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.

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<(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, array<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

// 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
*/

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

.NET Framework

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