Sdílet prostřednictvím

  • Článek

Expression<TDelegate> Class

Microsoft Silverlight will reach end of support after October 2021. Learn more.

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

Inheritance Hierarchy

System.Object
  System.Linq.Expressions.Expression
    System.Linq.Expressions.LambdaExpression
      System.Linq.Expressions.Expression<TDelegate>

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

Syntax

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

public sealed class Expression<TDelegate> : LambdaExpression

Type Parameters

  • TDelegate
    The type of the delegate that the Expression<TDelegate> represents.

The Expression<TDelegate> type exposes the following members.

Properties

  Name Description
Public propertySupported by Silverlight for Windows Phone Body Gets the body of the lambda expression. (Inherited from LambdaExpression.)
Public property CanReduce Indicates 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 property Name Gets the name of the lambda expression. (Inherited from LambdaExpression.)
Public propertySupported by Silverlight for Windows Phone NodeType Returns the node type of this Expression. (Inherited from LambdaExpression.)

In Silverlight for Windows Phone Windows Phone OS 7.1, this member is inherited from Expression.NodeType.

In XNA Framework Windows Phone OS 7.0, this member is inherited from Expression.NodeType.
Public propertySupported by Silverlight for Windows Phone Parameters Gets the parameters of the lambda expression. (Inherited from LambdaExpression.)
Public property ReturnType Gets the return type of the lambda expression. (Inherited from LambdaExpression.)
Public property TailCall Gets the value that indicates if the lambda expression will be compiled with the tail call optimization. (Inherited from LambdaExpression.)
Public propertySupported by Silverlight for Windows Phone Type Gets the static type of the expression that this Expression represents. (Inherited from LambdaExpression.)

In Silverlight for Windows Phone Windows Phone OS 7.1, this member is inherited from Expression.Type.

In XNA Framework Windows Phone OS 7.0, this member is inherited from Expression.Type.

Top

Methods

  Name Description
Protected method Accept Dispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall. (Inherited from Expression.)
Public methodSupported by Silverlight for Windows Phone Compile() Compiles the lambda expression described by the expression tree into executable code and produces a delegate that represents the lambda expression.
Public method Compile(DebugInfoGenerator) Produces a delegate that represents the lambda expression.
Public method CompileToMethod(MethodBuilder) Compiles the lambda into a method definition. (Inherited from LambdaExpression.)
Public method CompileToMethod(MethodBuilder, DebugInfoGenerator) Compiles the lambda into a method definition and custom debug information. (Inherited from LambdaExpression.)
Public methodSupported by Silverlight for Windows Phone Equals(Object) Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows Phone Finalize Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by Silverlight for Windows Phone GetHashCode Serves as a hash function for a particular type. (Inherited from Object.)
Public methodSupported by Silverlight for Windows Phone GetType Gets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows Phone MemberwiseClone Creates a shallow copy of the current Object. (Inherited from Object.)
Public method Reduce Reduces 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 method ReduceAndCheck Reduces 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 method ReduceExtensions Reduces 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 Silverlight for Windows Phone ToString Returns a textual representation of the Expression. (Inherited from Expression.)
Public method Update Creates 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 method VisitChildren Reduces 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.)

Top

Remarks

When a lambda expression is assigned to a variable, field, or parameter whose type is Expression<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<TDelegate>.

The NodeType of an Expression<TDelegate> is Lambda.

Use the Lambda<TDelegate>(Expression, IEnumerable<ParameterExpression>) or Lambda<TDelegate>(Expression, array<ParameterExpression[]) method to create an Expression<TDelegate> object.

Examples

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(i) i < 5
      ' Invoke the delegate and display the output.
      outputBlock.Text &= String.Format("deleg(4) = {0}", deleg(4)) & vbCrLf

      ' Lambda expression as data in the form of an expression tree.
      Dim expr As System.Linq.Expressions.Expression(Of Func(Of Integer, Boolean)) = Function(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.
      outputBlock.Text &= String.Format("deleg2(4) = {0}", deleg2(4)) & vbCrLf

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



      // Lambda expression as executable code.
      Func<int, bool> deleg = i => i < 5;
      // Invoke the delegate and display the output.
      outputBlock.Text += String.Format("deleg(4) = {0}", deleg(4)) + "\n";

      // Lambda expression as data in the form of an expression tree.
      System.Linq.Expressions.Expression<Func<int, bool>> expr = i => i < 5;
      // Compile the expression tree into executable code.
      Func<int, bool> deleg2 = expr.Compile();
      // Invoke the method and print the output.
      outputBlock.Text += String.Format("deleg2(4) = {0}", deleg2(4)) + "\n";

      /*  This code produces the following output:

          deleg(4) = True
          deleg2(4) = True
      */

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.

Thread Safety

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