If/Else and Ternary — Conditional Branching

Conditional branching — executing different code depending on whether a condition is true or false — is the foundation of all program logic. In C# the primary tools are the if/else statement and the ternary operator ? :. Every ASP.NET Core API controller method you write will contain conditional logic: check if the resource exists, check if the user is authorised, check if the input is valid. Writing these conditions correctly and readably is a skill you will use in every chapter of this series.

If / Else If / Else #

int score = 78;

if (score >= 90)
{
    Console.WriteLine("Grade: A");
}
else if (score >= 80)
{
    Console.WriteLine("Grade: B");
}
else if (score >= 70)
{
    Console.WriteLine("Grade: C");
}
else
{
    Console.WriteLine("Grade: F");
}
// Output: Grade: C

// Single-statement bodies — braces optional but recommended
if (score >= 60)
    Console.WriteLine("Pass");   // fine for very simple one-liners
else
    Console.WriteLine("Fail");

// Compound conditions
bool isLoggedIn   = true;
bool isAdmin      = false;

if (isLoggedIn && isAdmin)        // AND — both must be true
    Console.WriteLine("Admin panel");

if (!isLoggedIn || !isAdmin)       // OR — at least one must be true; ! = NOT
    Console.WriteLine("No access to admin panel");

Ternary Operator #

// Syntax: condition ? valueIfTrue : valueIfFalse
int age = 20;
string category = age >= 18 ? "Adult" : "Minor";
Console.WriteLine(category);   // "Adult"

// Ternary in string interpolation
bool isOnline = true;
Console.WriteLine($"Status: {(isOnline ? "Online" : "Offline")}");

// Nested ternary — use sparingly, prefer if/else for readability
int score2 = 85;
string grade = score2 >= 90 ? "A"
             : score2 >= 80 ? "B"
             : score2 >= 70 ? "C"
             : "F";
Console.WriteLine(grade);   // "B"

// Common use in ASP.NET Core — returning different status codes
// return isFound ? Ok(item) : NotFound();

Null-Coalescing and Null-Conditional in Conditions #

string? userName = null;

// Null-coalescing ?? — use a default when left side is null
string displayName = userName ?? "Anonymous";
Console.WriteLine(displayName);   // "Anonymous"

// Null-conditional ?. — short-circuits to null if left is null
string? upper = userName?.ToUpper();   // null (no NullReferenceException)
int?    len   = userName?.Length;      // null

// Combining with ??
string safe = userName?.Trim() ?? "Anonymous";

// Null-coalescing assignment ??= — only assign if currently null
userName ??= "Anonymous";
Console.WriteLine(userName);   // "Anonymous"

Common Mistakes #

Mistake 1 — Using = (assignment) instead of == (equality) #

❌ Wrong — silently assigns false to the variable:

bool isActive = true;
if (isActive = false)   // assigns false to isActive! always evaluates to false
    DoSomething();

✅ Correct:

if (isActive == false)   // or simply: if (!isActive)

Mistake 2 — Not short-circuiting null checks #

❌ Wrong — throws NullReferenceException when list is null:

if (list.Count > 0 && list != null)   // list.Count evaluated first — crash!

✅ Correct — null check must come first:

if (list != null && list.Count > 0)   // ✓ null checked first, Count only if not null

Quick Reference — Logical Operators #