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 four parameters, each of which is passed to it by value, and that it must return a value.
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To reference a method that has four parameters and that returns void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<(Of <(T1, T2, T3, T4>)>) delegate instead. |
When you use the Func<(Of <(T1, T2, T3, T4, TResult>)>) 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 generic delegate named Searcher and assigns a reference to the IndexOf method to its delegate instance.
Delegate Function Searcher(searchString As String, _
start As Integer, _
count As Integer, _
type As StringComparison) As Integer
Module DelegateExample
Public Sub Main()
Dim title As String = "The House of the Seven Gables"
Dim position As Integer = 0
Dim finder As Searcher = AddressOf title.IndexOf
Do
Dim characters As Integer = title.Length - position
position = finder("the", position, characters, _
StringComparison.InvariantCultureIgnoreCase)
If position >= 0 Then
position += 1
Console.WriteLine("'The' found at position {0} in {1}.", _
position, title)
End If
Loop While position > 0
End Sub
End Module
using System;
delegate int Searcher(string searchString, int start, int count,
StringComparison type);
public class DelegateExample
{
public static void Main()
{
string title = "The House of the Seven Gables";
int position = 0;
Searcher finder = title.IndexOf;
do
{
int characters = title.Length - position;
position = finder("the", position, characters,
StringComparison.InvariantCultureIgnoreCase);
if (position >= 0)
{
position++;
Console.WriteLine("'The' found at position {0} in {1}.",
position, title);
}
} while (position > 0);
}
}
The following example simplifies this code by instantiating the Func<(Of <(T1, T2, T3, T4, TResult>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
Module DelegateExample
Public Sub Main()
Dim title As String = "The House of the Seven Gables"
Dim position As Integer = 0
Dim finder As Func(Of String, Integer, Integer, StringComparison, Integer) _
= AddressOf title.IndexOf
Do
Dim characters As Integer = title.Length - position
position = finder("the", position, characters, _
StringComparison.InvariantCultureIgnoreCase)
If position >= 0 Then
position += 1
Console.WriteLine("'The' found at position {0} in {1}.", _
position, title)
End If
Loop While position > 0
End Sub
End Module
using System;
public class DelegateExample
{
public static void Main()
{
string title = "The House of the Seven Gables";
int position = 0;
Func<string, int, int, StringComparison, int> finder = title.IndexOf;
do
{
int characters = title.Length - position;
position = finder("the", position, characters,
StringComparison.InvariantCultureIgnoreCase);
if (position >= 0)
{
position++;
Console.WriteLine("'The' found at position {0} in {1}.",
position, title);
}
} while (position > 0);
}
}
You can use the Func<(Of <(T1, T2, T3, T4, TResult>)>) 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 DelegateExample
{
public static void Main()
{
string title = "The House of the Seven Gables";
int position = 0;
Func<string, int, int, StringComparison, int> finder =
delegate(string s, int pos, int chars, StringComparison type)
{ return title.IndexOf(s, pos, chars, type); };
do
{
int characters = title.Length - position;
position = finder("the", position, characters,
StringComparison.InvariantCultureIgnoreCase);
if (position >= 0)
{
position++;
Console.WriteLine("'The' found at position {0} in {1}.",
position, title);
}
} while (position > 0);
}
}
You can also assign a lambda expression to a Func<(Of <(T1, T2, T3, T4, TResult>)>) delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions (C# Programming Guide).)
Module DelegateExample
Public Sub Main()
Dim title As String = "The House of the Seven Gables"
Dim position As Integer = 0
Dim finder As Func(Of String, Integer, Integer, StringComparison, Integer) _
= Function(s, pos, chars, type) _
title.IndexOf(s, pos, chars, type)
Do
Dim characters As Integer = title.Length - position
position = finder("the", position, characters, _
StringComparison.InvariantCultureIgnoreCase)
If position >= 0 Then
position += 1
Console.WriteLine("'The' found at position {0} in {1}.", _
position, title)
End If
Loop While position > 0
End Sub
End Module
using System;
public class DelegateExample
{
public static void Main()
{
string title = "The House of the Seven Gables";
int position = 0;
Func<string, int, int, StringComparison, int> finder =
(s, pos, chars, type) => title.IndexOf(s, pos, chars, type);
do
{
int characters = title.Length - position;
position = finder("the", position, characters,
StringComparison.InvariantCultureIgnoreCase);
if (position >= 0)
{
position++;
Console.WriteLine("'The' found at position {0} in {1}.",
position, title);
}
} while (position > 0);
}
}
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 parameters, you can pass these methods a lambda expression without explicitly instantiating a Func delegate.