Func<(Of <(T1, T2, T3, TResult>)>) Delegate

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

Encapsulates a method that has three parameters and returns a value of the type specified by the TResult parameter.

Namespace: System
Assembly: mscorlib (in mscorlib.dll)

Syntax

Public Delegate Function Func(Of In T1, In T2, In T3, Out TResult) ( _
    arg1 As T1, _
    arg2 As T2, _
    arg3 As T3 _
) As TResult
public delegate TResult Func<in T1, in T2, in T3, out TResult>(
    T1 arg1,
    T2 arg2,
    T3 arg3
)

Type Parameters

  • inInininT1
    The type of the first parameter of the method that this delegate encapsulates.

    This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see [2678dc63-c7f9-4590-9ddc-0a4df684d42e].

  • inInininT2
    The type of the second parameter of the method that this delegate encapsulates.
  • inInininT3
    The type of the third parameter of the method that this delegate encapsulates.
  • outOutoutoutTResult
    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 [2678dc63-c7f9-4590-9ddc-0a4df684d42e].

Parameters

  • arg1
    Type: T1
    The first parameter of the method that this delegate encapsulates.
  • arg2
    Type: T2
    The second parameter of the method that this delegate encapsulates.
  • arg3
    Type: T3
    The third parameter of the method that this delegate encapsulates.

Return Value

Type: TResult
The return value of the method that this delegate encapsulates.

Remarks

You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have three parameters, each of which is passed to it by value, and that it must return a value.

Note

To reference a method that has three parameters and returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<(Of <(T1, T2, T3>)>) delegate instead.

When you use the Func<(Of <(T1, T2, T3, TResult>)>) delegate, you do not have to explicitly define a delegate that encapsulates a method with three parameters. For example, the following code explicitly declares a generic delegate named ParseNumber and assigns a reference to the Parse method to its delegate instance.

Imports System.Globalization

Delegate Function ParseNumber(Of T)(ByVal input As String, ByVal styles As NumberStyles, _
                                    ByVal provider As IFormatProvider) As T

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim numericString As String = "-1,234"
      Dim parser As ParseNumber(Of Integer) = AddressOf Integer.Parse
      outputBlock.Text += parser(numericString, _
                        NumberStyles.Integer Or NumberStyles.AllowThousands, _
                        CultureInfo.InvariantCulture) & vbCrLf
   End Sub
End Module
using System;
using System.Globalization;

delegate T ParseNumber<T>(string input, NumberStyles styles,
                         IFormatProvider provider);

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string numericString = "-1,234";
      ParseNumber<int> parser = int.Parse;
      outputBlock.Text += parser(numericString,
                        NumberStyles.Integer | NumberStyles.AllowThousands,
                        CultureInfo.InvariantCulture) + "\n";
   }
}

The following example simplifies this code by instantiating the Func<(Of <(T1, T2, T3, TResult>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.

Imports System.Globalization

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim numericString As String = "-1,234"
      Dim parser As Func(Of String, NumberStyles, IFormatProvider, Integer) _
                         = AddressOf Integer.Parse
      outputBlock.Text += parser(numericString, _
                        NumberStyles.Integer Or NumberStyles.AllowThousands, _
                        CultureInfo.InvariantCulture) & vbCrLf
   End Sub
End Module
using System;
using System.Globalization;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string numericString = "-1,234";
      Func<string, NumberStyles, IFormatProvider, int> parser = int.Parse;
      outputBlock.Text += parser(numericString,
                        NumberStyles.Integer | NumberStyles.AllowThousands,
                        CultureInfo.InvariantCulture) + "\n";
   }
}

You can use the Func<(Of <(T1, T2, T3, TResult>)>) delegate with anonymous methods in C#, as the following example illustrates.

using System;
using System.Globalization;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string numericString = "-1,234";
      Func<string, NumberStyles, IFormatProvider, int> parser =
           delegate(string s, NumberStyles sty, IFormatProvider p)
           { return int.Parse(s, sty, p); };
      outputBlock.Text += parser(numericString,
                        NumberStyles.Integer | NumberStyles.AllowThousands,
                        CultureInfo.InvariantCulture) + "\n";
   }
}

You can also assign a lambda expression to a Func<(Of <(T1, T2, T3, TResult>)>) delegate, as the following example illustrates.

Imports System.Globalization

Module Example
   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim numericString As String = "-1,234"
      Dim parser As Func(Of String, NumberStyles, IFormatProvider, Integer) _
                         = Function(s, sty, p) Integer.Parse(s, sty, p)
      outputBlock.Text += parser(numericString, _
                        NumberStyles.Integer Or NumberStyles.AllowThousands, _
                        CultureInfo.InvariantCulture) & vbCrLf
   End Sub
End Module
using System;
using System.Globalization;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string numericString = "-1,234";
      Func<string, NumberStyles, IFormatProvider, int> parser = (s, sty, p)
                   => int.Parse(s, sty, p);
      outputBlock.Text += parser(numericString,
                        NumberStyles.Integer | NumberStyles.AllowThousands,
                        CultureInfo.InvariantCulture) + "\n";
   }
}

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.

Examples

The following example demonstrates how to declare and use a Func<(Of <(T1, T2, TResult>)>) delegate. This example declares a Func<(Of <(T1, T2, TResult>)>) variable and assigns it a lambda expression that takes a String value and an Int32 value as parameters. The lambda expression returns true if the length of the String parameter is equal to the value of the Int32 parameter. The delegate that encapsulates this method is subsequently used in a query to filter strings in an array of strings.

Imports System.Collections.Generic
Imports System.Linq

Public Module Example

   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim predicate As Func(Of String, Integer, Boolean) = Function(str, index) str.Length = index

      Dim words() As String = {"orange", "apple", "Article", "elephant", "star", "and"}
      Dim aWords As IEnumerable(Of String) = words.Where(predicate)

      For Each word As String In aWords
         outputBlock.Text &= word & vbCrLf
      Next
   End Sub
End Module
using System;
using System.Collections.Generic;
using System.Linq;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Func<String, int, bool> predicate = (str, index) => str.Length == index;

      String[] words = { "orange", "apple", "Article", "elephant", "star", "and" };
      IEnumerable<String> aWords = words.Where(predicate).Select(str => str);

      foreach (String word in aWords)
         outputBlock.Text += word + "\n";
   }
}

Version Information

Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Platforms

Windows Phone

See Also

Reference

System Namespace