Instrukcja case w języku C# (jak działa dla programistów)

Czym jest instrukcja switch?

Instrukcja switch w języku C# to struktura kontrolna służąca do wyboru jednej z wielu ścieżek kodu do wykonania. Wybór opiera się na wartości lub wyrażeniu. Jest to wydajna alternatywa dla stosowania wielu warunków if-else, zwłaszcza w przypadku zmiennej, która może przyjmować kilka różnych wartości.

Składnia

The basic syntax of a switch statement is straightforward:

// Switch statement
switch (variable)
{
    case value1:
        // Code to execute if variable equals value1
        break;
    case value2:
        // Code to execute if variable equals value2
        break;
    // More cases as needed
    default:
        // Code to execute if variable doesn't match any case
        break;
}

// Switch statement
switch (variable)
{
    case value1:
        // Code to execute if variable equals value1
        break;
    case value2:
        // Code to execute if variable equals value2
        break;
    // More cases as needed
    default:
        // Code to execute if variable doesn't match any case
        break;
}

' Switch statement
Select Case variable
	Case value1
		' Code to execute if variable equals value1
	Case value2
		' Code to execute if variable equals value2
	' More cases as needed
	Case Else
		' Code to execute if variable doesn't match any case
End Select

• switch (variable): This specifies the variable or expression to evaluate.
• case value1: These are the different values or conditions you check against the variable.
• break: This keyword is used to exit the switch block once a matching case is executed.
• default statement: This block executes if none of the specified cases match the variable.

Zrozumienie instrukcji break

The break statement in the switch is crucial. It prevents "fall through" behavior, where execution moves to the subsequent case even if the matching condition is already met. Each case block typically ends with a break statement to ensure that only the code under the matching case is executed.

Porównanie instrukcji switch i if-else

While the structure of the if-else statement involves checking a condition and executing a block of code if the condition is true, switch statements compare a single variable or expression against multiple potential values. This makes the switch statement more concise and easier to read when you have many conditions or case patterns to check.

Przykład: użycie instrukcji switch

int number = 3;
switch (number)
{
    case 1:
        Console.WriteLine("One");
        break;
    case 2:
        Console.WriteLine("Two");
        break;
    case 3:
        Console.WriteLine("Three");
        break;
    default:
        Console.WriteLine("Other Number"); // Print to console
        break;
}

int number = 3;
switch (number)
{
    case 1:
        Console.WriteLine("One");
        break;
    case 2:
        Console.WriteLine("Two");
        break;
    case 3:
        Console.WriteLine("Three");
        break;
    default:
        Console.WriteLine("Other Number"); // Print to console
        break;
}

Dim number As Integer = 3
Select Case number
	Case 1
		Console.WriteLine("One")
	Case 2
		Console.WriteLine("Two")
	Case 3
		Console.WriteLine("Three")
	Case Else
		Console.WriteLine("Other Number") ' Print to console
End Select

In this example, the program will print "Three" as the output since the number matches case 3.

Zrozumienie przypadku default w bloku switch

In a switch statement, the default case plays a crucial role. It serves as a catch-all option that is executed when none of the specified case labels matches the switch expression's value. While it's optional, including a default case is good practice to handle unexpected or unknown values.

Jak i kiedy używać instrukcji default

The default case is used when you want to execute a block of code if none of the specific cases match. It ensures that the switch statement always has a defined behavior, regardless of the input. The default case is declared using the default keyword, followed by a colon.

default:
    // Code to execute if no case matches
    break;

default:
    // Code to execute if no case matches
    break;

Case Else
	' Code to execute if no case matches
	break

The default case can be placed anywhere within the switch block but is typically placed at the end for readability.

Przykład: instrukcja switch z przypadkiem default

Consider a scenario where you're evaluating a day of the week:

int day = 5;
string dayName;
switch (day)
{
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    case 7:
        dayName = "Sunday";
        break;
    default:
        dayName = "Invalid day";
        break;
}
Console.WriteLine(dayName);

int day = 5;
string dayName;
switch (day)
{
    case 1:
        dayName = "Monday";
        break;
    case 2:
        dayName = "Tuesday";
        break;
    case 3:
        dayName = "Wednesday";
        break;
    case 4:
        dayName = "Thursday";
        break;
    case 5:
        dayName = "Friday";
        break;
    case 6:
        dayName = "Saturday";
        break;
    case 7:
        dayName = "Sunday";
        break;
    default:
        dayName = "Invalid day";
        break;
}
Console.WriteLine(dayName);

Dim day As Integer = 5
Dim dayName As String
Select Case day
	Case 1
		dayName = "Monday"
	Case 2
		dayName = "Tuesday"
	Case 3
		dayName = "Wednesday"
	Case 4
		dayName = "Thursday"
	Case 5
		dayName = "Friday"
	Case 6
		dayName = "Saturday"
	Case 7
		dayName = "Sunday"
	Case Else
		dayName = "Invalid day"
End Select
Console.WriteLine(dayName)

In this example, if day has a value other than 1 to 7, the default case is executed, setting dayName to "Invalid day".

Najlepsze praktyki dotyczące przypadku default

• Always Include a Default: Even if you believe you have covered all possible cases, include a default case to handle unforeseen values.
• Meaningful Actions: Use the default case to perform meaningful actions, like logging an error, setting a default value, or notifying the user of an unknown value.

Wprowadzenie do wyrażeń switch w C

With the evolution of C#, switch expressions were introduced as a more concise and expressive way of handling multiple conditional branches. Unlike traditional switch statements, switch expressions return a value and are more streamlined, making them a powerful tool in modern C# programming.

Składnia wyrażeń switch

The syntax of a switch expression in C# is a more compact form of the switch case statement. Here's a basic structure:

var result = variable switch
{
    value1 => result1,
    value2 => result2,
    _ => defaultResult
};

var result = variable switch
{
    value1 => result1,
    value2 => result2,
    _ => defaultResult
};

'INSTANT VB TODO TASK: The following 'switch expression' was not converted by Instant VB:
'var result = variable switch
'{
'	value1 => result1,
'	value2 => result2,
'	_ => defaultResult
'};

The underscore (_) symbol represents the default case in switch expressions, functioning similarly to the default block in traditional switch statements.

Przykład: użycie wyrażenia switch

Consider a scenario where you need to categorize a temperature reading:

int temperature = 25;
string weatherDescription = temperature switch
{
    <= 0 => "Freezing",
    < 20 => "Cold",
    < 30 => "Mild",
    _ => "Hot"
};
Console.WriteLine(weatherDescription);

int temperature = 25;
string weatherDescription = temperature switch
{
    <= 0 => "Freezing",
    < 20 => "Cold",
    < 30 => "Mild",
    _ => "Hot"
};
Console.WriteLine(weatherDescription);

Dim temperature As Integer = 25
'INSTANT VB TODO TASK: The following 'switch expression' was not converted by Instant VB:
'string weatherDescription = temperature switch
'{
'	<= 0 => "Freezing",
'	< 20 => "Cold",
'	< 30 => "Mild",
'	_ => "Hot"
'};
Console.WriteLine(weatherDescription)

In this example, the switch expression succinctly categorizes the temperature, with the default case (_) covering any scenario not matched by the other cases.

Dopasowywanie wzorców z wyrażeniami switch

Switch expressions in C# allow for pattern matching, making them even more versatile. You can match types, values, or even patterns:

object obj = // some object;
string description = obj switch
{
    int i => $"Integer: {i}",
    string s => $"String: {s}",
    _ => "Unknown type"
};

object obj = // some object;
string description = obj switch
{
    int i => $"Integer: {i}",
    string s => $"String: {s}",
    _ => "Unknown type"
};

'INSTANT VB TODO TASK: The following 'switch expression' was not converted by Instant VB:
'object obj = string description = obj switch
'{
'	int i => $"Integer: {i}",
'	string s => $"String: {s}",
'	_ => "Unknown type"
'};

Instrukcja switch a wyrażenie switch w C

• C# Switch Statement: Traditionally used for executing different blocks of code based on a variable's value. It requires a break statement for each case.
• Switch Expression: Introduced in C# 8.0, this provides a more concise syntax and is typically used when a value needs to be returned based on a condition.

Przykład: warunkowe znakowanie wodne z IronPDF i instrukcjami switch

Let's consider a scenario where you have a PDF document, and you want to apply different watermarks based on page count based on the number of pages in the document. Here's how you can achieve this using IronPDF in combination with a C# switch statement:

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        IronPdf.License.LicenseKey = "Your-License-Code";
        PdfDocument pdf = PdfDocument.FromFile("sample.pdf");
        // Define different watermark HTML for each case
        string watermarkHtmlOnePage = "<div style='color:red;'>One Page Document</div>";
        string watermarkHtmlTwoPage = "<div style='color:blue;'>Two Page Document</div>";
        switch (pdf.PageCount)
        {
            case 1:
                // Apply watermark for one-page document
                pdf.ApplyWatermark(watermarkHtmlOnePage);
                break;
            case 2:
                // Apply watermark for two-page documents
                pdf.ApplyWatermark(watermarkHtmlTwoPage);
                break;
            default:
                // Apply a default watermark for other cases
                pdf.ApplyWatermark("<div style='color:green;'>Multiple Page Document</div>");
                break;
        }
        // Save the watermarked PDF
        pdf.SaveAs("watermarked.pdf");
    }
}

using IronPdf;

class Program
{
    static void Main(string[] args)
    {
        IronPdf.License.LicenseKey = "Your-License-Code";
        PdfDocument pdf = PdfDocument.FromFile("sample.pdf");
        // Define different watermark HTML for each case
        string watermarkHtmlOnePage = "<div style='color:red;'>One Page Document</div>";
        string watermarkHtmlTwoPage = "<div style='color:blue;'>Two Page Document</div>";
        switch (pdf.PageCount)
        {
            case 1:
                // Apply watermark for one-page document
                pdf.ApplyWatermark(watermarkHtmlOnePage);
                break;
            case 2:
                // Apply watermark for two-page documents
                pdf.ApplyWatermark(watermarkHtmlTwoPage);
                break;
            default:
                // Apply a default watermark for other cases
                pdf.ApplyWatermark("<div style='color:green;'>Multiple Page Document</div>");
                break;
        }
        // Save the watermarked PDF
        pdf.SaveAs("watermarked.pdf");
    }
}

Imports IronPdf

Friend Class Program
	Shared Sub Main(ByVal args() As String)
		IronPdf.License.LicenseKey = "Your-License-Code"
		Dim pdf As PdfDocument = PdfDocument.FromFile("sample.pdf")
		' Define different watermark HTML for each case
		Dim watermarkHtmlOnePage As String = "<div style='color:red;'>One Page Document</div>"
		Dim watermarkHtmlTwoPage As String = "<div style='color:blue;'>Two Page Document</div>"
		Select Case pdf.PageCount
			Case 1
				' Apply watermark for one-page document
				pdf.ApplyWatermark(watermarkHtmlOnePage)
			Case 2
				' Apply watermark for two-page documents
				pdf.ApplyWatermark(watermarkHtmlTwoPage)
			Case Else
				' Apply a default watermark for other cases
				pdf.ApplyWatermark("<div style='color:green;'>Multiple Page Document</div>")
		End Select
		' Save the watermarked PDF
		pdf.SaveAs("watermarked.pdf")
	End Sub
End Class

Here is the output PDF file of one page: