ms_learn_csharp/015_switch_case/015_csharp.md

Introduction

The C# programming language is similar to any human written or spoken language. They each support different ways of expressing the same idea. In spoken languages, some words and phrases are more descriptive, accurate, or succinct than others. In the C# programming language, there is more than one way to create branching logic. For example, selections that use if statements and selections that use switch statements. Depending on the context of your application, one type of selection statement might be more expressive and succinct than the other.

Suppose working on applications that make extensive use of selections statements. In some cases, if-elseif-else constructs are used to produce succinct and expressive code that is easy to read and maintain. In other cases, the if-elseif-else constructs produce the required result, but are difficult to read and maintain. You have been tasked with reviewing the code and determining when it is suitable to use a switch statement rather than an if statement.

In this module, you'll investigate the use of a switch statement to implement branching logic as an alternative to an if statement. You'll also work on converting an if-elseif-else construct to a switch-case construct. During this process, you'll learn to recognize the benefits of choosing one type of selection statement over the other.

By the end of this module, you'll be able to implement switch statements in your application, judge when to use a switch statement over an if-elseif-else construct, and convert if-elseif-else constructs to switch statements.

Learning objectives

In this module, you will:

• Use the switch-case construct to match a variable or expression against several possible outcomes.
• Convert code that uses an if-elseif-else construct into a switch-case construct.

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Exercise

Implement a switch statement

A switch statement is a C# selection statement that provides an alternative to an if-elseif-else branching construct. The switch statement provides advantages over an if-elseif-else construct when evaluating a single value against a list of known matching values.

Consider the following scenario:

• You're working on an application related to food nutrition. A section of the code deals with fruits.
• Your code includes a variable named fruit that's used to hold the name of different types of fruit.
• You have a list of the 20 fruits that your application is focused on.
• You want to branch your code based on the value assigned to fruit.

In this scenario, you can use a switch statement to create a separate branch for each type of fruit.

How does a switch statement work?

The switch statement chooses one section of code to execute from a list of possible switch sections. The selected switch section is chosen based on a pattern match with the statement's match expression.

Consider the following code sample that shows the basic structure of switch statement:

switch (fruit) {
    case "apple":
        Console.WriteLine($"App will display information for apple.");
        break;
    case "banana":
        Console.WriteLine($"App will display information for banana.");
        break;
    case "cherry":
        Console.WriteLine($"App will display information for cherry.");
        break;
}

The match expression (which may also be referred to as the switch expression) is the value following the switch keyword, in this case (fruit). Each switch section is defined by a case pattern. Case patterns are constructed using the keyword case followed by a value. The first case pattern in this example is: case "apple":. Case patterns are Boolean expressions that evaluate to either true or false. Each switch section includes a small number of code lines that will be executed if the case pattern is a match for the match expression. In this example, if fruit is assigned a value of "apple", the first case pattern will evaluate as true and that switch section will be executed.

A switch statement must include at least one switch section, but will normally contain three or more switch sections.

The switch is best used when:

• You have a single value (variable or expression) that you want to match against many possible values.
• For any given match, you need to execute a couple of lines of code at most.

Note
This first example of a switch statement is purposefully simple and your examination of the syntax was brief. You will be examining additional features of the switch statement when you work through some more advanced scenarios in the sections below.

It's time to prepare your coding environment and begin developing your own switch statements.

Prepare your coding environment

At the Terminal command prompt, to create a new console application in a specified folder, type dotnet new console -o ./path_to/TestProject and then press Enter.

This .NET CLI command uses a .NET program template to create a new C# console application project in the specified folder location.

Create and test a switch statement

Type the following code into the editor:

int employeeLevel = 200;
string employeeName = "John Smith";

string title = "";

switch (employeeLevel) {
    case 100:
        title = "Junior Associate";
        break;
    case 200:
        title = "Senior Associate";
        break;
    case 300:
        title = "Manager";
        break;
    case 400:
        title = "Senior Manager";
        break;
    default:
        title = "Associate";
        break;
}

Console.WriteLine($"{employeeName}, {title}");

The Program.cs file must be saved before building or running the code.

At the Terminal command prompt, to run your code, type dotnet run and then press Enter.

Output 

John Smith, Senior Associate

Take a minute to review the switch statement that you entered.

Notice that the switch statement defines a single code block.

The switch statement defines a single code block that includes a list of switch sections. To the right of the switch keyword is a switch expression that's enclosed in parentheses.

Notice the list of switch sections inside the code block.

The switch code block contains a list of switch sections, each of which includes one or more switch labels. In addition, each switch section includes a statement list that will execute if the label is equal to the switch expression defined at the top of the switch statement.

The switch expression is evaluated against the case labels from top to bottom until a value that is equal to the switch expression is found. If none of the labels are a match, the statement list for the default case will be executed. If no default is included, control is transferred to the end point of the switch statement. Each label must provide a value type that matches the type specified in the switch expression.

Note
The optional default label can appear at any position within the list of switch sections. However, most developers choose to put it last because it makes more sense (logically) to position default as the final option. Regardless of the position, the default section will be evaluated last.

In our example:

• the switch expression is (employeeLevel)
• each switch section has a single switch label (case or default).
• the matching switch section is defined by case: 200, since employeeLevel = 200.

Notice that each switch section is separated from the next.

Only one switch section is allowed to be executed. This means that execution of a switch section is not permitted to “fall through” to the next switch section. The break keyword is one of several ways to end a switch section before it gets to the next section. If you forget the break keyword (or, optionally, the return keyword) the compiler will generate an error.

Change the level variable value to see how the switch statement evaluates it

To exercise the default case, let's change the employee's level by modifying the value assignment.

To modify the value assigned to employeeLevel, update your code as follows:

int employeeLevel = 201;

Save your code file, and then run your code.

Enter dotnet run from the Terminal command prompt to run your code.

Notice that the output has changed.

Now, when you run the code, you should see the more generic title used.

John Smith, Associate

Since the employeeLevel doesn't match any labels, the default label is matched.

Modify a switch section to include multiple labels

Suppose our company decided to give all level 100 employees the title "Senior Associate" -- the same title as level 200 employees. As the developer, you decide to implement this by removing the first switch section belonging to the label case 100:, and instead allow both the case 100: and case 200: labels to execute the same switch section.

To modify the value assigned to employeeLevel, update your code as follows:

int employeeLevel = 100;

To assign multiple labels to the first switch section, update your code as follows:

case 100:
case 200:
    title = "Senior Associate";
    break;

When you're finished making changes, your modifications should match the following code:

int employeeLevel = 100;
string employeeName = "John Smith";

string title = "";

switch (employeeLevel) {
    case 100:
    case 200:
        title = "Senior Associate";
        break;
    case 300:
        title = "Manager";
        break;
    case 400:
        title = "Senior Manager";
        break;
    default:
        title = "Associate";
        break;
}

Console.WriteLine($"{employeeName}, {title}");

Save your code file, and then run your code.

Enter dotnet run from the Terminal command prompt to run your code.

You should see the following output:

John Smith, Senior Associate

Both of the case labels 100 and 200 are now paired with the switch section that sets the title to the string value Senior Associate.

Recap

Here's the main takeaways you learned about the switch statement:

• Use the switch statement when you have one value with many possible matche s, each match requiring a branch in your code logic.
• A single switch section containing code logic can be matched using one or more labels defined by the case keyword.
• Use the optional default keyword to create a label and a switch section that will be used when no other case labels match.

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Exercise

Complete a challenge activity using switch statements

Code challenges will reinforce what you've learned, and help you gain some confidence before continuing on.

Convert to switch statements challenge

In this challenge, you'll rewrite an if-elseif-else construct as a switch statement. This challenge should help you see the strengths/weaknesses of the switch statement when compared to an if-elseif-else construct. Good luck.

Code challenge: rewrite if-elseif-else using a switch statement

You'll start with code that uses an if-elseif-else construct to evaluate the components of a product SKU. The SKU (Stock Keeping Unit) is formatted using three coded values: <product #>-<2-letter color code>-<size code>. For example, a SKU value of 01-MN-L corresponds to (sweat shirt)-(maroon)-(large), and the code outputs a description that appears as "Product: Large Maroon Sweat shirt".

Your challenge is to convert the if statement code to a switch statement that achieves the same result as the initial code.

Enter the following code into the code editor:

// SKU = Stock Keeping Unit. 
// SKU value format: <product #>-<2-letter color code>-<size code>
string sku = "01-MN-L";

string[] product = sku.Split('-');

string type = "";
string color = "";
string size = "";

if (product[0] == "01") {
    type = "Sweat shirt";
} else if (product[0] == "02") {
    type = "T-Shirt";
} else if (product[0] == "03") {
    type = "Sweat pants";
} else {
    type = "Other";
}

if (product[1] == "BL") {
    color = "Black";
} else if (product[1] == "MN") {
    color = "Maroon";
} else {
    color = "White";
}

if (product[2] == "S") {
    size = "Small";
} else if (product[2] == "M") {
    size = "Medium";
} else if (product[2] == "L") {
    size = "Large";
} else {
    size = "One Size Fits All";
}

Console.WriteLine($"Product: {size} {color} {type}");

Update the code to use a switch statement in place of the if-elseif-else construct.

Verify that your output hasn't changed.

No matter how you do it, your code should produce the following output:

Output 

Product: Large Maroon Sweat shirt

string sku = "01-MN-L";

string[] product = sku.Split('-');

string type = "";
string color = "";
string size = "";

switch (product[0]) {
    case "01":
        type = "Sweat shirt";
        break;
    case "02":
        type = "T-Shirt";
        break;
    case "03":
        type = "Sweat pants";
        break;
    default:
        type = "Other";
        break;
}

switch (product[1]) {
    case "BL":
        color = "Black";
        break;
    case "MN":
        color = "Maroon";
        break;
    default:
        color = "White";
        break;
}

switch (product[2]) {
    case "S":
        size = "Small";
        break;
    case "M":
        size = "Medium";
        break;
    case "L":
        size = "Large";
        break;
    default:
        size = "One Size Fits All";
        break;
}

Console.WriteLine($"Product: {size} {color} {type}");