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Methods

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Methods define where behavior begins and what data may cross the boundary. The parameter list is part of that contract. By-value parameters isolate the caller’s variable, ref and out expose it for reassignment, and in exposes storage as readonly. params changes how an expanded argument list is collected at the call site.

Inheritance adds a separate question: which implementation runs? virtual and override use runtime dispatch. new hides a member and leaves selection to the compile-time type of the reference. Similar syntax, very different behavior.

Input Parameters

Ref

ref aliases the caller’s storage rather than copying its current value into a new parameter variable.

  • The caller must initialize the variable.
  • The method can read and write the variable.

For reference types, ref allows the callee to replace the caller’s reference rather than only mutate the referenced object:

class MyClass {}
 
static void ModifyReference(ref MyClass obj)
{
    obj = new MyClass();
}
 
var myObj = new MyClass();
ModifyReference(ref myObj);

For value types, ref allows the callee to update the caller’s variable:

static void InitializeAndModify(ref int value)
{
    value = 10;
}
 
int num = 0;
InitializeAndModify(ref num);

In

in is a readonly by-reference parameter. It can avoid copying a large struct, though the compiler may still create a temporary for argument expressions that are not variables.

  • The caller must initialize the argument.
  • The method can read, but cannot assign to the parameter.
  • It is most useful when measurements show that copying a large readonly struct matters.
static void ProcessData(in int value)
{
    // value = 10; // Compile-time error
    Console.WriteLine(value);
}

Out

out also aliases caller storage, but the value flows outward. The caller need not initialize it, and every normal return path must assign it. This supports Try* APIs that separate success from the produced value.

static bool TryDivide(int a, int b, out int result)
{
    if (b == 0) { result = 0; return false; }
    result = a / b;
    return true;
}
 
if (TryDivide(10, 2, out var quotient))   // 'quotient' declared inline
    Console.WriteLine(quotient);           // 5

Dictionary.TryGetValue shows the boundary well: a missing key is expected input, so a Boolean result is clearer than an exception.

Params

params lets callers provide zero or more arguments without constructing the parameter collection explicitly. C# 13 extended the feature beyond arrays to recognized collection types.

  • Must be the last parameter in the method signature.
  • Individual arguments or no arguments use expanded form, which constructs the declared params collection at the call site.
  • Normal form accepts one expression implicitly convertible to the declared parameter type. An array works only when that array-to-parameter conversion exists, including the applicable array-to-span conversion.
static int Sum(params int[] numbers)
{
    int total = 0;
    foreach (var n in numbers)
        total += n;
    return total;
}
 
Sum(1, 2, 3);   // 6
Sum();           // 0
Sum(new[] { 4, 5 }); // 9 — explicit array also works

C# 13 permits collection types such as Span<T>, ReadOnlySpan<T>, and types with a suitable collection-builder shape. Allocation behavior still depends on the declared parameter type and whether the call uses expanded or normal form.

static int Sum(params ReadOnlySpan<int> numbers)
{
    int total = 0;
    foreach (var n in numbers)
        total += n;
    return total;
}

Inheritance Method Keywords

Virtual

virtual marks a base implementation as an extension point. Calls through a base reference still select the most-derived override at runtime.

  • Enables runtime polymorphism.
  • Calls are resolved by the runtime type, not just the variable type.
class Animal
{
    public virtual string Speak() => "...";
}

Override

override supplies the derived implementation of a virtual or abstract member while keeping the same dispatch slot.

  • Signature must match the base member.
  • The derived method can still call the base implementation with base.MethodName().
class Dog : Animal
{
    public override string Speak() => "Woof";
}

New

new declares a separate member that hides one inherited with the same name. It does not join the base member’s dispatch slot.

  • Behavior depends on the compile-time type of the variable.
  • Use intentionally only when member hiding, rather than polymorphism, is the required behavior.
class Animal
{
    public string Category() => "Animal";
}
 
class Dog : Animal
{
    public new string Category() => "Dog";
}

Virtual Vs Override Vs New in One Example

class Animal
{
    public virtual string Speak() => "...";
    public string Category() => "Animal";
}
 
class Dog : Animal
{
    public override string Speak() => "Woof";
    public new string Category() => "Dog";
}
 
Animal asAnimal = new Dog();
Dog asDog = new Dog();
 
Console.WriteLine(asAnimal.Speak());    // Woof (runtime dispatch)
Console.WriteLine(asDog.Speak());       // Woof
Console.WriteLine(asAnimal.Category()); // Animal (member hiding)
Console.WriteLine(asDog.Category());    // Dog

The object is the same in both calls. Only the lookup rule changes: runtime type for Speak, compile-time reference type for Category.

Other Method Forms

  • ref return and ref readonly return return an alias to existing storage. The writable form lets a caller update that storage. The readonly form avoids a copy without granting mutation. Ref-safety rules prevent the alias from outliving its source.
  • Local functions name behavior inside another method. A static local function cannot capture outer state, and a direct call needs no delegate. A lambda is the natural form when the method must produce a Func or Action value.
  • Extension methods are static methods whose first parameter has the this modifier. They participate in normal static overload resolution and do not modify the extended type.
  • Expression-bodied members use shorter syntax for a single expression. Their call and dispatch semantics do not change.

Overload resolution compares the applicable candidates and their required conversions. Optional and named arguments affect applicability, while an unresolved tie is a compile error. Modern compilers can cache some static method-group conversions, but capturing lambdas and instance targets still need their own lifetime analysis.

Pitfalls

  • Optional defaults are copied into the caller at compile time. Changing a library’s default does not affect an already compiled consumer, so a default-value change can create version-dependent behavior.
  • An in parameter does not guarantee zero copies. Conversions and non-variable arguments can require temporaries, and readonly access to a mutable struct may trigger defensive copies.
  • Member hiding with new makes the result depend on the reference’s compile-time type. That split is hard to reason about in public APIs unless compatibility requires it.

Tradeoffs

DecisionOption AOption BWhen AWhen B
By-value vs inBy-value copies the valuein passes a readonly aliasSmall values and ordinary APIsLarge readonly structs on a measured hot path
in vs refin forbids reassignment through the parameterref permits itThe callee only readsMutation or rebinding is part of the contract
override vs newoverride uses runtime polymorphismnew uses compile-time hidingDerived behavior must work through base referencesA deliberate compatibility split requires a separate member
params T[] vs params ReadOnlySpan<T>Array form is broadly compatible and can be retained by the calleeSpan form is stack-only and cannot escapeThe method or caller needs array semanticsSynchronous processing benefits from span-based calls

Default to by-value parameters and ordinary virtual dispatch. Add by-reference semantics when aliasing is part of the API or profiling shows that struct copies matter. Use new only when compile-time hiding is the intended contract.

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