Im folgenden Codebeispiel wird die Verwendung des Comparison-Delegaten mit der Sort(Comparison (generisch))-Methodenüberladung veranschaulicht.
Im Codebeispiel wird eine alternative Vergleichsmethode für Zeichenfolgen mit der Bezeichnung CompareDinosByLength definiert. Dieses Verfahren funktioniert folgendermaßen: Zunächst werden die zu vergleichenden Objekte auf NULL (Nothing in Visual Basic) getestet, und ein NULL-Verweis wird als kleiner angesehen als ein Nicht-NULL-Verweis. Dann werden die Zeichenfolgenlängen verglichen, und die längere Zeichenfolge wird als größer betrachtet. In einem dritten Schritt wird der gewöhnliche Zeichenfolgenvergleich verwendet, wenn die Längen übereinstimmen.
Es wird eine List von Zeichenfolgen erstellt und ohne bestimmte Reihenfolge mit vier Zeichenfolgen aufgefüllt. Die Liste enthält auch eine leere Zeichenfolge und einen NULL-Verweis. Die Liste wird angezeigt, mithilfe eines generischen Comparison-Delegaten, der die CompareDinosByLength-Methode darstellt, sortiert und erneut angezeigt.
Imports System
Imports System.Collections.Generic
Public Class Example
Private Shared Function CompareDinosByLength( _
ByVal x As String, ByVal y As String) As Integer
If x Is Nothing Then
If y Is Nothing Then
' If x is Nothing and y is Nothing, they're
' equal.
Return 0
Else
' If x is Nothing and y is not Nothing, y
' is greater.
Return -1
End If
Else
' If x is not Nothing...
'
If y Is Nothing Then
' ...and y is Nothing, x is greater.
Return 1
Else
' ...and y is not Nothing, compare the
' lengths of the two strings.
'
Dim retval As Integer = _
x.Length.CompareTo(y.Length)
If retval <> 0 Then
' If the strings are not of equal length,
' the longer string is greater.
'
Return retval
Else
' If the strings are of equal length,
' sort them with ordinary string comparison.
'
Return x.CompareTo(y)
End If
End If
End If
End Function
Public Shared Sub Main()
Dim dinosaurs As New List(Of String)
dinosaurs.Add("Pachycephalosaurus")
dinosaurs.Add("Amargasaurus")
dinosaurs.Add("")
dinosaurs.Add(Nothing)
dinosaurs.Add("Mamenchisaurus")
dinosaurs.Add("Deinonychus")
Display(dinosaurs)
Console.WriteLine(vbLf & "Sort with generic Comparison(Of String) delegate:")
dinosaurs.Sort(AddressOf CompareDinosByLength)
Display(dinosaurs)
End Sub
Private Shared Sub Display(ByVal lis As List(Of String))
Console.WriteLine()
For Each s As String In lis
If s Is Nothing Then
Console.WriteLine("(Nothing)")
Else
Console.WriteLine("""{0}""", s)
End If
Next
End Sub
End Class
' This code example produces the following output:
'
'"Pachycephalosaurus"
'"Amargasaurus"
'""
'(Nothing)
'"Mamenchisaurus"
'"Deinonychus"
'
'Sort with generic Comparison(Of String) delegate:
'
'(Nothing)
'""
'"Deinonychus"
'"Amargasaurus"
'"Mamenchisaurus"
'"Pachycephalosaurus"
using System;
using System.Collections.Generic;
public class Example
{
private static int CompareDinosByLength(string x, string y)
{
if (x == null)
{
if (y == null)
{
// If x is null and y is null, they're
// equal.
return 0;
}
else
{
// If x is null and y is not null, y
// is greater.
return -1;
}
}
else
{
// If x is not null...
//
if (y == null)
// ...and y is null, x is greater.
{
return 1;
}
else
{
// ...and y is not null, compare the
// lengths of the two strings.
//
int retval = x.Length.CompareTo(y.Length);
if (retval != 0)
{
// If the strings are not of equal length,
// the longer string is greater.
//
return retval;
}
else
{
// If the strings are of equal length,
// sort them with ordinary string comparison.
//
return x.CompareTo(y);
}
}
}
}
public static void Main()
{
List<string> dinosaurs = new List<string>();
dinosaurs.Add("Pachycephalosaurus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("");
dinosaurs.Add(null);
dinosaurs.Add("Mamenchisaurus");
dinosaurs.Add("Deinonychus");
Display(dinosaurs);
Console.WriteLine("\nSort with generic Comparison<string> delegate:");
dinosaurs.Sort(CompareDinosByLength);
Display(dinosaurs);
}
private static void Display(List<string> list)
{
Console.WriteLine();
foreach( string s in list )
{
if (s == null)
Console.WriteLine("(null)");
else
Console.WriteLine("\"{0}\"", s);
}
}
}
/* This code example produces the following output:
"Pachycephalosaurus"
"Amargasaurus"
""
(null)
"Mamenchisaurus"
"Deinonychus"
Sort with generic Comparison<string> delegate:
(null)
""
"Deinonychus"
"Amargasaurus"
"Mamenchisaurus"
"Pachycephalosaurus"
*/
using namespace System;
using namespace System::Collections::Generic;
int CompareDinosByLength(String^ x, String^ y)
{
if (x == nullptr)
{
if (y == nullptr)
{
// If x is null and y is null, they're
// equal.
return 0;
}
else
{
// If x is null and y is not null, y
// is greater.
return -1;
}
}
else
{
// If x is not null...
//
if (y == nullptr)
// ...and y is null, x is greater.
{
return 1;
}
else
{
// ...and y is not null, compare the
// lengths of the two strings.
//
int retval = x->Length.CompareTo(y->Length);
if (retval != 0)
{
// If the strings are not of equal length,
// the longer string is greater.
//
return retval;
}
else
{
// If the strings are of equal length,
// sort them with ordinary string comparison.
//
return x->CompareTo(y);
}
}
}
};
void Display(List<String^>^ list)
{
Console::WriteLine();
for each(String^ s in list)
{
if (s == nullptr)
Console::WriteLine("(null)");
else
Console::WriteLine("\"{0}\"", s);
}
};
void main()
{
List<String^>^ dinosaurs = gcnew List<String^>();
dinosaurs->Add("Pachycephalosaurus");
dinosaurs->Add("Amargasaurus");
dinosaurs->Add("");
dinosaurs->Add(nullptr);
dinosaurs->Add("Mamenchisaurus");
dinosaurs->Add("Deinonychus");
Display(dinosaurs);
Console::WriteLine("\nSort with generic Comparison<String^> delegate:");
dinosaurs->Sort(
gcnew Comparison<String^>(CompareDinosByLength));
Display(dinosaurs);
}
/* This code example produces the following output:
"Pachycephalosaurus"
"Amargasaurus"
""
(null)
"Mamenchisaurus"
"Deinonychus"
Sort with generic Comparison<String^> delegate:
(null)
""
"Deinonychus"
"Amargasaurus"
"Mamenchisaurus"
"Pachycephalosaurus"
*/