• Articolo

Expression.Lambda Method (Type, Expression, IEnumerable<ParameterExpression>)

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Creates a LambdaExpression by first constructing a delegate type. It can be used when the delegate type is not known at compile time.

Namespace:  System.Linq.Expressions
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
Public Shared Function Lambda ( _
    delegateType As Type, _
    body As Expression, _
    parameters As IEnumerable(Of ParameterExpression) _
) As LambdaExpression

public static LambdaExpression Lambda(
    Type delegateType,
    Expression body,
    IEnumerable<ParameterExpression> parameters
)

Parameters

  • delegateType
    Type: System.Type
    A Type that represents a delegate signature for the lambda.

Return Value

Type: System.Linq.Expressions.LambdaExpression
An object that represents a lambda expression which has the NodeType property equal to Lambda and the Body and Parameters properties set to the specified values.

Exceptions

Exception Condition
ArgumentNullException

delegateType or body is nulla null reference (Nothing in Visual Basic).

-or-

One or more elements in parameters are nulla null reference (Nothing in Visual Basic).
ArgumentException

delegateType does not represent a delegate type.

-or-

body.Type represents a type that is not assignable to the return type of the delegate type represented by delegateType.

-or-

parameters does not contain the same number of elements as the list of parameters for the delegate type represented by delegateType.

-or-

The Type property of an element of parameters is not assignable from the type of the corresponding parameter type of the delegate type represented by delegateType.

Remarks

The object that is returned from this function is of type Expression<TDelegate>. The LambdaExpression type is used to represent the returned object because the concrete type of the lambda expression is not known at compile time.

The number of parameters for the delegate type represented by delegateType must equal the length of parameters.

The elements of parameters must be reference equal to the parameter expressions in body.

The Type property of the resulting object is equal to delegateType. If parameters is nulla null reference (Nothing in Visual Basic), the Parameters property of the resulting object is an empty collection.

Examples

The following example demonstrates how to create an expression that represents a lambda expression that adds 1 to the passed argument.

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

' A parameter for the lambda expression.
Dim paramExpr As ParameterExpression = Expression.Parameter(GetType(Integer), "arg")

' This expression represents a lambda expression
' that adds 1 to the parameter value.
Dim lambdaExpr As LambdaExpression = Expression.Lambda(
        Expression.Add(
            paramExpr,
            Expression.Constant(1)
        ),
        New List(Of ParameterExpression)() From {paramExpr}
    )

' Print out the expression.
outputBlock.Text &= lambdaExpr.ToString() & vbCrLf

' Compile and run the lamda expression.
' The value of the parameter is 1.
outputBlock.Text &= lambdaExpr.Compile().DynamicInvoke(1).ToString() & vbCrLf

' This code example produces the following output:
'
' arg => (arg +1)
' 2

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

// A parameter for the lambda expression.
ParameterExpression paramExpr = Expression.Parameter(typeof(int), "arg");

// This expression represents a lambda expression
// that adds 1 to the parameter value.
LambdaExpression lambdaExpr = Expression.Lambda(
    Expression.Add(
        paramExpr,
        Expression.Constant(1)
    ),
    new List<ParameterExpression>() { paramExpr }
);

// Print out the expression.
outputBlock.Text += lambdaExpr + "\n";

// Compile and run the lamda expression.
// The value of the parameter is 1.
outputBlock.Text += lambdaExpr.Compile().DynamicInvoke(1) + "\n";

// This code example produces the following output:
//
// arg => (arg +1)
// 2

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.