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

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

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BlockExpression 包含一个表达式序列的块，表达式中可定义变量

 1                 BlockExpression blockExpr = Expression.Block(
 2                      Expression.Call(null, typeof(Console).GetMethod("Write", new Type[] { typeof(String) }), Expression.Constant("Hello ")),
 3                      Expression.Call(null, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }), Expression.Constant("World!")),
 4                      Expression.Constant(42)
 5                 );
 6                 var result = Expression.Lambda<Func<int>>(blockExpr).Compile()();
 7                 Console.WriteLine("**************************");
 8                 foreach (var expr in blockExpr.Expressions)
 9                     Console.WriteLine(expr.ToString());
10                 Console.WriteLine("**************************");
11                 Console.WriteLine(result);

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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

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

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DefaultExpression 类型或空表达式的默认值

1                 Expression defaultExpr = Expression.Default(typeof(byte));
2 
3                 // Print out the expression.
4                 Console.WriteLine(defaultExpr.ToString());//  等价于 default(byte)
5 
6                 // The following statement first creates an expression tree,
7                 // then compiles it, and then executes it.
8                 Console.WriteLine(Expression.Lambda<Func<byte>>(defaultExpr).Compile()());//0

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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

1                 ParameterExpression arrayExpr = Expression.Parameter(typeof(int[]), "Array");
2                 ParameterExpression indexExpr = Expression.Parameter(typeof(int), "Index");
3                 ParameterExpression valueExpr = Expression.Parameter(typeof(int), "Value");
4                 Expression arrayAccessExpr = Expression.ArrayAccess(
5                     arrayExpr,
6                     indexExpr
7                 );//Array[Index]

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

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

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

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

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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

 1                 string tree1 = "maple";
 2                 string tree2 = "oak";
 3 
 4                 MethodInfo addMethod = typeof(Dictionary<int, string>).GetMethod("Add");
 5 
 6                 // Create two ElementInit objects that represent the
 7                 // two key-value pairs to add to the Dictionary.
 8                 ElementInit elementInit1 =Expression.ElementInit(addMethod,Expression.Constant(tree1.Length),Expression.Constant(tree1));
 9                 ElementInit elementInit2 =Expression.ElementInit(addMethod,Expression.Constant(tree2.Length),Expression.Constant(tree2));
10 
11                 // Create a NewExpression that represents constructing
12                 // a new instance of Dictionary<int, string>.
13                 NewExpression newDictionaryExpression = Expression.New(typeof(Dictionary<int, string>));//等价 new Dictionary<int, string>();
14                 
15                 // Create a ListInitExpression that represents initializing
16                 // a new Dictionary<> instance with two key-value pairs.
17                 ListInitExpression listInitExpression = Expression.ListInit(newDictionaryExpression, elementInit1, elementInit2);//等价 var dic= new Dictionary<int, string>{}; dic.Add(5,"maple");dic.Add(3,"oak");
18                 
19                 Console.WriteLine(listInitExpression.ToString());

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LoopExpression 无限循环。可以使用“break”退出它

LabelTarget 表示 GotoExpression 的目标

 1                 ParameterExpression value = Expression.Parameter(typeof(int), "value");
 2                 ParameterExpression result = Expression.Parameter(typeof(int), "result");
 3                 LabelTarget label = Expression.Label(typeof(int));
 4                 BlockExpression block = Expression.Block(
 5                     new[] { result },
 6                     Expression.Assign(result, Expression.Constant(1)),
 7                         Expression.Loop(
 8                            Expression.IfThenElse(
 9                                Expression.GreaterThan(value, Expression.Constant(1)),
10                                Expression.MultiplyAssign(result,
11                                    Expression.PostDecrementAssign(value)),
12                                Expression.Break(label, result)
13                            ),
14                        label
15                     )
16                 );
17                 //var s =value=>
18                 //{
19                 //    var result = 1;
20                 //    for (int i = value; i >1; i--)
21                 //    {
22                 //        result *= i;
23                 //    }
24                 //    return result;
25                 //};

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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

 1     public class BaseEntity
 2     {
 3         /// <summary>
 4         /// 创建人账号
 5         /// </summary>    
 6         [DataMember]
 7         [Display(Name = "创建人账号")]
 8         [Column]
 9         public string CreateMan { get; set; }
10         /// <summary>
11         /// 创建时间
12         /// </summary>    
13         [DataMember]
14         [Display(Name = "创建时间")]
15         public DateTime CreateDateTime { get; set; }
16         /// <summary>
17         /// 异动人账号
18         /// </summary>    
19         [DataMember]
20         [Display(Name = "异动人账号")]
21         public string TrMan { get; set; }
22         /// <summary>
23         /// 异动时间
24         /// </summary>    
25         [DataMember]
26         [Display(Name = "异动时间")]
27         public DateTime TrDateTime { get; set; }
28         /// <summary>
29         /// 时间戳
30         /// </summary>    
31         [DataMember]
32         [Display(Name = "时间戳")]
33         public DateTime? TrVersion { get; set; }
34     }

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 1                 BaseEntity entity = new BaseEntity();
 2                 NewExpression newExp = Expression.New(typeof(BaseEntity));
 3 
 4                 MemberInfo createMan = typeof(BaseEntity).GetMember("CreateMan")[0];
 5                 MemberInfo trMan = typeof(BaseEntity).GetMember("TrMan")[0];
 6                 MemberBinding createManMemberBinding = Expression.Bind(createMan, Expression.Constant("horse"));
 7                 MemberBinding trManMemberBinding = Expression.Bind(trMan, Expression.Constant("admin"));
 8                 MemberInitExpression memberInitExpression = Expression.MemberInit(newExp, createManMemberBinding, trManMemberBinding);
 9 
10                 Console.WriteLine(memberInitExpression.ToString());

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NewArrayExpression 创建新数组并可能初始化该新数组的元素

 1                 List<Expression> trees =new List<Expression>()
 2                 { 
 3                     Expression.Constant("oak"),
 4                     Expression.Constant("fir"),
 5                     Expression.Constant("spruce"),
 6                     Expression.Constant("alder") 
 7                 };
 8                 NewArrayExpression newArrayExpression =Expression.NewArrayInit(typeof(string), trees);
 9                 
10                 // new [] {"oak", "fir", "spruce", "alder"}
11                 Console.WriteLine(newArrayExpression.ToString());

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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

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

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TryExpression try/catch/finally/fault 块

CatchBlock try 块中的 catch 语句

1                     TryExpression tryCatchExpr =Expression.TryCatch(
2                     Expression.Block(
3                         Expression.Throw(Expression.Constant(new DivideByZeroException())),
4                         Expression.Constant("Try block")
5                     ),
6                     Expression.Catch(
7                         typeof(DivideByZeroException),
8                         Expression.Constant("Catch block") 
9                 ));