Blocks and the conditional statement

Code blocks and scope

A block is a sequence of statements enclosed in braces { }. A block can appear wherever the language allows one statement, so blocks form the bodies of branches and loops.

A variable declared inside a block is visible only within it: from its declaration to the closing brace. After leaving the block, the name is unavailable (CS0103); a nested block cannot declare a variable with the same name as one in an outer block (CS0136):

int total = 0;
{
    int bonus = 5;        // visible only in this block
    total += bonus;
}
Console.WriteLine(total); // 5
// Console.WriteLine(bonus);  – error CS0103

C# convention uses Allman style: each brace goes on its own line, and block contents are indented by 4 spaces. Visual Studio formats code automatically (Ctrl+K, Ctrl+D). Use braces even around a single statement: this prevents mistakes when adding lines (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions).

The if statement

if executes a block if the condition, an expression of type bool, is true. The optional else part executes if the condition is false (Fig. 3.1):

int temperature = -3;

if (temperature < 0)
{
    Console.WriteLine("It is freezing. Dress warmly.");
}
else
{
    Console.WriteLine("The temperature is above zero.");
}

Figure 3.1. Flowchart of an if/else branch

A C# condition always has type bool: if (count) with an integer count causes CS0029, and the compiler also rejects assignment if (x = 5) in place of comparison if (x == 5). Build complex conditions with &&, ||, and ! (Topic 2) (https://learn.microsoft.com/dotnet/csharp/language-reference/statements/selection-statements).

else if chains and nested conditions

For more than two alternatives, use an else if chain. Conditions are checked in order, and only the block for the first true condition executes, so order matters:

double bmi = 23.4;
string category;

if (bmi < 18.5)
{
    category = "underweight";
}
else if (bmi < 25)
{
    category = "normal";
}
else if (bmi < 30)
{
    category = "overweight";
}
else
{
    category = "obesity";
}
Console.WriteLine(category);   // normal

The second condition, bmi < 25, does not check bmi >= 18.5: a lower index would have matched the first condition. The compiler also sees that category is assigned in every branch; removing the final else would make use of the variable cause CS0165.

You can nest one if inside another. Deep nesting is difficult to read, so an early return is often used: check error cases first and exit with return, leaving the main logic unindented:

Console.Write("Age: ");
if (!int.TryParse(Console.ReadLine(), out int age))
{
    Console.WriteLine("An integer is required.");
    return;
}
if (age < 0 || age > 150)
{
    Console.WriteLine("Invalid age.");
    return;
}
Console.WriteLine(age >= 18 ? "Adult" : "Minor");

Common mistake

A semicolon after the condition in if (x > 0); creates an empty statement: the braced block that follows always executes. The compiler warns with CS0642, Possible mistaken empty statement.

The ternary operator ?:

The conditional (ternary) operator condition ? expression1 : expression2 returns expression1 if the condition is true, otherwise expression2. Unlike if, it is an expression: it has a value that can be assigned, passed to a method, or inserted into an interpolated string:

int n = 7;
string parity = n % 2 == 0 ? "even" : "odd";
int abs = n >= 0 ? n : -n;
Console.WriteLine($"{n} – {parity}, absolute value {abs}");
Console.WriteLine($"Found {n} file{(n == 1 ? "" : "s")}");

Both expressions must have the same type or convert to a common type. Parenthesize the ternary operator inside an interpolated string because a colon otherwise introduces a format. The ternary operator is convenient for choosing between two simple values; nested ?: expressions are hard to read, so prefer if or a switch expression in those cases.