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Function Expression

Declares the parameters and code that define a lambda expression.

Function ( [ parameterlist ] ) expression
- or -
Function ( [ parameterlist ] ) _
  expression
parameterlist

Optional. A list of local variable names that represent the parameters of this procedure. The parentheses must be present even when the list is empty. See Parameter List.

expression

Required. A single expression. The value of the expression determines the type of the function and the value returned when the function is called.

A lambda expression is a function without a name that calculates and returns a single value. You can use a lambda expression anywhere you can use a delegate type, except as an argument to RemoveHandler. For more information about delegates, and the use of lambda expressions with delegates, see Delegate Statement and Relaxed Delegate Conversion.

Lambda Expression Syntax

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

  • A lambda expression does not have a name.

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

  • Lambda expressions do not use an As clause to designate the return type of the function. 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 Where cust.City = "London", its return type is Boolean.

  • The body of the function must be an expression, not a statement. The body can consist of a call to a function procedure, but not a call to a sub procedure.

  • There is no Return statement. The value returned by the function is the value of the expression in the body of the function.

  • There is no End Function statement.

  • 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.

As a result of these restrictions, and of the ways in which lambda expressions are used, they usually are short and uncomplicated.

The following examples show two ways to create simple lambda expressions. The first uses a Dim to provide a name for the function. To call the function, you send in a value for the parameter.

Dim add1 = Function(num As Integer) num + 1
' The following line prints 6.
Console.WriteLine(add1(5))

Alternatively, you can declare and run the function at the same time.

Console.WriteLine((Function(num As Integer) num + 1)(5))

Lambda expressions underlie many of the query operators in Language-Integrated Query (LINQ), and can be used explicitly in method-based queries. The following example shows a typical LINQ query, followed by the translation of the query into method format.

Dim londonCusts = From cust In db.Customers 
                  Where cust.City = "London" 
                  Select cust

' This query is compiled to the following code:
Dim londonCusts = db.Customers _
    .Where(Function(cust) cust.City = "London") _
    .Select(Function(cust) cust)

For more information about query methods, see Queries (Visual Basic). For more information about standard query operators, see Standard Query Operators Overview.

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