Func<TResult> 代理人

定義

命名空間:

System

組件:

netstandard.dll, System.Runtime.dll

來源:

Function.cs

封裝一個沒有參數的方法，並回傳由 TResult 參數指定的值。

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

public delegate TResult Func<out TResult>() where TResult : allows ref struct;

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

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

類型參數

TResult

此代理所封裝的方法的回傳值類型。

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

此代理所封裝的方法的回傳值。

範例

以下範例示範如何使用一個不接受參數的代理。 此程式碼會建立一個名為 LazyValue 的泛型類別，其欄位 Func<TResult>為 。 這個代理欄位可以儲存對任何回傳與物件類型參數對應的值 LazyValue 的函式的參考。 該 LazyValue 型別也有一個 Value 屬性，可以執行該函式（如果尚未執行過），並回傳結果值。

範例中建立了兩個方法，並實 LazyValue 例化兩個物件，使用lambda表達式來呼叫這些方法。 lambda 表達式不接受參數，因為它們只需要呼叫一個方法。 如輸出所示，這兩種方法僅在取得每個 LazyValue 物件值時執行。

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

你可以用這個代理子來表示一個方法，可以直接以參數方式傳遞，而不必明確宣告自訂代理。 封裝方法必須對應於此代理所定義的方法簽名。 這表示封裝方法必須沒有參數，且必須回傳一個值。

備註

若要參考一個沒有參數且回傳 void（unit，F# 中）（或在 Visual Basic 中宣告為 Sub 而非 Function 的方法），請使用 Action 代理。

使用 Func<TResult> 代理時，不必明確定義一個封裝無參數方法的代理。 例如，以下程式碼明確宣告一個名為代理 WriteMethod 的代理，並將該代理方法的參考 OutputTarget.SendToFile 指派給其代理實例。

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

以下範例簡化了此程式碼，透過實 Func<TResult> 例化代理，而非明確定義新代理並指派命名方法。

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

你可以在 C# 中使用匿名方法的 Func<TResult> 代理，如下範例所示。 （關於匿名方法的入門，請參見 匿名方法。）

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;
      }
   }
}

你也可以將 lambda 運算式指派給 Func<T,TResult> 代理，如下範例所示。 （關於 lambda 表達式的介紹，請參見 Lambda 表達式 （VB）、Lambda 表達式 （C#） 及 Lambda 表達式 （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 表達式的底層類型是其中一種通用 Func 代理。 這使得可以將 lambda 運算式作為參數傳遞，而不必明確指派給代理。 特別是，因為命名空間中 System.Linq 許多型別的方法都有 Func 參數，你可以在不明確實 Func 例化代理的情況下，將 lambda 表達式傳遞給這些方法。

如果你有一個昂貴的計算，只想在結果真正需要時執行，你可以將昂貴的函數指派給 Func<TResult> 代理。 此時，函式的執行可以延遲，直到在表達式中使用存取該值的屬性。 下一節的範例示範如何做到這一點。

擴充方法

名稱	Description
GetMethodInfo(Delegate)	取得一個代表指定代理所代表方法的物件。

另請參閱

• Lambda 表達式（C# 程式設計指南）
• Lambda 表達式：有趣的關鍵字（F#）
• Lambda 表達式
• Delegates（C# 程式設計指南）
• 代表人數（F#）
• 代表們Visual Basic