Func<TResult> Func<TResult> Func<TResult> Func<TResult> Delegate

定義

封裝沒有參數並傳回 TResult 參數所指定之型別值的方法。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>();
type Func<'Result> = delegate of unit -> 'Result
Public Delegate Function Func(Of Out TResult)() As TResult 

類型參數

TResult

這個委派所封裝之方法的傳回值之型別。The type of the return value of the method that this delegate encapsulates.

傳回值

TResult

這個委派所封裝之方法的傳回值。The return value of the method that this delegate encapsulates.

繼承
Func<TResult>Func<TResult>Func<TResult>Func<TResult>

範例

下列範例示範如何使用未採用參數的委派。The following example demonstrates how to use a delegate that takes no parameters. 此程式碼會建立名為泛型類別LazyValue具有型別的欄位Func<TResult>This code creates a generic class named LazyValue that has a field of type Func<TResult>. 此委派欄位可以儲存任何型別參數中傳回值的型別對應的函式的參考LazyValue物件。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. LazyValue型別也有Value執行函式 (如果它已經尚未執行) 的屬性,並傳回產生的值。The LazyValue type also has a Value property that executes the function (if it has not already been executed) and returns the resulting value.

此範例會建立兩種方法,並具現化兩個LazyValue呼叫這些方法的 lambda 運算式的物件。The example creates two methods and instantiates two LazyValue objects with lambda expressions that call these methods. 因為它們只需要呼叫方法的 lambda 運算式不使用參數。The lambda expressions do not take parameters because they just need to call a method. 如輸出所示,會執行兩種方法時,才每個值LazyValue擷取物件。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
*/    
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

您可以使用此委派来表示的方法,可以做為參數傳遞,而不必明確宣告自訂委派。You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. 封裝的方法必須對應到這個委派所定義的方法簽章。The encapsulated method must correspond to the method signature that is defined by this delegate. 這表示封裝的方法必須有任何參數,而且必須傳回值。This means that the encapsulated method must have no parameters and must return a value.

注意

若要參考的方法沒有參數並傳回void(或在 Visual Basic 中宣告為SubFunction),使用Action改為委派。To reference a method that has no parameters and returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the Action delegate instead.

當您使用Func<TResult>委派,您不需要明確地定義封裝的無參數方法的委派。When you use the Func<TResult> delegate, you do not have to explicitly define a delegate that encapsulates a parameterless method. 例如,下列程式碼明確宣告委派,名為WriteMethod,並將指派的參考OutputTarget.SendToFile執行個體給它的委派執行個體的方法。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;
      }
   }
}
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

下列範例中簡化這個程式碼,以具現化Func<TResult>而不是明確地定義新的委派,並為其指派具名的方法的委派。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;
      }
   }
}
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

您可以使用Func<TResult>具有在 C# 中,匿名方法委派,如下列範例所示。You can use the Func<TResult> delegate with anonymous methods in C#, as the following example illustrates. (如匿名方法的簡介,請參閱 < 匿名方法。)(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;
      }
   }
}

您也可以指派至 lambda 運算式Func<T,TResult>委派,如下列範例所示。You can also assign a lambda expression to a Func<T,TResult> delegate, as the following example illustrates. (如需 lambda 運算式,請參閱 < Lambda 運算式Lambda 運算式。)(For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions.)

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;
      }
   }
}
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。The underlying type of a lambda expression is one of the generic Func delegates. 這讓您能夠傳遞做為參數的 lambda 運算式,而不需要明確地將它指派給委派。This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. 特別是,因為有許多方法中的型別System.Linq命名空間一定Func參數,您可以將這些方法的 lambda 運算式而不需要明確具現化Func委派。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.

如果您有想要的結果實際需要時,才執行昂貴的計算,您可以將耗費資源的函式,來指派Func<TResult>委派。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. 直到存取值的屬性運算式中使用,然後會延遲執行函式。The execution of the function can then be delayed until a property that accesses the value is used in an expression. 下一節中的範例示範如何執行這項操作。The example in the next section demonstrates how to do this.

擴充方法

GetMethodInfo(Delegate) GetMethodInfo(Delegate) GetMethodInfo(Delegate) GetMethodInfo(Delegate)

取得表示特定委派所代表之方法的物件。Gets an object that represents the method represented by the specified delegate.

適用於

另請參閱