    Expression.Block(
                        Expression.Throw(Expression.Constant(new DivideByZeroException())),
                        Expression.Constant("Try block")
                    ),
                    Expression.Catch(
                        typeof(DivideByZeroException),
                        Expression.Constant("Catch block") 
                ));

UnaryExpression 包含一元运算符的表达式

UnaryExpression typeAsExpression =Expression.TypeAs(Expression.Constant(34, typeof(int)),typeof(int?));//等价    34 as int?;

微软文档地址：

System.Linq.Expressions：https://docs.microsoft.com/zh-cn/dotnet/api/system.linq.expressions?view=netframework-4.8

ExpressionType：https://docs.microsoft.com/zh-cn/dotnet/api/system.linq.expressions.expressiontype?view=netframework-4.8#System_Linq_Expressions_ExpressionType_Add