Func(Of TResult) Delegate
Encapsulates a method that has no parameters and returns a value of the type specified by the TResult parameter.
Assembly: System.Core (in System.Core.dll)
'Declaration Public Delegate Function Func(Of TResult) As TResult 'Usage Dim instance As New Func(Of TResult)(AddressOf HandlerMethod)
Type Parameters
- TResult
The type of the return value of the method that this delegate encapsulates.
Return Value
Type: TResultThe return value of the method that this delegate encapsulates.
You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have no parameters and must return a value.
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To reference a method that has no parameters and that returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the Action delegate instead. |
When you use the Func(Of TResult) delegate, you do not have to explicitly define a delegate that encapsulates a parameterless method. For example, the following code explicitly declares a delegate named WriteMethod and assigns a reference to the OutputTarget.SendToFile instance method to its delegate instance.
Imports System.IO Delegate Function WriteMethod As Boolean Module TestDelegate Public Sub Main() Dim output As New OutputTarget() Dim methodCall As WriteMethod = AddressOf output.SendToFile If methodCall() Then Console.WriteLine("Success!") Else Console.WriteLine("File write operation failed.") End If End Sub End Module Public Class OutputTarget Public Function SendToFile() As Boolean Try Dim fn As String = Path.GetTempFileName Dim sw As StreamWriter = New StreamWriter(fn) sw.WriteLine("Hello, World!") sw.Close Return True Catch Return False End Try End Function End Class
The following example simplifies this code by instantiating the Func(Of TResult) delegate rather than explicitly defining a new delegate and assigning a named method to it.
Imports System.IO Module TestDelegate Public Sub Main() Dim output As New OutputTarget() Dim methodCall As Func(Of Boolean) = AddressOf output.SendToFile If methodCall() Then Console.WriteLine("Success!") Else Console.WriteLine("File write operation failed.") End If End Sub End Module Public Class OutputTarget Public Function SendToFile() As Boolean Try Dim fn As String = Path.GetTempFileName Dim sw As StreamWriter = New StreamWriter(fn) sw.WriteLine("Hello, World!") sw.Close Return True Catch Return False End Try End Function End Class
You can use the Func(Of TResult) delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)
using System; using System.IO; public class Anonymous { public static void Main() { OutputTarget output = new OutputTarget(); Func<bool> methodCall = delegate() { return output.SendToFile(); }; if (methodCall()) Console.WriteLine("Success!"); else Console.WriteLine("File write operation failed."); } } public class OutputTarget { public bool SendToFile() { try { string fn = Path.GetTempFileName(); StreamWriter sw = new StreamWriter(fn); sw.WriteLine("Hello, World!"); sw.Close(); return true; } catch { return false; } } }
You can also assign a lambda expression to a Func(Of T, TResult) delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions (C# Programming Guide).)
Imports System.IO Module TestDelegate Public Sub Main() Dim output As New OutputTarget() Dim methodCall As Func(Of Boolean) = Function() output.SendToFile() If methodCall() Then Console.WriteLine("Success!") Else Console.WriteLine("File write operation failed.") End If End Sub End Module Public Class OutputTarget Public Function SendToFile() As Boolean Try Dim fn As String = Path.GetTempFileName Dim sw As StreamWriter = New StreamWriter(fn) sw.WriteLine("Hello, World!") sw.Close Return True Catch Return False End Try End Function End Class
The underlying type of a lambda expression is one of the generic Func delegates. This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. In particular, because many methods of types in the System.Linq namespace have Func parameters, you can pass these methods a lambda expression without explicitly instantiating a Func delegate.
If you have an expensive computation that you want to execute only if the result is actually needed, you can assign the expensive function to a Func(Of TResult) delegate. The execution of the function can then be delayed until a property that accesses the lazy value is used in an expression. The example in the next section demonstrates how to do this.
The following example demonstrates how to use a delegate that takes no parameters. This code creates a generic class named LazyValue that has a field of type Func(Of TResult). This delegate field can store a reference to any function that returns a value of the type that corresponds to the type parameter of the LazyValue object. The LazyValue type also has a Value property that executes the function (if it has not already been executed) and returns the resulting value.
The example creates two methods and instantiates two LazyValue objects with lambda expressions that call these methods. The lambda expressions do not take parameters because they just need to call a method. As the output shows, the two methods are executed only when the value of each LazyValue object is retrieved.
Public Module Func Public Sub Main() ' Note that each lambda expression has no parameters. Dim lazyOne As New LazyValue(Of Integer)(Function() ExpensiveOne()) Dim lazyTwo As New LazyValue(Of Long)(Function() ExpensiveTwo("apple")) Console.WriteLine("LazyValue objects have been created.") ' Get the values of the LazyValue objects. Console.WriteLine(lazyOne.Value) Console.WriteLine(lazyTwo.Value) End Sub Public Function ExpensiveOne() As Integer Console.WriteLine() Console.WriteLine("ExpensiveOne() is executing.") Return 1 End Function Public Function ExpensiveTwo(input As String) As Long Console.WriteLine() Console.WriteLine("ExpensiveTwo() is executing.") Return input.Length End Function End Module Public Class LazyValue(Of T As Structure) Private val As Nullable(Of T) Private getValue As Func(Of T) ' Constructor. Public Sub New(func As Func(Of T)) Me.val = Nothing Me.getValue = func End Sub Public ReadOnly Property Value() As T Get If Me.val Is Nothing Then ' Execute the delegate. Me.val = Me.getValue() End If Return CType(val, T) End Get End Property End Class
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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