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Func<T1, T2, T3, T4, TResult> Delegate

July 28, 2014

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

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

public delegate TResult Func<in T1, in T2, in T3, in T4, out TResult>(
	T1 arg1,
	T2 arg2,
	T3 arg3,
	T4 arg4
)

Type Parameters

in T1

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].

in T2

The type of the second parameter of the method that this delegate encapsulates.

in T3

The type of the third parameter of the method that this delegate encapsulates.

in T4

The type of the fourth parameter of the method that this delegate encapsulates.

out TResult

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.
arg4
Type: T4
The fourth parameter of the method that this delegate encapsulates.

Return Value

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

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 four parameters, each of which is passed to it by value, and that it must return a value.

NoteNote:

To reference a method that has four parameters and that returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<T1, T2, T3, T4> delegate instead.

When you use the Func<T1, T2, T3, T4, TResult> delegate, you do not have to explicitly define a delegate that encapsulates a method with four parameters. For example, the following code explicitly declares a generic delegate named Searcher and assigns a reference to the IndexOf method to its delegate instance.


using System;

delegate int Searcher(string searchString, int start, int count,
                         StringComparison type);

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string title = "The House of the Seven Gables";
      int position = 0;
      Searcher finder = title.IndexOf;
      do
      {
         int characters = title.Length - position;
         position = finder("the", position, characters,
                         StringComparison.InvariantCultureIgnoreCase);
         if (position >= 0)
         {
            position++;
            outputBlock.Text += String.Format("'The' found at position {0} in {1}.",
                              position, title) + "\n";
         }
      } while (position > 0);
   }
}


The following example simplifies this code by instantiating the Func<T1, T2, T3, T4, TResult> delegate rather than explicitly defining a new delegate and assigning a named method to it.


using System;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string title = "The House of the Seven Gables";
      int position = 0;
      Func<string, int, int, StringComparison, int> finder = title.IndexOf;
      do
      {
         int characters = title.Length - position;
         position = finder("the", position, characters,
                         StringComparison.InvariantCultureIgnoreCase);
         if (position >= 0)
         {
            position++;
            outputBlock.Text += String.Format("'The' found at position {0} in {1}.",
                              position, title) + "\n";
         }
      } while (position > 0);
   }
}


You can use the Func<T1, T2, T3, T4, TResult> delegate with anonymous methods in C#, as the following example illustrates.


using System;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string title = "The House of the Seven Gables";
      int position = 0;
      Func<string, int, int, StringComparison, int> finder =
           delegate(string s, int pos, int chars, StringComparison type)
           { return title.IndexOf(s, pos, chars, type); };
      do
      {
         int characters = title.Length - position;
         position = finder("the", position, characters,
                         StringComparison.InvariantCultureIgnoreCase);
         if (position >= 0)
         {
            position++;
            outputBlock.Text += String.Format("'The' found at position {0} in {1}.",
                              position, title) + "\n";
         }
      } while (position > 0);
   }
}


You can also assign a lambda expression to a Func<T1, T2, T3, T4, TResult> delegate, as the following example illustrates.


using System;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      string title = "The House of the Seven Gables";
      int position = 0;
      Func<string, int, int, StringComparison, int> finder =
           (s, pos, chars, type) => title.IndexOf(s, pos, chars, type);
      do
      {
         int characters = title.Length - position;
         position = finder("the", position, characters,
                         StringComparison.InvariantCultureIgnoreCase);
         if (position >= 0)
         {
            position++;
            outputBlock.Text += String.Format("'The' found at position {0} in {1}.",
                              position, title) + "\n";
         }
      } while (position > 0);
   }
}


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.

The following example demonstrates how to declare and use a Func<T1, T2, TResult> delegate. This example declares a Func<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.


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";
   }
}


Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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