VB.NET Func, Action and Predicate Examples

Use Func, Action and Predicate. See the lambda syntax, and invoke the functions.

Func, Action, Predicate. In VB.NET, we sometimes need to create function objects to pass to other methods (like List methods). We can create Func, Action and Predicate objects with lambdas.LambdaList

Some syntax examples. We use the Function or Sub keywords to create these function objects. Funcs and Predicates return a value, so use the Function keyword.

And: Actions are like Subs in that they return no values, so we use the Sub keyword to specify an Action type.


Func example. Let us begin with the Func type. When we create a Func, we specify its argument types (if any) and then its return type. Only one return type can be used.

Here: We create 2 Funcs and store them in local variables. We could pass these as arguments, or use the lambda directly as arguments.

VB.NET program that uses Func Module Module1 Sub Main() ' Return an Integer with no arguments. Dim test As Func(Of Integer) = Function() 10 Dim result = test.Invoke() Console.WriteLine($"TEST RESULT: {result}") ' Return an Integer with a String argument. ' ... Returns the string length multiplied by 2. Dim test2 As Func(Of String, Integer) = Function(x) x.Length * 2 Dim result2 = test2.Invoke("bird") Console.WriteLine($"TEST2 RESULT: {result2}") End Sub End Module Output TEST RESULT: 10 TEST2 RESULT: 8

Action example. Action is the same as a Func, but it returns no value—it is like a Subroutine instead of a Function (in VB.NET terms). Here we create an action that receives 2 integers.Integer

And: The Action is specified with a 1-line Sub. It returns no value. Instead it just calls Console.WriteLine.

VB.NET program that uses Action Module Module1 Sub Main() ' Use Sub to create an action. Dim test As Action(Of Integer, Integer) = Sub(x, y) Console.WriteLine("X is {0}, Y is {1}", x, y) ' Call the Action 3 times. test.Invoke(10, 20) test.Invoke(20, 30) test.Invoke(30, -1) End Sub End Module Output X is 10, Y is 20 X is 20, Y is 30 X is 30, Y is -1

Predicate example. A Predicate is a function object that receives arguments, and always returns a Boolean. It tells us whether something is true or false.Boolean

Here: We see 2 Predicates. The first tells us whether an integer is positive—it returns True if the value is greater than or equal to 0.

VB.NET program that uses Predicate Module Module1 Sub Main() ' This Predicate receives an Integer, and returns a Boolean. ' ... It tells us if an Integer is positive. Dim isPositive As Predicate(Of Integer) = Function(x) x >= 0 If isPositive.Invoke(10) Then Console.WriteLine("NUMBER IS POSITIVE") End If ' This Predicate receives a String. ' ... It returns true if the string is short (less than 10 chars). Dim isShortString As Predicate(Of String) = Function(value) value.Length <= 9 If isShortString.Invoke("this is a long string") Then ' This is not reached. Console.WriteLine("NOT REACHED") End If End Sub End Module Output NUMBER IS POSITIVE

Argument example. We usually want to pass things like a Predicate to another Function. Consider the FirstOrDefault Function from LINQ. It can accept a Predicate argument.

Here: We use a Lambda expression to create a Predicate that returns True when a String begins with the letter F.

StartsWith, EndsWith

Result: The first string that begins with the letter F, "frog," is returned by the FirstOrDefault Function. The Predicate worked.

VB.NET program that uses Predicate as argument, LINQ Module Module1 Sub Main() Dim values() As String = {"bird", "frog", "dog", "cat"} ' Use Predicate to find first item starting with lowercase F. Dim firstLetterF = values.FirstOrDefault(Function(x) x.StartsWith("f")) ' Result. Console.WriteLine("FIRST ITEM STARTING WITH F: {0}", firstLetterF) End Sub End Module Output FIRST ITEM STARTING WITH F: frog

Notes. Usually when we need a Func, Action or Predicate, it is as an argument to another method. Many methods in LINQ (like FirstOrDefault) can optionally accept a function object.LINQ

A summary. Often we encounter a Function that requires a Func, Action or Predicate argument. We can create these with Lambda syntax—the arguments and return value must be specified.
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