Action<T1,T2> Delegate

Definition

generic <typename T1, typename T2>
public delegate void Action(T1 arg1, T2 arg2);

public delegate void Action<in T1,in T2>(T1 arg1, T2 arg2);

public delegate void Action<in T1,in T2>(T1 arg1, T2 arg2) where T1 : allows ref struct where T2 : allows ref struct;

public delegate void Action<T1,T2>(T1 arg1, T2 arg2);

type Action<'T1, 'T2> = delegate of 'T1 * 'T2 -> unit

Public Delegate Sub Action(Of In T1, In T2)(arg1 As T1, arg2 As T2)

Public Delegate Sub Action(Of T1, T2)(arg1 As T1, arg2 As T2)

Type Parameters

Parameters

Remarks

You can use the Action<T1,T2> delegate to pass a method as a parameter without explicitly declaring a custom delegate. The encapsulated method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have two parameters that are both passed to it by value, and it must not return a value. (In C#, the method must return void. In F#, the method or function must return unit. In Visual Basic, it must be defined by the Sub…End Sub construct. It can also be a method that returns a value that is ignored.) Typically, such a method is used to perform an operation.

When you use the Action<T1,T2> delegate, you do not have to explicitly define a delegate that encapsulates a method with two parameters. For example, the following code explicitly declares a delegate named ConcatStrings. It then assigns a reference to either of two methods to its delegate instance. One method writes two strings to the console; the second writes two strings to a file.

using System;
using System.IO;

delegate void ConcatStrings(string string1, string string2);

public class TestDelegate
{
 public static void Main()
 {
 string message1 = "The first line of a message.";
 string message2 = "The second line of a message.";
 ConcatStrings concat;

 if (Environment.GetCommandLineArgs().Length > 1)
 concat = WriteToFile;
 else
 concat = WriteToConsole;

 concat(message1, message2);
 }

 private static void WriteToConsole(string string1, string string2)
 {
 Console.WriteLine("{0}\n{1}", string1, string2);
 }

 private static void WriteToFile(string string1, string string2)
 {
 StreamWriter writer = null;
 try
 {
 writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);
 writer.WriteLine("{0}\n{1}", string1, string2);
 }
 catch
 {
 Console.WriteLine("File write operation failed...");
 }
 finally
 {
 if (writer != null) writer.Close();
 }
 }
}

open System
open System.IO

type ConcatStrings = delegate of string1: string * string1: string -> unit

let message1 = "The first line of a message"
let message2 = "The second line of a message"

let writeToConsole string1 string2 = 
 printfn $"{string1}\n{string2}"

let writeToFile string1 string2 =
 use writer = new StreamWriter(Environment.GetCommandLineArgs().[1], false)
 writer.WriteLine $"{string1}\n{string2}"

let concat =
 ConcatStrings(fun string1 string2 ->
 if Environment.GetCommandLineArgs().Length > 1 then
 writeToFile string1 string2
 else
 writeToConsole string1 string2
 )

concat.Invoke(message1, message2)

Imports System.IO

Delegate Sub ConcatStrings(string1 As String, string2 As String)

Module TestDelegate
 Public Sub Main()
 
 Dim message1 As String = "The first line of a message."
 Dim message2 As String = "The second line of a message."
 Dim concat As ConcatStrings
 
 If Environment.GetCommandLineArgs().Length > 1 Then
 concat = AddressOf WriteToFile
 Else
 concat = AddressOf WriteToConsole
 End If 
 concat(message1, message2) 
 End Sub
 
 Private Sub WriteToConsole(string1 As String, string2 As String)
 Console.WriteLine("{0}{1}{2}", string1, vbCrLf, string2)
 End Sub
 
 Private Sub WriteToFile(string1 As String, string2 As String)
 Dim writer As StreamWriter = Nothing 
 Try
 writer = New StreamWriter(Environment.GetCommandLineArgs(1), False)
 writer.WriteLine("{0}{1}{2}", string1, vbCrLf, string2)
 Catch
 Console.WriteLine("File write operation failed...")
 Finally
 If writer IsNot Nothing Then writer.Close
 End Try 
 End Sub
End Module

The following example simplifies this code by instantiating the Action<T1,T2> delegate instead of explicitly defining a new delegate and assigning a named method to it.

using System;
using System.IO;

public class TestAction2
{
 public static void Main()
 {
 string message1 = "The first line of a message.";
 string message2 = "The second line of a message.";
 Action<string, string> concat;

 if (Environment.GetCommandLineArgs().Length > 1)
 concat = WriteToFile;
 else
 concat = WriteToConsole;

 concat(message1, message2);
 }

 private static void WriteToConsole(string string1, string string2)
 {
 Console.WriteLine("{0}\n{1}", string1, string2);
 }

 private static void WriteToFile(string string1, string string2)
 {
 StreamWriter writer = null;
 try
 {
 writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);
 writer.WriteLine("{0}\n{1}", string1, string2);
 }
 catch
 {
 Console.WriteLine("File write operation failed...");
 }
 finally
 {
 if (writer != null) writer.Close();
 }
 }
}

open System
open System.IO

let message1 = "The first line of a message"
let message2 = "The second line of a message"

let writeToConsole string1 string2 = 
 printfn $"{string1}\n{string2}"

let writeToFile string1 string2 =
 use writer = new StreamWriter(Environment.GetCommandLineArgs().[1], false)
 writer.WriteLine $"{string1}\n{string2}"

let concat =
 Action<string, string>(fun string1 string2 ->
 if Environment.GetCommandLineArgs().Length > 1 then
 writeToFile string1 string2
 else
 writeToConsole string1 string2
 )

concat.Invoke(message1, message2)

Imports System.IO

Module TestAction2
 Public Sub Main()
 
 Dim message1 As String = "The first line of a message."
 Dim message2 As String = "The second line of a message."
 Dim concat As Action(Of String, String)
 
 If Environment.GetCommandLineArgs().Length > 1 Then
 concat = AddressOf WriteToFile
 Else
 concat = AddressOf WriteToConsole
 End If 
 concat(message1, message2) 
 End Sub
 
 Private Sub WriteToConsole(string1 As String, string2 As String)
 Console.WriteLine("{0}{1}{2}", string1, vbCrLf, string2)
 End Sub
 
 Private Sub WriteToFile(string1 As String, string2 As String)
 Dim writer As StreamWriter = Nothing 
 Try
 writer = New StreamWriter(Environment.GetCommandLineArgs(1), False)
 writer.WriteLine("{0}{1}{2}", string1, vbCrLf, string2)
 Catch
 Console.WriteLine("File write operation failed...")
 Finally
 If writer IsNot Nothing Then writer.Close
 End Try 
 End Sub
End Module

You can also use the Action<T1,T2> delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods.)

using System;
using System.IO;

public class TestAnonymousMethod
{
 public static void Main()
 {
 string message1 = "The first line of a message.";
 string message2 = "The second line of a message.";
 Action<string, string> concat;

 if (Environment.GetCommandLineArgs().Length > 1)
 concat = delegate(string s1, string s2) { WriteToFile(s1, s2); };
 else
 concat = delegate(string s1, string s2) { WriteToConsole(s1, s2);} ;

 concat(message1, message2);
 }

 private static void WriteToConsole(string string1, string string2)
 {
 Console.WriteLine("{0}\n{1}", string1, string2);
 }

 private static void WriteToFile(string string1, string string2)
 {
 StreamWriter writer = null;
 try
 {
 writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);
 writer.WriteLine("{0}\n{1}", string1, string2);
 }
 catch
 {
 Console.WriteLine("File write operation failed...");
 }
 finally
 {
 if (writer != null) writer.Close();
 }
 }
}

You can also assign a lambda expression to an Action<T1,T2> delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C#), or Lambda Expressions (F#).)

using System;
using System.IO;

public class TestLambdaExpression
{
 public static void Main()
 {
 string message1 = "The first line of a message.";
 string message2 = "The second line of a message.";
 Action<string, string> concat;

 if (Environment.GetCommandLineArgs().Length > 1)
 concat = (s1, s2) => WriteToFile(s1, s2);
 else
 concat = (s1, s2) => WriteToConsole(s1, s2);

 concat(message1, message2);
 }

 private static void WriteToConsole(string string1, string string2)
 {
 Console.WriteLine("{0}\n{1}", string1, string2);
 }

 private static void WriteToFile(string string1, string string2)
 {
 StreamWriter writer = null;
 try
 {
 writer = new StreamWriter(Environment.GetCommandLineArgs()[1], false);
 writer.WriteLine("{0}\n{1}", string1, string2);
 }
 catch
 {
 Console.WriteLine("File write operation failed...");
 }
 finally
 {
 if (writer != null) writer.Close();
 }
 }
}

open System
open System.IO

let message1 = "The first line of a message"
let message2 = "The second line of a message"

let writeToConsole string1 string2 = 
 printfn $"{string1}\n{string2}"

let writeToFile string1 string2 =
 use writer = new StreamWriter(Environment.GetCommandLineArgs().[1], false)
 writer.WriteLine $"{string1}\n{string2}"

let concat =
 Action<string,string>(
 if Environment.GetCommandLineArgs().Length > 1 then
 fun s1 s2 -> writeToFile s1 s2
 else
 fun s1 s2 -> writeToConsole s1 s2
 )

concat.Invoke(message1, message2)

Imports System.IO

Public Module TestLambdaExpression

 Public Sub Main()
 Dim message1 As String = "The first line of a message."
 Dim message2 As String = "The second line of a message."
 Dim concat As Action(Of String, String)
 
 If Environment.GetCommandLineArgs().Length > 1 Then
 concat = Sub(s1, s2) WriteToFile(s1, s2)
 Else
 concat = Sub(s1, s2) WriteToConsole(s1, s2)
 End If
 
 concat(message1, message2)
 End Sub
 
 Private Function WriteToConsole(string1 As String, string2 As String) As Integer
 Dim message As String = String.Format("{0}{1}{2}", string1, vbCrLf, string2)
 Console.WriteLine(message)
 Return message.Length
 End Function

 Private Function WriteToFile(string1 As String, string2 As String) As Integer
 Dim writer As StreamWriter = Nothing 
 Dim message As String = String.Format("{0}{1}{2}", string1, vbCrLf, string2)
 Dim charsWritten As Integer
 Try
 writer = New StreamWriter(Environment.GetCommandLineArgs()(1), False)
 writer.WriteLine(message)
 Catch
 Console.WriteLine("File write operation failed...")
 Finally
 If writer IsNot Nothing Then 
 writer.Close()
 charsWritten = message.Length
 Else
 charsWritten = 0
 End If
 End Try 
 Return charsWritten
 End Function
End Module

See also

• Func<T1,T2,TResult>