Using for and foreach with arrays, arraylists and other collections

Overview

Most people are familiar with working with lists like arrays, array lists, hashtables and other collections. A common action with these types of objects is to loop through them and there have always been a variety of ways of doing this with operators like for, while, do/until etc. Many people are not familiar with the foreach construct though.

Standard For

In a standard for loop, you would do something like this. Let's say that you have a standard string array with 6 names in it as follows:

CSharp
    // An array of names...     string[] namesArray = new string[] { "John", "Jack", "James",                                         "Mary", "Mindy", "Mandy" };
VB
    ' An array of names...     String() namesArray = New String() { "John", "Jack", "James",                                         "Mary", "Mindy", "Mandy"                                         }

Now, let's say that we wanted to print these names onto a web page using Response.Write. To do this using a regular for loop, the code would look something like this:

CSharp
    // Print string array     for (int i = 0; i < namesArray.Length; i++)     {         Response.Write(namesArray[i] + "...<br>");     }
VB
    ' Print string array     Dim i As Integer     For i = 0 To namesArray.Length- 1 Step i + 1         Response.Write(namesArray(i) + "...<br>")     Next

Now lets say that we wanted to print these names from an array list. The following code would do this. Notice that in the first for loop, we had to check for namesArray.Length, while this time, it has to use nameArrayList.Count.

CSharp

    ArrayList namesArrayList = new ArrayList(namesArray);
    for (int i = 0; i < namesArrayList.Count; i++)
    {
        Response.Write(namesArray[i] + "...<br>");
    }

VB

    Dim namesArrayList As ArrayList =  New ArrayList(namesArray) 
    Dim i As Integer
    For  i = 0 To  namesArrayList.Count- 1  Step  i + 1
        Response.Write(namesArray(i) + "...<br>")
    Next

This is just one of many problems with using a for loop. You always have to "know" the count/length. You have to know if it's a 0 or 1 based array, etc. In this case we're working with strings, so it's pretty straightforward, but things also get more complicated when working with complex object types since we have to explicitly convert each object before we can use it.

Now let's look at this same functionality using the foreach construct.

CSharp

    // Print string array
    foreach (string name in namesArray)
    {
        Response.Write(name + "...<br>");
    }

    // Print array list
    ArrayList namesArrayList = new ArrayList(namesArray);
    foreach (string name in namesArrayList)
    {
        Response.Write(name + "...<br>");
    }

VB

    ' Print string array
    Dim name As String
    For Each name In namesArray
        Response.Write(name + "...<br>")
    Next
 
    ' Print array list
    Dim namesArrayList As ArrayList =  New ArrayList(namesArray) 
    Dim name As String
    For Each name In namesArrayList
        Response.Write(name + "...<br>")
    Next

There are a few things that should be pointed out about this code as compared with the for syntax.

You don't have to know the bounds/dimentions of the array
It automatically gives you an instance (of the right type) of whatever is collected by the array (a string in this case).
The syntax (and therefore the resulting code) is just so much cleaner
This same syntax will work whether you're iterating over an array, ArrayList, Hashtable or any other type of collection.

The complete code for this sample application is listed below.

default.aspx.cs/vb

CSharp
using System; using System.Data; using System.Configuration; using System.Web; using System.Web.Security; using System.Web.UI; using System.Web.UI.WebControls; using System.Web.UI.WebControls.WebParts; using System.Web.UI.HtmlControls; using System.Collections; public partial class _Default : System.Web.UI.Page {     protected void Page_Load(object sender, EventArgs e)     {         PrintNamesWithFor();         PrintNamesWithForeach();     }     // An array of names...     string[] namesArray = new string[] { "John", "Jack", "James",                                         "Mary", "Mindy", "Mandy" };     // Print each name on it's own line     private void PrintNamesWithFor()     {         // Print string array         for (int i = 0; i < namesArray.Length; i++)         {             Response.Write(namesArray[i] + "...<br>");         }         // Print array list         ArrayList namesArrayList = new ArrayList(namesArray);         for (int i = 0; i < namesArrayList.Count; i++)         {             Response.Write(namesArray[i] + "...<br>");         }     }     private void PrintNamesWithForeach()     {         // Print string array         foreach (string name in namesArray)         {             Response.Write(name + "...<br>");         }         // Print array list         ArrayList namesArrayList = new ArrayList(namesArray);         foreach (string name in namesArrayList)         {             Response.Write(name + "...<br>");         }     } }
VB
Imports System Imports System.Data Imports System.Configuration Imports System.Web Imports System.Web.Security Imports System.Web.UI Imports System.Web.UI.WebControls Imports System.Web.UI.WebControls.WebParts Imports System.Web.UI.HtmlControls Imports System.Collections Public partial Class _Default Inherits System.Web.UI.Page     Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)         PrintNamesWithFor()         PrintNamesWithForeach()     End Sub     ' An array of names...     String() namesArray = New String() { "John", "Jack", "James",                                         "Mary", "Mindy", "Mandy"                                         }     ' Print each name on it's own line     Private Sub PrintNamesWithFor()         ' Print string array         Dim i As Integer         For i = 0 To namesArray.Length- 1 Step i + 1             Response.Write(namesArray(i) + "...<br>")         Next         ' Print array list         Dim namesArrayList As ArrayList = New ArrayList(namesArray)         Dim i As Integer         For i = 0 To namesArrayList.Count- 1 Step i + 1             Response.Write(namesArray(i) + "...<br>")         Next     End Sub     Private Sub PrintNamesWithForeach()         ' Print string array         Dim name As String         For Each name In namesArray             Response.Write(name + "...<br>")         Next         ' Print array list         Dim namesArrayList As ArrayList = New ArrayList(namesArray)         Dim name As String         For Each name In namesArrayList             Response.Write(name + "...<br>")         Next     End Sub End Class

default.aspx

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server">     <title>Untitled Page</title> </head> <body>     <form id="form1" runat="server">     <div>         </div>     </form> </body> </html>

'----------------------------------------------------------------
' Converted from C# to VB .NET using CSharpToVBConverter(1.2).
' Developed by: Kamal Patel (http://www.KamalPatel.net)
'----------------------------------------------------------------



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ASP FAQ

Using for and foreach with arrays, arraylists and other collections

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