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