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

定义

封装一个不具有参数但却返回 TResult 参数指定的类型值的方法。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 

类型参数

TResult

此委托封装的方法的返回值类型。The type of the return value of the method that this delegate encapsulates.

返回值

TResult

此委托封装的方法的返回值。The return value of the method that this delegate encapsulates.

继承
Func<TResult>Func<TResult>Func<TResult>Func<TResult>

示例

下面的示例演示如何使用不带参数的委托。The following example demonstrates how to use a delegate that takes no parameters. 此代码将创建一个名为的泛型类LazyValue具有一个类型的字段Func<TResult>This code creates a generic class named LazyValue that has a field of type Func<TResult>. 此委托字段可以存储的任何函数相对应的类型值返回到的类型参数的引用LazyValue对象。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类型还具有Value(如果它已不执行),则执行该函数的属性,并返回生成的值。The LazyValue type also has a Value property that executes the function (if it has not already been executed) and returns the resulting value.

该示例创建两种方法,并实例化两个LazyValue具有调用这些方法的 lambda 表达式的对象。The example creates two methods and instantiates two LazyValue objects with lambda expressions that call these methods. Lambda 表达式不带参数,因为它们只需调用的方法。The lambda expressions do not take parameters because they just need to call a method. 如输出所示,将执行两个方法仅当每个值LazyValue检索对象。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

注解

可以使用此委托来表示可以作为参数传递而无需显式声明自定义委托的方法。You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. 封装的方法必须对应于此委托定义的方法签名。The encapsulated method must correspond to the method signature that is defined by this delegate. 这意味着,封装的方法必须具有任何参数,并且必须返回值。This means that the encapsulated method must have no parameters and must return a value.

备注

若要引用的方法,没有任何参数,并返回void(或在 Visual Basic 中的声明为Sub而不是作为Function),使用Action改为委托。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.

当你使用Func<TResult>委托时,您无需显式定义用于封装无参数方法的委托。When you use the Func<TResult> delegate, you do not have to explicitly define a delegate that encapsulates a parameterless method. 例如,下面的代码显式声明名为的委托WriteMethod,并将分配到的引用OutputTarget.SendToFile实例到其委托实例的方法。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

下面的示例简化了此代码实例化Func<TResult>而不是显式定义一个新委托,并为其赋值命名的方法的委托。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

可以使用Func<TResult>委托与匿名方法在 C# 中,如以下示例所示。You can use the Func<TResult> delegate with anonymous methods in C#, as the following example illustrates. (有关匿名方法的介绍,请参阅匿名方法。)(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;
      }
   }
}

你还可以分配一个 lambda 表达式到Func<T,TResult>委托,如以下示例所示。You can also assign a lambda expression to a Func<T,TResult> delegate, as the following example illustrates. (有关 lambda 表达式的简介,请参阅Lambda 表达式Lambda 表达式。)(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 表达式的基础类型是一个泛型Func委托。The underlying type of a lambda expression is one of the generic Func delegates. 这样,可以将 lambda 表达式作为参数传递而无需显式将其分配给委托。This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. 具体而言,因为许多方法中的类型System.Linq命名空间具有Func参数,您可以将传递这些方法的 lambda 表达式无需显式实例化Func委托。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.

如果有想要执行实际需要结果时才进行耗时的计算,则可以分配到函数开销Func<TResult>委托。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. 然后可以推迟到在表达式中使用访问值属性时的函数的执行。The execution of the function can then be delayed until a property that accesses the value is used in an expression. 下一节中的示例演示如何执行此操作。The example in the next section demonstrates how to do this.

扩展方法

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

获取指示指定委托表示的方法的对象。Gets an object that represents the method represented by the specified delegate.

适用于

另请参阅