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

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) > ;
                 InvocationExpression invocationExpression =Expression.Invoke(largeSumTest,Expression.Constant(),Expression.Constant());
                 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()),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()),
                         Expression.Loop(
                            Expression.IfThenElse(
                                Expression.GreaterThan(value, Expression.Constant()),
                                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")[];
                 MemberInfo trMan = typeof(BaseEntity).GetMember("TrMan")[];
                 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();
                 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()),
                     Expression.SwitchCase(Expression.Call(null,
                     typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                     Expression.Constant("Second")),Expression.Constant())});
                 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