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Back to Basics: Conditional Statements

This is the second in my Back to Basics series, and this time we are going way back. Conditional statements are some of the very first things you learn in any introductory programing class. In this post I will show you the C# and VB syntax for if, if/else, and switch statements.

If-statement

We will start off with the simplest of conditional statements, the if-statement. This is just simple boolean logic that will evaluate true or false. If the result is true, it will execute the block, if false it will skip it. In this example we will check to see if a person is over the age of 21. We will be using the same person class as in my last post

C#

if (myPerson.Age >= 21) 
     canDrink = true; //Whatever logic you want

Notice that in C# you can omit the curly braces ({}) if the logic under the if statement is one line long. If it is longer than one line you must use the braces. If you omit those, then the line directly beneath the if statement will be the only one evaluated on a conditional basis the rest of the statements will always be executed.

However in the VB.Net syntax you must have a closing End If, even if the statement is a single line.

VB

If myPerson.Age >= 21 Then
    canDrink = true
End If

If/Else

The next example is of an if/else statement. The same rules apply as before, we are just extending it a bit. You can chain together as many if/else statements as you wish, just by adding more conditions. However all of the conditions must before the else, with the else being the final statement (if needed, don't have to have an else).

Once the compiler finds a match for your statement it will not execute any more of the statements in your if/else block. This lets the compiler be more efficient.

C#

if (myPerson.Age < 18)
    canDrink = canVote = false;
else if (18 <= myPerson.Age < 21) {
    canVote = true;
    canDrink = false;
}
else {
    canDrink = canVote = true;
}

Note in this example that I mixed my curly braces. First condition I didn't need them since I had one line. Second one I did since I have more than one line. And the third I didn't need them but put them anyway.

VB

If myPerson.Age < 18 Then
    canVote = canDrink = False
ElseIf 18 <= myPerson.Age < 21 Then
    canVote = True
    canDrink = False
Else
    canVote = canDrink = True
End If

Switch Statement

The switch statement is a little more complicated than the if and if/else statements but it can be very useful and make your code look cleaner in a lot of places. They can be very useful when evaluating enumerations. In the example below lets assume that our person class has an enumeration of hair colors called (strangely) HairColors. So lets use an switch statement to evaluate our public property hair, and do some logic based on hair color. If the hair color is not in our list the default case will be evaluated. I realize that since we are using an enum we would never evaluate the default case, but just bear with me.

C#

switch (myPerson.hair) { 
    case HairColors.brown:
       //You are a brunette
       break;
    case HairColors.blonde:
       // you are blonde, duh
       break;
    case HairColors.red:
       //You are a redhead
       break;
    case HairColors.black:
       //You have black hair
       break;
    default: 
       //You have colorful hair
       break;
}

You must use the break statement to end each specific case. This will exit you out of the enclosing block, in this case the switch. The break statement can be used in any control flow statement, and it will have the same effect. In the switch statement though it is required, and the compiler will throw an error if you omit this step.

VB

Select Case myPerson.hair
    Case HairColors.brown
 'You are a brunette
    Case HairColors.blonde
 ' you are blonde, duh
    Case HairColors.red
 'You are a redhead
    Case HairColors.black
 'You have black hair
    Case Else
 'You have colorful hair
End Select

In VB, the switch statement is known as a Select Case, and as you can see by the syntax it is very similar to C# other than the naming. In this case however, you do not need to include the break statement, the flow will stop when it hits the next case keyword.

As a bonus I will give you one more statement called the ternary operator. This works very well in C# because of short-circuiting, but in VB you have to be careful because VB won't short circuit.

You can use the ternary operator to evaluate something and assign a value on one line. You can easily re-write it to be an if/else statement. Lets re-write our first statement using a ternary operator. The first part is your condition. So in this case we are checking to make sure person is over 21. After the question mark is the true part of the statement and the last is the false.

C#

canDrink = myPerson.Age >= 21 ? true : false;

VB

canDrink = IIf(myPerson.Age >= 21, True, False)

The syntax is very similar, however this isn't a true ternary operator since we are calling a function (iif), but it functions almost exactly the same. The difference being that in VB the second part of the statement (the false section) will still be evaluated. In this case it wouldn't matter if it is evaluated since nothing can go wrong. Check here for some other reading on the subject of VB and iif.

Conclusion

So there are your basic control flow and conditional statements. I hope this was useful, and good luck.

Labels: .NET, Back to Basics

posted by Tom Becker at 11/09/2009