Expression(Of TDelegate) Class

Represents a strongly typed lambda expression as a data structure in the form of an expression tree. This class cannot be inherited.

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

'Declaration
Public NotInheritable Class Expression(Of TDelegate) _
	Inherits LambdaExpression

Type Parameters

TDelegate

The type of the delegate that the Expression(Of TDelegate) represents.

The Expression(Of TDelegate) type exposes the following members.

  NameDescription
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsBodyGets the body of the lambda expression. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsCanReduceIndicates that the node can be reduced to a simpler node. If this returns true, Reduce() can be called to produce the reduced form. (Inherited from Expression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsNameGets the name of the lambda expression. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsNodeTypeReturns the node type of this Expression. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsParametersGets the parameters of the lambda expression. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsReturnTypeGets the return type of the lambda expression. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsTailCallGets the value that indicates if the lambda expression will be compiled with the tail call optimization. (Inherited from LambdaExpression.)
Public propertySupported by Portable Class LibrarySupported in .NET for Windows Store appsTypeGets the static type of the expression that this Expression represents. (Inherited from LambdaExpression.)
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  NameDescription
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsAcceptDispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall. (Inherited from Expression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsCompileCompiles the lambda expression described by the expression tree into executable code and produces a delegate that represents the lambda expression.
Public methodCompile(DebugInfoGenerator)Produces a delegate that represents the lambda expression.
Public methodCompileToMethod(MethodBuilder)Compiles the lambda into a method definition. (Inherited from LambdaExpression.)
Public methodCompileToMethod(MethodBuilder, DebugInfoGenerator)Compiles the lambda into a method definition and custom debug information. (Inherited from LambdaExpression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsReduceReduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsReduceAndCheckReduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsReduceExtensionsReduces the expression to a known node type (that is not an Extension node) or just returns the expression if it is already a known type. (Inherited from Expression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringReturns a textual representation of the Expression. (Inherited from Expression.)
Public methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsUpdateCreates a new expression that is like this one, but using the supplied children. If all of the children are the same, it will return this expression.
Protected methodSupported by Portable Class LibrarySupported in .NET for Windows Store appsVisitChildrenReduces the node and then calls the visitor delegate on the reduced expression. The method throws an exception if the node is not reducible. (Inherited from Expression.)
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When a lambda expression is assigned to a variable, field, or parameter whose type is Expression(Of TDelegate), the compiler emits instructions to build an expression tree.

NoteNote

A conversion from a lambda expression to type Expression<D> (Expression(Of D) in Visual Basic) exists if a conversion from the lambda expression to a delegate of type D exists. However, the conversion may fail, for example, if the body of the lambda expression is a block. This means that delegates and expression trees behave similarly with regard to overload resolution.

The expression tree is an in-memory data representation of the lambda expression. The expression tree makes the structure of the lambda expression transparent and explicit. You can interact with the data in the expression tree just as you can with any other data structure.

The ability to treat expressions as data structures enables APIs to receive user code in a format that can be inspected, transformed, and processed in a custom manner. For example, the LINQ to SQL data access implementation uses this facility to translate expression trees to Transact-SQL statements that can be evaluated by the database.

Many standard query operators defined in the Queryable class have one or more parameters of type Expression(Of TDelegate).

The NodeType of an Expression(Of TDelegate) is Lambda.

Use the Lambda(Of TDelegate)(Expression, IEnumerable(Of ParameterExpression)) or Lambda(Of TDelegate)(Expression, ParameterExpression()) method to create an Expression(Of TDelegate) object.

The following code example demonstrates how to represent a lambda expression both as executable code in the form of a delegate and as data in the form of an expression tree. It also demonstrates how to turn the expression tree back into executable code by using the Compile method.


        ' Lambda expression as executable code. 
        Dim deleg As Func(Of Integer, Boolean) = Function(ByVal i) i < 5
        ' Invoke the delegate and display the output.
        MsgBox(String.Format("deleg(4) = {0}", deleg(4)))

        ' Lambda expression as data in the form of an expression tree. 
        Dim expr As System.Linq.Expressions.Expression(Of Func(Of Integer, Boolean)) = Function(ByVal i) i < 5
        ' Compile the expression tree into executable code. 
        Dim deleg2 As Func(Of Integer, Boolean) = expr.Compile()
        ' Invoke the method and print the output.
        MsgBox(String.Format("deleg2(4) = {0}", deleg2(4)))

        ' This code produces the following output: 
        
        ' deleg(4) = True 
        ' deleg2(4) = True

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
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