You can use the Action<(Of <(T1, T2, T3, T4>)>) 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 four parameters that are all passed to it by value, and it 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, T3, T4>)>) 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 delegate named StringCopy and assigns a reference to the CopyStrings method to its delegate instance.
Delegate Sub StringCopy(stringArray1() As String, _
stringArray2() As String, _
indexToStart As Integer, _
numberToCopy As Integer)
Module TestDelegate
Public Sub Main()
Dim ordinals() As String = {"First", "Second", "Third", "Fourth", _
"Fifth", "Sixth", "Seventh", "Eighth", _
"Ninth", "Tenth"}
Dim copiedOrdinals(ordinals.Length - 1) As String
Dim copyOperation As StringCopy = AddressOf CopyStrings
copyOperation(ordinals, copiedOrdinals, 3, 5)
For Each ordinal As String In copiedOrdinals
Console.WriteLine(ordinal)
Next
End Sub
Private Sub CopyStrings(source() As String, target() As String, _
startPos As Integer, number As Integer)
If source.Length <> target.Length Then
Throw New IndexOutOfRangeException("The source and target arrays" & _
" must have the same number of elements.")
End If
For ctr As Integer = startPos to startpos + number - 1
target(ctr) = String.Copy(source(ctr))
Next
End Sub
End Module
using System;
delegate void StringCopy(string[] stringArray1,
string[] stringArray2,
int indexToStart,
int numberToCopy);
public class TestDelegate
{
public static void Main()
{
string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth",
"Sixth", "Seventh", "Eighth", "Ninth", "Tenth"};
string[] copiedOrdinals = new string[ordinals.Length];
StringCopy copyOperation = CopyStrings;
copyOperation(ordinals, copiedOrdinals, 3, 5);
foreach (string ordinal in copiedOrdinals)
Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
}
private static void CopyStrings(string[] source, string[] target,
int startPos, int number)
{
if (source.Length != target.Length)
throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.");
for (int ctr = startPos; ctr <= startPos + number - 1; ctr++)
target[ctr] = String.Copy(source[ctr]);
}
}
The following example simplifies this code by instantiating the Action<(Of <(T1, T2, T3>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
Module TestAction4
Public Sub Main()
Dim ordinals() As String = {"First", "Second", "Third", "Fourth", _
"Fifth", "Sixth", "Seventh", "Eighth", _
"Ninth", "Tenth"}
Dim copiedOrdinals(ordinals.Length - 1) As String
Dim copyOperation As Action(Of String(), String(), Integer, Integer) = _
AddressOf CopyStrings
copyOperation(ordinals, copiedOrdinals, 3, 5)
For Each ordinal As String In copiedOrdinals
Console.WriteLine(ordinal)
Next
End Sub
Private Sub CopyStrings(source() As String, target() As String, _
startPos As Integer, number As Integer)
If source.Length <> target.Length Then
Throw New IndexOutOfRangeException("The source and target arrays" & _
" must have the same number of elements.")
End If
For ctr As Integer = startPos to startpos + number - 1
target(ctr) = String.Copy(source(ctr))
Next
End Sub
End Module
using System;
public class TestAction4
{
public static void Main()
{
string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth",
"Sixth", "Seventh", "Eighth", "Ninth", "Tenth"};
string[] copiedOrdinals = new string[ordinals.Length];
Action<string[], string[], int, int> copyOperation = CopyStrings;
copyOperation(ordinals, copiedOrdinals, 3, 5);
foreach (string ordinal in copiedOrdinals)
Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
}
private static void CopyStrings(string[] source, string[] target,
int startPos, int number)
{
if (source.Length != target.Length)
throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.");
for (int ctr = startPos; ctr <= startPos + number - 1; ctr++)
target[ctr] = String.Copy(source[ctr]);
}
}
You can also use the Action<(Of <(T1, T2, T3, T4>)>) 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;
public class TestAnonymousMethod
{
public static void Main()
{
string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth",
"Sixth", "Seventh", "Eighth", "Ninth", "Tenth"};
string[] copiedOrdinals = new string[ordinals.Length];
Action<string[], string[], int, int> copyOperation =
delegate(string[] s1, string[] s2,
int pos, int num)
{ CopyStrings(s1, s2, pos, num); };
copyOperation(ordinals, copiedOrdinals, 3, 5);
foreach (string ordinal in copiedOrdinals)
Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
}
private static void CopyStrings(string[] source, string[] target,
int startPos, int number)
{
if (source.Length != target.Length)
throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.");
for (int ctr = startPos; ctr <= startPos + number - 1; ctr++)
target[ctr] = String.Copy(source[ctr]);
}
}
You can also assign a lambda expression to an Action<(Of <(T1, T2, T3, T4>)>) delegate instance, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions (C# Programming Guide).)
Public Module TestLambdaExpression
Public Sub Main()
Dim ordinals() As String = {"First", "Second", "Third", "Fourth", "Fifth", _
"Sixth", "Seventh", "Eighth", "Ninth", "Tenth"}
Dim copiedOrdinals(ordinals.Length - 1) As String
Dim copyOperation As Action(Of String(), String(), Integer, Integer) = _
Function(s1, s2, pos, num) CopyStrings(s1, s2, pos, num)
copyOperation(ordinals, copiedOrdinals, 3, 5)
For Each ordinal As String In copiedOrdinals
If String.IsNullOrEmpty(ordinal) Then
Console.WriteLine("<None>")
Else
Console.WriteLine(ordinal)
End If
Next
End Sub
Private Function CopyStrings(source() As String, target() As String, _
startPos As Integer, number As Integer) As Integer
If source.Length <> target.Length Then
throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.")
End If
For ctr As Integer = startPos To startPos + number - 1
target(ctr) = String.Copy(source(ctr))
Next
Return number
End Function
End Module
' The example displays the following output:
' <None>
' <None>
' <None>
' Fourth
' Fifth
' Sixth
' Seventh
' Eighth
' <None>
' <None>
using System;
public class TestLambdaExpression
{
public static void Main()
{
string[] ordinals = {"First", "Second", "Third", "Fourth", "Fifth",
"Sixth", "Seventh", "Eighth", "Ninth", "Tenth"};
string[] copiedOrdinals = new string[ordinals.Length];
Action<string[], string[], int, int> copyOperation = (s1, s2, pos, num)
=> CopyStrings(s1, s2, pos, num);
copyOperation(ordinals, copiedOrdinals, 3, 5);
foreach (string ordinal in copiedOrdinals)
Console.WriteLine(String.IsNullOrEmpty(ordinal) ? "<None>" : ordinal);
}
private static void CopyStrings(string[] source, string[] target,
int startPos, int number)
{
if (source.Length != target.Length)
throw new IndexOutOfRangeException("The source and target arrays must have the same number of elements.");
for (int ctr = startPos; ctr <= startPos + number - 1; ctr++)
target[ctr] = String.Copy(source[ctr]);
}
}
.gif) Note: |
|---|
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, T3, T4>)>) delegate. |