Expression.Label 方法

参考

定义

命名空间:: System.Linq.Expressions

程序集:: System.Linq.Expressions.dll

程序集:: System.Core.dll

程序集:: netstandard.dll

重要

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创建一个表示标签的 LabelTarget。

重载

Label()	创建一个 LabelTarget，它表示具有 void 类型但没有名称的标签。
Label(LabelTarget)	创建一个LabelExpression，它表示不带默认值的标签。
Label(String)	创建一个 LabelTarget，它表示具有 void 类型和给定名称的标签。
Label(Type)	创建一个 LabelTarget，它表示具有给定类型的标签。
Label(LabelTarget, Expression)	创建一个 LabelExpression，它表示具有给定默认值的标签。
Label(Type, String)	创建一个 LabelTarget，它表示具有给定类型和名称的标签。

Label()

Source:: LabelTarget.cs

Source:: LabelTarget.cs

Source:: LabelTarget.cs

创建一个 LabelTarget，它表示具有 void 类型但没有名称的标签。

public:
 static System::Linq::Expressions::LabelTarget ^ Label();

public static System.Linq.Expressions.LabelTarget Label ();

static member Label : unit -> System.Linq.Expressions.LabelTarget

Public Shared Function Label () As LabelTarget

LabelTarget

新的 LabelTarget。

示例

以下示例演示如何创建包含 LabelTarget 对象的表达式。

// Add the following directive to the file:
// using System.Linq.Expressions;

// A label expression of the void type that is the target for Expression.Return().
LabelTarget returnTarget = Expression.Label();

// This block contains a GotoExpression that represents a return statement with no value.
// It transfers execution to a label expression that is initialized with the same LabelTarget as the GotoExpression.
// The types of the GotoExpression, label expression, and LabelTarget must match.
BlockExpression blockExpr =
    Expression.Block(
        Expression.Call(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }), Expression.Constant("Return")),
        Expression.Return(returnTarget),
        Expression.Call(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }), Expression.Constant("Other Work")),
        Expression.Label(returnTarget)
    );

// The following statement first creates an expression tree,
// then compiles it, and then runs it.
Expression.Lambda<Action>(blockExpr).Compile()();

// This code example produces the following output:
//
// Return

// "Other Work" is not printed because
// the Return expression transfers execution from Expression.Return(returnTarget)
// to Expression.Label(returnTarget).

' Add the following directive to the file:
' Imports System.Linq.Expressions  

' A label expression of the void type that is the target for Expression.Return().
Dim returnTarget As LabelTarget = Expression.Label()

' This block contains a GotoExpression that represents a return statement with no value.
' It transfers execution to a label expression that is initialized with the same LabelTarget as the GotoExpression.
' The types of the GotoExpression, label expression, and LabelTarget must match.
Dim blockExpr As BlockExpression =
      Expression.Block(
          Expression.Call(GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}), Expression.Constant("Return")),
          Expression.Return(returnTarget),
          Expression.Call(GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}), Expression.Constant("Other Work")),
          Expression.Label(returnTarget)
      )

' The following statement first creates an expression tree,
' then compiles it, and then runs it.
Expression.Lambda(Of Action)(blockExpr).Compile()()

' This code example produces the following output:
'
' Return

' "Other Work" is not printed because 
' the Return expression transfers execution from Return(returnTarget)
' to Expression.Label(returnTarget).

适用于

Label(LabelTarget)

Source:: LabelExpression.cs

Source:: LabelExpression.cs

Source:: LabelExpression.cs

创建一个LabelExpression，它表示不带默认值的标签。

public:
 static System::Linq::Expressions::LabelExpression ^ Label(System::Linq::Expressions::LabelTarget ^ target);

public static System.Linq.Expressions.LabelExpression Label (System.Linq.Expressions.LabelTarget target);

static member Label : System.Linq.Expressions.LabelTarget -> System.Linq.Expressions.LabelExpression

Public Shared Function Label (target As LabelTarget) As LabelExpression

参数

target: LabelTarget

此 LabelTarget 将关联的 LabelExpression。

LabelExpression

不带默认值的 LabelExpression。

适用于

Label(String)

Source:: LabelTarget.cs

Source:: LabelTarget.cs

Source:: LabelTarget.cs

创建一个 LabelTarget，它表示具有 void 类型和给定名称的标签。

public:
 static System::Linq::Expressions::LabelTarget ^ Label(System::String ^ name);

public static System.Linq.Expressions.LabelTarget Label (string name);

public static System.Linq.Expressions.LabelTarget Label (string? name);

static member Label : string -> System.Linq.Expressions.LabelTarget

Public Shared Function Label (name As String) As LabelTarget

参数

name: String

标签的名称。

LabelTarget

新的 LabelTarget。

适用于

Label(Type)

Source:: LabelTarget.cs

Source:: LabelTarget.cs

Source:: LabelTarget.cs

创建一个 LabelTarget，它表示具有给定类型的标签。

public:
 static System::Linq::Expressions::LabelTarget ^ Label(Type ^ type);

public static System.Linq.Expressions.LabelTarget Label (Type type);

static member Label : Type -> System.Linq.Expressions.LabelTarget

Public Shared Function Label (type As Type) As LabelTarget

参数

type: Type

跳转到标签时传递的值的类型。

LabelTarget

新的 LabelTarget。

示例

以下示例演示如何在循环表达式中使用 LabelTarget 对象。

// Add the following directive to the file:
// using System.Linq.Expressions;

// Creating a parameter expression.
ParameterExpression value = Expression.Parameter(typeof(int), "value");

// Creating an expression to hold a local variable.
ParameterExpression result = Expression.Parameter(typeof(int), "result");

// Creating a label to jump to from a loop.
LabelTarget label = Expression.Label(typeof(int));

// Creating a method body.
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
    )
);

// Compile and run an expression tree.
int factorial = Expression.Lambda<Func<int, int>>(block, value).Compile()(5);

Console.WriteLine(factorial);

// This code example produces the following output:
//
// 120

' Add the following directive to the file:
' Imports System.Linq.Expressions  

' Creating a parameter expression.
Dim value As ParameterExpression =
    Expression.Parameter(GetType(Integer), "value")

' Creating an expression to hold a local variable. 
Dim result As ParameterExpression =
    Expression.Parameter(GetType(Integer), "result")

' Creating a label to jump to from a loop.
Dim label As LabelTarget = Expression.Label(GetType(Integer))

' Creating a method body.
Dim block As BlockExpression = Expression.Block(
    New ParameterExpression() {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
    )
)

' Compile an expression tree and return a delegate.
Dim factorial As Integer =
    Expression.Lambda(Of Func(Of Integer, Integer))(block, value).Compile()(5)

Console.WriteLine(factorial)

' This code example produces the following output:
'
' 120

适用于

Label(LabelTarget, Expression)

Source:: LabelExpression.cs

Source:: LabelExpression.cs

Source:: LabelExpression.cs

创建一个 LabelExpression，它表示具有给定默认值的标签。

public:
 static System::Linq::Expressions::LabelExpression ^ Label(System::Linq::Expressions::LabelTarget ^ target, System::Linq::Expressions::Expression ^ defaultValue);

public static System.Linq.Expressions.LabelExpression Label (System.Linq.Expressions.LabelTarget target, System.Linq.Expressions.Expression defaultValue);

public static System.Linq.Expressions.LabelExpression Label (System.Linq.Expressions.LabelTarget target, System.Linq.Expressions.Expression? defaultValue);

static member Label : System.Linq.Expressions.LabelTarget * System.Linq.Expressions.Expression -> System.Linq.Expressions.LabelExpression

Public Shared Function Label (target As LabelTarget, defaultValue As Expression) As LabelExpression

参数

target: LabelTarget

此 LabelTarget 将关联的 LabelExpression。

defaultValue: Expression

当通过常规控制流到达标签时，此 LabelExpression 的值。

LabelExpression

具有给定默认值的 LabelExpression。

适用于

Label(Type, String)

Source:: LabelTarget.cs

Source:: LabelTarget.cs

Source:: LabelTarget.cs

创建一个 LabelTarget，它表示具有给定类型和名称的标签。

public:
 static System::Linq::Expressions::LabelTarget ^ Label(Type ^ type, System::String ^ name);

public static System.Linq.Expressions.LabelTarget Label (Type type, string name);

public static System.Linq.Expressions.LabelTarget Label (Type type, string? name);

static member Label : Type * string -> System.Linq.Expressions.LabelTarget

Public Shared Function Label (type As Type, name As String) As LabelTarget

参数

type: Type

跳转到标签时传递的值的类型。

name: String

标签的名称。

LabelTarget

新的 LabelTarget。

适用于

反馈

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通过

Label()

返回

示例

适用于

Label(LabelTarget)

参数

返回

适用于

Label(String)

参数

返回

适用于

Label(Type)

参数

返回

示例

适用于

Label(LabelTarget, Expression)

参数

返回

适用于

Label(Type, String)

参数

返回

适用于