Func<TResult> Delegate
Definition
Important
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Encapsulates a method that has no parameters and returns a value of the type specified by the TResult parameter.
generic <typename TResult>
public delegate TResult Func();public delegate TResult Func<out TResult>();public delegate TResult Func<TResult>();type Func<'Result> = delegate of unit -> 'ResultPublic Delegate Function Func(Of Out TResult)() As TResult Public Delegate Function Func(Of TResult)() As TResult Type Parameters
- TResult
The type of the return value of the method that this delegate encapsulates.
This type parameter is covariant. That is, you can use either the type you specified or any type that is more derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.Return Value
The return value of the method that this delegate encapsulates.
Examples
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<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.
using System;
static class Func1
{
public static void Main()
{
// Note that each lambda expression has no parameters.
LazyValue<int> lazyOne = new LazyValue<int>(() => ExpensiveOne());
LazyValue<long> lazyTwo = new LazyValue<long>(() => ExpensiveTwo("apple"));
Console.WriteLine("LazyValue objects have been created.");
// Get the values of the LazyValue objects.
Console.WriteLine(lazyOne.Value);
Console.WriteLine(lazyTwo.Value);
}
static int ExpensiveOne()
{
Console.WriteLine("\nExpensiveOne() is executing.");
return 1;
}
static long ExpensiveTwo(string input)
{
Console.WriteLine("\nExpensiveTwo() is executing.");
return (long)input.Length;
}
}
class LazyValue<T> where T : struct
{
private Nullable<T> val;
private Func<T> getValue;
// Constructor.
public LazyValue(Func<T> func)
{
val = null;
getValue = func;
}
public T Value
{
get
{
if (val == null)
// Execute the delegate.
val = getValue();
return (T)val;
}
}
}
/* The example produces the following output:
LazyValue objects have been created.
ExpensiveOne() is executing.
1
ExpensiveTwo() is executing.
5
*/
open System
type LazyValue<'T>(func: Func<'T>) =
let mutable value = ValueNone
member _.Value =
match value with
| ValueSome v -> v
| ValueNone ->
// Execute the delegate.
let v = func.Invoke()
value <- ValueSome v
v
let expensiveOne () =
printfn "\nExpensiveOne() is executing."
1
let expensiveTwo (input: string) =
printfn "\nExpensiveTwo() is executing."
int64 input.Length
// Note that each lambda expression has no parameters.
let lazyOne = LazyValue(fun () -> expensiveOne ())
let lazyTwo = LazyValue(fun () -> expensiveTwo "apple")
printfn "LazyValue objects have been created."
// Get the values of the LazyValue objects.
printfn $"{lazyOne.Value}"
printfn $"{lazyTwo.Value}"
// The example produces the following output:
// LazyValue objects have been created.
//
// ExpensiveOne() is executing.
// 1
//
// ExpensiveTwo() is executing.
// 5
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
Remarks
You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The encapsulated 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.
Note
To reference a method that has no parameters and returns void (unit, in F#) (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<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.
using System;
using System.IO;
delegate bool WriteMethod();
public class TestDelegate
{
public static void Main()
{
OutputTarget output = new OutputTarget();
WriteMethod methodCall = 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;
}
}
}
open System.IO
type WriteMethod = delegate of unit -> bool
type OutputTarget() =
member _.SendToFile() =
try
let fn = Path.GetTempFileName()
use sw = new StreamWriter(fn)
sw.WriteLine "Hello, World!"
true
with _ ->
false
let output = new OutputTarget()
let methodCall = WriteMethod output.SendToFile
if methodCall.Invoke() then
printfn "Success!"
else
printfn "File write operation failed."
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<TResult> delegate instead of explicitly defining a new delegate and assigning a named method to it.
using System;
using System.IO;
public class TestDelegate
{
public static void Main()
{
OutputTarget output = new OutputTarget();
Func<bool> methodCall = 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;
}
}
}
open System
open System.IO
type OutputTarget() =
member _.SendToFile() =
try
let fn = Path.GetTempFileName()
use sw = new StreamWriter(fn)
sw.WriteLine "Hello, World!"
true
with _ ->
false
let output = OutputTarget()
let methodCall = Func<bool> output.SendToFile
if methodCall.Invoke() then
printfn "Success!"
else
printfn "File write operation failed."
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<TResult> delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods.)
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<T,TResult> delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (VB), Lambda Expressions (C#), and Lambda Expressions (F#).)
using System;
using System.IO;
public class Anonymous
{
public static void Main()
{
OutputTarget output = new OutputTarget();
Func<bool> methodCall = () => 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;
}
}
}
open System
open System.IO
type OutputTarget() =
member _.SendToFile() =
try
let fn = Path.GetTempFileName()
use sw = new StreamWriter(fn)
sw.WriteLine "Hello, World!"
true
with _ ->
false
let output = OutputTarget()
let methodCall = Func<bool>(fun () -> output.SendToFile())
if methodCall.Invoke() then
printfn "Success!"
else
printfn "File write operation failed."
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<TResult> delegate. The execution of the function can then be delayed until a property that accesses the value is used in an expression. The example in the next section demonstrates how to do this.
Extension Methods
| GetMethodInfo(Delegate) |
Gets an object that represents the method represented by the specified delegate. |
