Action<T1,T2> Delegate
Definition
Important
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Encapsulates a method that has two parameters and does not return a value.
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<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
- 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 Covariance and Contravariance in Generics.- T2
The type of the second 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 Covariance and Contravariance in Generics.Parameters
- arg1
- T1
The first parameter of the method that this delegate encapsulates.
- arg2
- T2
The second parameter of the method that this delegate encapsulates.
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.
Note
To reference a method that has two parameters and returns a value, use the generic Func<T1,T2,TResult> delegate instead.
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
Extension Methods
GetMethodInfo(Delegate) |
Gets an object that represents the method represented by the specified delegate. |
Applies to
See also
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