Func<TResult> 代理人

參考

定義

命名空間:: System

組件:: System.Runtime.dll

組件:: System.Core.dll

組件:: mscorlib.dll

組件:: netstandard.dll

重要

部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。

封裝沒有參數並傳回 TResult 參數所指定之類型值的方法。

generic <typename TResult>
public delegate TResult Func();

public delegate TResult Func<out TResult>();

public delegate TResult Func<TResult>();

type Func<'Result> = delegate of unit -> 'Result

Public Delegate Function Func(Of Out TResult)() As TResult

Public Delegate Function Func(Of TResult)() As TResult

類型參數

TResult

這個委派所封裝之方法的傳回值之類型。

這是共變數的型別參數。也就是說，您可以使用您指定的類型，或衍生程度較高的任何類型。如需共變數與反變數的詳細資訊，請參閱泛型中的共變數與反變數。

傳回值

TResult

這個委派所封裝之方法的傳回值。

範例

下列範例示範如何使用不採用任何參數的委派。此程式碼會建立名為 LazyValue 的泛型類別，此類別具有類型的 Func<TResult> 欄位。這個委派欄位可以儲存任何函式的參考，該函式會傳回對應至物件之 type 參數 LazyValue 的值。如果函式尚未執行) 並傳回產生的值，此 LazyValue 類型也會有屬性 Value (執行函式。

此範例會建立兩個方法，並使用呼叫這些方法的 Lambda 運算式具現化兩 LazyValue 個物件。 Lambda 運算式不會採用參數，因為它們只需要呼叫方法。如輸出所示，只有在擷取每個 LazyValue 物件的值時，才會執行這兩種方法。

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

備註

您可以使用這個委派來表示可以當做參數傳遞的方法，而不需明確宣告自訂委派。封裝的方法必須對應至這個委派所定義的方法簽章。這表示封裝的方法必須沒有任何參數，而且必須傳回值。

注意

若要參考沒有參數並傳回 void (unit 的方法，請在 F#) (或 Visual Basic 中宣告為 Sub ，而不是宣告為 Function) ，請改用 Action 委派。

當您使用委派時 Func<TResult> ，不需要明確定義封裝無參數方法的委派。例如，下列程式碼會明確宣告名為 WriteMethod 的委派，並將實例方法的 OutputTarget.SendToFile 參考指派給其委派實例。

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

下列範例藉由具現化 Func<TResult> 委派，而不是明確定義新的委派，並將具名方法指派給它，藉此簡化此程式碼。

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

您可以在 C# 中搭配 Func<TResult> 匿名方法使用委派，如下列範例所示。 (如需匿名方法的簡介，請參閱 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;
      }
   }
}

您也可以將 Lambda 運算式指派給 Func<T,TResult> 委派，如下列範例所示。 (如需 Lambda 運算式簡介，請參閱Lambda 運算式 (VB) 、Lambda 運算式 (C#) 和Lambda 運算式 (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

Lambda 運算式的基礎類型是其中一個泛型 Func 委派。這可讓您將 Lambda 運算式當做參數傳遞，而不明確地將它指派給委派。特別是，因為命名空間中的 System.Linq 許多型別方法都有 Func 參數，所以您可以傳遞這些方法 Lambda 運算式，而不需明確具現化 Func 委派。

如果您只有實際需要結果時才想要執行的昂貴計算，您可以將昂貴的函式指派給 Func<TResult> 委派。然後，函式的執行可能會延遲，直到存取值的屬性用於運算式為止。下一節中的範例示範如何執行這項操作。

擴充方法

GetMethodInfo(Delegate)

取得表示特定委派所代表之方法的物件。

適用於

共用方式為

Func<TResult> 代理人

定義

類型參數

傳回值

範例

備註

擴充方法

適用於

另請參閱

其他資源