As you develop ASP.NET applications, there will be many instances in which you’ll need to perform an action only if a certain condition is met, for instance, if the user has checked a certain checkbox, selected a certain item from a DropDownList control, or typed a certain string into a TextBox control.

We check for such things using conditionals, the simplest of which is probably the If statement. This statement is often used in conjunction with an Else statement, which specifies what should happen if the condition is not met. So, for instance, we may wish to check if the name entered in a text box is "Zak," redirecting to one page if it is, or else redirecting to an error page:

If (txtUsername.Text = "Zak") Then 
  Response.Redirect("ZaksPage.aspx")
Else 
  Response.Redirect("errorPage.aspx")
End If

if (txtUsername.Text == "Zak") { 
  Response.Redirect("ZaksPage.aspx");
} else {
  Response.Redirect("errorPage.aspx");
}

Often, we want to check for one of many possibilities, and perform a particular action in each case. In that event, we can use the Switch Case (VB.NET) or switch (C#) construct:

Dim strName As String = txtUsername.Text
Select Case strName
  Case "Zak" 
    Response.Redirect("ZaksPage.aspx")
  Case "Mark" 
    Response.Redirect("MarksPage.aspx")
  Case "Fred" 
    Response.Redirect("FredsPage.aspx")
  Case Else 
    Response.Redirect("errorPage.aspx")
End Select

string strName = txtUsername.Text;
switch (strName) {
  case "Zak":
    Response.Redirect("ZaksPage.aspx");
    break;
  case "Mark":
    Response.Redirect("MarksPage.aspx");
    break;
  case "Fred":
    Response.Redirect("FredsPage.aspx");
    break;
  default:
    Response.Redirect("errorPage.aspx");
    break;
}

As you’ve just seen, an If statement causes a code block to execute once if the value of its test expression is true. Loops, on the other hand, cause a code block to execute repeatedly for as long as the test expression remains true. There are two basic kinds of loop:

A While loop is the simplest form of loop; it makes a block of code repeat for as long as a particular condition is true. Here’s an example:

Dim Counter As Integer = 0

Do While Counter <= 10

  ' Convert out Integer to a String
  lblMessage.Text = Counter.ToString()

  ' Below we use the += operator to increase our variable by 1
  Counter += 1
Loop

int counter = 0;

while (counter <= 10) {

  // Below we use a sneaky way to convert our int to a string
  lblMessage.Text = counter + "";

  // C# has the operator ++ to increase a variable by 1
  counter++;
}

You can try out this code—enter it inside a Page_Load subroutine of one of the pages you’ve already created. The page illustrating Page_Load at the start of this chapter would be ideal. Make sure you remove any other code in the subroutine, and that there is an ASP.NET Label control in the HTML of the page with the ID lblMessage. When you open the page, the label will be set to show the number 0, then 1, then 2, all the way to 10. Of course, since all this happens in Page_Load (i.e. before any output is sent to the browser), you’ll only see the last value assigned, 10.

This demonstrates that the loop repeats until the condition is no longer met. Try changing the code so that the counter variable is initialized to 20 instead of 10. When you open the page now, you won’t see anything on screen, because the loop condition was never met.

There is another form of the While loop, called a Do While loop, which checks if the condition has been met at the end of the code block, rather than at the beginning:

Dim Counter As Integer = 0

Do 

  ' Convert our Integer to a String
  lblMessage.Text = Counter.toString()

  ' Below we use the += operator to increase our variable by 1
  Counter += 1
Loop While Counter <= 10

int counter = 0;

do {

  // Below we use a sneaky way to convert our int to a string
  lblMessage.Text = counter + "";

  // C# has the operator ++ to increase a variable by 1
  counter++;
} while (counter <= 10);

If you run this code, you’ll see it provides the exact same output we saw when we tested the condition before the code block. However, we can see the crucial difference if we again change it so the counter variable is initialized to 20. In this case, we will, in fact, see 20 on screen, because the loop code is executed once before the condition is even checked! There are some instances when this is just what we want, so being able to place the condition at the end of the loop can be very handy.

A For loop is similar to a While loop, but is typically used when the number of times we need it to execute is known beforehand. The following example displays the count of items within a DropDownList control called ddlProducts:

Dim i As Integer 
For i = 1 To ddlProducts.Items.Count
  lblMessage.Text = i.toString()
Next

int i;
for (i = 1; i <= ddlProducts.Items.Count; i++) {
  lblMessage.Text = Convert.ToString(i);
}

In VB.NET, the loop syntax specifies the starting and ending values for our counter variable in the For statement itself. In C#, we assign a starting value (i = 1), a condition to be tested each time through the loop, just like a While loop (i <= ddlProducts.Items.Count), and how the counter variable should be incremented after each loop (i++). While this allows for some powerful variations on the theme in C#, it can be confusing at first. In VB.NET, the syntax is considerably simpler, but can be a bit limiting in exceptional cases.

The other type of For loop is For Each, which loops through every item within a collection. The following example loops through an array called arrayName:

For Each item In arrayName
  lblMessage.Text = item
Next

foreach (string item in arrayName) {
  lblMessage.Text = item;
}

You may also come across instances in which you need to exit a loop prematurely. In this case, you would use Exit (VB.NET) or break (C#) to terminate the loop:

Dim i As Integer
For i = 0 To 10
  If (i = 5) Then
    Response.Write("Oh no! Not the number 5!!")
    Exit For
  End If
Next

int i;
for (i = 0; i <= 10; i++) {
  if (i == 5) {
    Response.Write("Oh no! Not the number 5!!");
    break;
  }
}

In this case, as soon as our For loop hits 5, it displays a warning message, using the Response.Write() method that will be familiar to those with past ASP experience, and exits the loop so that no further passes through the loop will be made.

Although we have only scratched the surface, VB.NET and C# provide a great deal of power and flexibility to the Web developer, and time spent learning the basics now will more than pay off in the future.