Expression<TDelegate> Class

Definition

Namespace:

System.Linq.Expressions

Assembly:

System.Linq.Expressions.dll

Source:

LambdaExpression.cs

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

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

TDelegate

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

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.

Note

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

Properties

Body	Gets the body of the lambda expression. (Inherited from LambdaExpression)
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)
Name	Gets the name of the lambda expression. (Inherited from LambdaExpression)
NodeType	Returns the node type of this Expression. (Inherited from LambdaExpression)
Parameters	Gets the parameters of the lambda expression. (Inherited from LambdaExpression)
ReturnType	Gets the return type of the lambda expression. (Inherited from LambdaExpression)
TailCall	Gets the value that indicates if the lambda expression will be compiled with the tail call optimization. (Inherited from LambdaExpression)
Type	Gets the static type of the expression that this Expression represents. (Inherited from LambdaExpression)

Methods

Accept(ExpressionVisitor)	Dispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall(MethodCallExpression). (Inherited from Expression)
Compile()	Compiles the lambda expression described by the expression tree into executable code and produces a delegate that represents the lambda expression.
Compile(Boolean)	Compiles the lambda expression described by the expression tree into interpreted or compiled code and produces a delegate that represents the lambda expression.
Compile(DebugInfoGenerator)	Produces a delegate that represents the lambda expression.
CompileToMethod(MethodBuilder)	Compiles the lambda into a method definition. (Inherited from LambdaExpression)
CompileToMethod(MethodBuilder, DebugInfoGenerator)	Compiles the lambda into a method definition and custom debug information. (Inherited from LambdaExpression)
Equals(Object)	Determines whether the specified object is equal to the current object. (Inherited from Object)
GetHashCode()	Serves as the default hash function. (Inherited from Object)
GetType()	Gets the Type of the current instance. (Inherited from Object)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
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)
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)
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)
ToString()	Returns a textual representation of the Expression. (Inherited from Expression)
Update(Expression, IEnumerable<ParameterExpression>)	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.
VisitChildren(ExpressionVisitor)	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)

See also

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