You can use the Action<(Of <(T1, T2>)>) delegate to pass a method 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 two parameters that are both passed to it by value, and must not return a value. (In C#, the method must return void. In Visual Basic, it must be defined by the Sub…End Sub construct.) Typically, such a method is used to perform an operation.
When you use the Action<(Of <(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.
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
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();
}
}
}
The following example simplifies this code by instantiating the Action<(Of <(T1, T2>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
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
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();
}
}
}
You can also use the Action<(Of <(T1, T2>)>) delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)
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<(Of <(T1, T2>)>) delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)
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 = Function(s1, s2) WriteToFile(s1, s2)
Else
concat = Function(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
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();
}
}
}
.gif) Note: |
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Visual Basic requires that a lambda expression return a value. As a result, that return value must be discarded if the lambda expression is to be used with the Action<(Of <(T1, T2>)>) delegate. |