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 two parameters, each of which is passed to it by value, and that it must return a value.
.gif "Note") Note: |
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To reference a method that has
two
parameter
s
and returns
void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<(Of <(T1, T2>)>) delegate instead.
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When you use the Func<(Of <(T1, T2, TResult>)>) 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 ExtractMethod and assigns a reference to the ExtractWords method to its delegate instance.
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' Declare a delegate to represent string extraction method
Delegate Function ExtractMethod(ByVal stringToManipulate As String, _
ByVal maximum As Integer) As String()
Module DelegateExample
Public Sub Main()
' Instantiate delegate to reference ExtractWords method
Dim extractMeth As ExtractMethod = AddressOf ExtractWords
Dim title As String = "The Scarlet Letter"
' Use delegate instance to call ExtractWords method and display result
For Each word As String In extractMeth(title, 5)
Console.WriteLine(word)
Next
End Sub
Private Function ExtractWords(phrase As String, limit As Integer) As String()
Dim delimiters() As Char = {" "c}
If limit > 0 Then
Return phrase.Split(delimiters, limit)
Else
Return phrase.Split(delimiters)
End If
End Function
End Module
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using System;
delegate string[] ExtractMethod(string stringToManipulate, int maximum);
public class DelegateExample
{
public static void Main()
{
// Instantiate delegate to reference ExtractWords method
ExtractMethod extractMeth = ExtractWords;
string title = "The Scarlet Letter";
// Use delegate instance to call ExtractWords method and display result
foreach (string word in extractMeth(title, 5))
Console.WriteLine(word);
}
private static string[] ExtractWords(string phrase, int limit)
{
char[] delimiters = new char[] {' '};
if (limit > 0)
return phrase.Split(delimiters, limit);
else
return phrase.Split(delimiters);
}
}
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The following example simplifies this code by instantiating a Func<(Of <(T1, T2, TResult>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
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Module GenericFunc
Public Sub Main()
' Instantiate delegate to reference ExtractWords method
Dim extractMeth As Func(Of String, Integer, String()) = AddressOf ExtractWords
Dim title As String = "The Scarlet Letter"
' Use delegate instance to call ExtractWords method and display result
For Each word As String In extractMeth(title, 5)
Console.WriteLine(word)
Next
End Sub
Private Function ExtractWords(phrase As String, limit As Integer) As String()
Dim delimiters() As Char = {" "c}
If limit > 0 Then
Return phrase.Split(delimiters, limit)
Else
Return phrase.Split(delimiters)
End If
End Function
End Module
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using System;
public class GenericFunc
{
public static void Main()
{
// Instantiate delegate to reference ExtractWords method
Func<string, int, string[]> extractMethod = ExtractWords;
string title = "The Scarlet Letter";
// Use delegate instance to call ExtractWords method and display result
foreach (string word in extractMethod(title, 5))
Console.WriteLine(word);
}
private static string[] ExtractWords(string phrase, int limit)
{
char[] delimiters = new char[] {' '};
if (limit > 0)
return phrase.Split(delimiters, limit);
else
return phrase.Split(delimiters);
}
}
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You can use the Func<(Of <(T1, T2, TResult>)>) delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)
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using System;
public class Anonymous
{
public static void Main()
{
Func<string, int, string[]> extractMeth = delegate(string s, int i)
{ char[] delimiters = new char[] {' '};
return i > 0 ? s.Split(delimiters, i) : s.Split(delimiters);
};
string title = "The Scarlet Letter";
// Use Func instance to call ExtractWords method and display result
foreach (string word in extractMeth(title, 5))
Console.WriteLine(word);
}
}
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You can also assign a lambda expression to a Func<(Of <(T1, T2, TResult>)>) delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions (C# Programming Guide).)
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Module LambdaExpression
Public Sub Main()
Dim separators() As Char = {" "c}
Dim extract As Func(Of String, Integer, String()) = Function(s, i) _
CType(iif(i > 0, s.Split(separators, i), s.Split(separators)), String())
Dim title As String = "The Scarlet Letter"
For Each word As String In extract(title, 5)
Console.WriteLine(word)
Next
End Sub
End Module
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using System;
public class LambdaExpression
{
public static void Main()
{
char[] separators = new char[] {' '};
Func<string, int, string[]> extract = (s, i) =>
i > 0 ? s.Split(separators, i) : s.Split(separators) ;
string title = "The Scarlet Letter";
// Use Func instance to call ExtractWords method and display result
foreach (string word in extract(title, 5))
Console.WriteLine(word);
}
}
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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<(Of <(T1, T2, TResult>)>) parameters, you can pass these methods a lambda expression without explicitly instantiating a Func<(Of <(T1, T2, TResult>)>) delegate.