lambda-expressions.md

title	Lambda Expressions (Visual Basic)
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Lambda Expressions (Visual Basic)

A lambda expression is a function or subroutine without a name that can be used wherever a delegate is valid. Lambda expressions can be functions or subroutines and can be single-line or multi-line. You can pass values from the current scope to a lambda expression.

Note

The RemoveHandler statement is an exception. You cannot pass a lambda expression in for the delegate parameter of RemoveHandler.

You create lambda expressions by using the Function or Sub keyword, just as you create a standard function or subroutine. However, lambda expressions are included in a statement.

The following example is a lambda expression that increments its argument and returns the value. The example shows both the single-line and multi-line lambda expression syntax for a function.

[!code-vbVbVbalrLambdas#14]

The following example is a lambda expression that writes a value to the console. The example shows both the single-line and multi-line lambda expression syntax for a subroutine.

[!code-vbVbVbalrLambdas#15]

Notice that in the previous examples the lambda expressions are assigned to a variable name. Whenever you refer to the variable, you invoke the lambda expression. You can also declare and invoke a lambda expression at the same time, as shown in the following example.

[!code-vbVbVbalrLambdas#3]

A lambda expression can be returned as the value of a function call (as is shown in the example in the Context section later in this topic), or passed in as an argument to a parameter that takes a delegate type, as shown in the following example.

[!code-vbVbVbalrLambdas#8]

Lambda Expression Syntax

The syntax of a lambda expression resembles that of a standard function or subroutine. The differences are as follows:

• A lambda expression does not have a name.

• Lambda expressions cannot have modifiers, such as Overloads or Overrides.

• Single-line lambda functions do not use an As clause to designate the return type. Instead, the type is inferred from the value that the body of the lambda expression evaluates to. For example, if the body of the lambda expression is cust.City = "London", its return type is Boolean.

• In multi-line lambda functions, you can either specify a return type by using an As clause, or omit the As clause so that the return type is inferred. When the As clause is omitted for a multi-line lambda function, the return type is inferred to be the dominant type from all the Return statements in the multi-line lambda function. The dominant type is a unique type that all other types can widen to. If this unique type cannot be determined, the dominant type is the unique type that all other types in the array can narrow to. If neither of these unique types can be determined, the dominant type is Object. In this case, if Option Strict is set to On, a compiler error occurs.

  For example, if the expressions supplied to the Return statement contain values of type Integer, Long, and Double, the resulting array is of type Double. Both Integer and Long widen to Double and only Double. Therefore, Double is the dominant type. For more information, see Widening and Narrowing Conversions.

• The body of a single-line function must be an expression that returns a value, not a statement. There is no Return statement for single-line functions. The value returned by the single-line function is the value of the expression in the body of the function.

• The body of a single-line subroutine must be single-line statement.

• Single-line functions and subroutines do not include an End Function or End Sub statement.

• You can specify the data type of a lambda expression parameter by using the As keyword, or the data type of the parameter can be inferred. Either all parameters must have specified data types or all must be inferred.

• Optional and Paramarray parameters are not permitted.

• Generic parameters are not permitted.

Async Lambdas

You can easily create lambda expressions and statements that incorporate asynchronous processing by using the Async and Await Operator keywords. For example, the following Windows Forms example contains an event handler that calls and awaits an async method, ExampleMethodAsync.

Public Class Form1  
  
    Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click  
        ' ExampleMethodAsync returns a Task.  
        Await ExampleMethodAsync()  
        TextBox1.Text = vbCrLf & "Control returned to button1_Click."  
    End Sub  
  
    Async Function ExampleMethodAsync() As Task  
        ' The following line simulates a task-returning asynchronous process.  
        Await Task.Delay(1000)  
    End Function  
  
End Class  

You can add the same event handler by using an async lambda in an AddHandler Statement. To add this handler, add an Async modifier before the lambda parameter list, as the following example shows.

Public Class Form1  
  
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load  
        AddHandler Button1.Click,   
            Async Sub(sender1, e1)  
                ' ExampleMethodAsync returns a Task.  
                Await ExampleMethodAsync()  
                TextBox1.Text = vbCrLf & "Control returned to Button1_ Click."  
            End Sub  
    End Sub  
  
    Async Function ExampleMethodAsync() As Task  
        ' The following line simulates a task-returning asynchronous process.  
        Await Task.Delay(1000)  
    End Function  
  
End Class  

For more information about how to create and use async methods, see Asynchronous Programming with Async and Await.

Context

A lambda expression shares its context with the scope within which it is defined. It has the same access rights as any code written in the containing scope. This includes access to member variables, functions and subs, Me, and parameters and local variables in the containing scope.

Access to local variables and parameters in the containing scope can extend beyond the lifetime of that scope. As long as a delegate referring to a lambda expression is not available to garbage collection, access to the variables in the original environment is retained. In the following example, variable target is local to makeTheGame, the method in which the lambda expression playTheGame is defined. Note that the returned lambda expression, assigned to takeAGuess in Main, still has access to the local variable target.

[!code-vbVbVbalrLambdas#12]

The following example demonstrates the wide range of access rights of the nested lambda expression. When the returned lambda expression is executed from Main as aDel, it accesses these elements:

• A field of the class in which it is defined: aField

• A property of the class in which it is defined: aProp

• A parameter of method functionWithNestedLambda, in which it is defined: level1

• A local variable of functionWithNestedLambda: localVar

• A parameter of the lambda expression in which it is nested: level2

[!code-vbVbVbalrLambdas#9]

Converting to a Delegate Type

A lambda expression can be implicitly converted to a compatible delegate type. For information about the general requirements for compatibility, see Relaxed Delegate Conversion. For example, the following code example shows a lambda expression that implicitly converts to Func(Of Integer, Boolean) or a matching delegate signature.

[!code-vbVbVbalrLambdas#16]

The following code example shows a lambda expression that implicitly converts to Sub(Of Double, String, Double) or a matching delegate signature.

[!code-vbVbVbalrLambdas#23]

When you assign lambda expressions to delegates or pass them as arguments to procedures, you can specify the parameter names but omit their data types, letting the types be taken from the delegate.

Examples

• The following example defines a lambda expression that returns True if the nullable argument has an assigned value, and False if its value is Nothing.

  [!code-vbVbVbalrLambdas#4]

• The following example defines a lambda expression that returns the index of the last element in an array.

  [!code-vbVbVbalrLambdas#5]

See Also

Procedures
Introduction to LINQ in Visual Basic
Delegates
Function Statement
Sub Statement
Nullable Value Types
How to: Pass Procedures to Another Procedure in Visual Basic
How to: Create a Lambda Expression
Relaxed Delegate Conversion