Expression.Label Method

Definition

Namespace:

System.Linq.Expressions

Assembly:

System.Linq.Expressions.dll

Creates a LabelTarget representing a label.

Overloads

Label()	Creates a LabelTarget representing a label with void type and no name.
Label(LabelTarget)	Creates a LabelExpression representing a label without a default value.
Label(String)	Creates a LabelTarget representing a label with void type and the given name.
Label(Type)	Creates a LabelTarget representing a label with the given type.
Label(LabelTarget, Expression)	Creates a LabelExpression representing a label with the given default value.
Label(Type, String)	Creates a LabelTarget representing a label with the given type and name.

Label()

Source:

LabelTarget.cs

Creates a LabelTarget representing a label with void type and no name.

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

Returns

LabelTarget

The new LabelTarget.

Examples

The following example demonstrates how to create an expression that contains a LabelTarget object.

// 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).

Label(LabelTarget)

Source:

LabelExpression.cs

Creates a LabelExpression representing a label without a default value.

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

Parameters

target

LabelTarget

The LabelTarget which this LabelExpression will be associated with.

Returns

LabelExpression

A LabelExpression without a default value.

Label(String)

Source:

LabelTarget.cs

Creates a LabelTarget representing a label with void type and the given name.

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

Parameters

name

String

The name of the label.

Returns

LabelTarget

The new LabelTarget.

Label(Type)

Source:

LabelTarget.cs

Creates a LabelTarget representing a label with the given type.

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

Parameters

type

Type

The type of value that is passed when jumping to the label.

Returns

LabelTarget

The new LabelTarget.

Examples

The following example demonstrates how to use a LabelTarget object in a loop expression.

// 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

Label(LabelTarget, Expression)

Source:

LabelExpression.cs

Creates a LabelExpression representing a label with the given default value.

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

Parameters

target

LabelTarget

The LabelTarget which this LabelExpression will be associated with.

defaultValue

Expression

The value of this LabelExpression when the label is reached through regular control flow.

Returns

LabelExpression

A LabelExpression with the given default value.

Label(Type, String)

Source:

LabelTarget.cs

Creates a LabelTarget representing a label with the given type and name.

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

Parameters

type

Type

The type of value that is passed when jumping to the label.

name

String

The name of the label.

Returns

LabelTarget

The new LabelTarget.