Expression<TDelegate> Class

Definition

generic <typename TDelegate>
public ref class Expression sealed : System::Linq::Expressions::LambdaExpression

public sealed class Expression<TDelegate> : System.Linq.Expressions.LambdaExpression

type Expression<'Delegate> = class
    inherit LambdaExpression

Public NotInheritable Class Expression(Of TDelegate)
Inherits LambdaExpression

Type Parameters

Inheritance

Object

Expression

LambdaExpression

Expression<TDelegate>

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.
Func<int, bool> deleg = i => i < 5;
// Invoke the delegate and display the output.
Console.WriteLine("deleg(4) = {0}", deleg(4));

// 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.
Console.WriteLine("deleg2(4) = {0}", deleg2(4));

/*  This code produces the following output:

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

' 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

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.

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, ParameterExpression[]) method to create an Expression<TDelegate> object.

Properties

Methods

Applies to

See also

• Lambda Expressions (C# Programming Guide)
• Expression Trees