주석: ?, !, MaybeNull과 NotNull

?와 !를 넘어서: Nullability 특성

MaybeNull과 NotNull

using System.Diagnostics.CodeAnalysis;

// [MaybeNull]: return might be null even though T is non-nullable
[return: MaybeNull]
public T Find<T>(int id)
{
    // Returns default(T) which is null for reference types
    return _cache.TryGetValue(id, out var val) ? val : default!;
}

// [NotNull]: after this call, output is guaranteed non-null
public static void EnsureNotNull<T>(
    [NotNull] ref T? value,
    T defaultValue) where T : class
{
    value ??= defaultValue;
}

NotNullWhen: 조건부 null 비허용

using System.Diagnostics.CodeAnalysis;

public static bool TryParseEmail(
    string? input,
    [NotNullWhen(true)] out string? email)
{
    if (input?.Contains('@') == true)
    {
        email = input.Trim().ToLower();
        return true;
    }
    email = null;
    return false;
}

// Usage — no warning after true check:
if (TryParseEmail(raw, out var email))
    Console.WriteLine(email.Length); // safe — email is non-null here

MaybeNullWhen: 조건부 null 허용

// This is how Dictionary<K,V>.TryGetValue is annotated:
public bool TryGetValue(
    TKey key,
    [MaybeNullWhen(false)] out TValue value)
{ ... }

// Usage:
if (!dict.TryGetValue("key", out var value))
    return; // early return — value is null in this branch

Console.WriteLine(value.Length); // safe after the guard

AllowNull과 DisallowNull

public class Config
{
    private string _name = "";

    // Allow setting null (setter converts null -> empty)
    [AllowNull]
    public string Name
    {
        get => _name;
        set => _name = value ?? "";
    }

    // Getter always returns non-null: fine
    // Setter accepts null: [AllowNull] tells compiler that's OK
}

NotNullIfNotNull: null 허용성 전파

[return: NotNullIfNotNull(nameof(value))]
public static string? Normalize(string? value)
{
    return value?.Trim().ToLower();
}

// Usage:
string  name = "  Alice  ";
string  norm1 = Normalize(name)!; // guaranteed non-null
string? raw   = GetRaw();
string? norm2 = Normalize(raw);   // still nullable (raw might be null)

DoesNotReturn

using System.Diagnostics.CodeAnalysis;

[DoesNotReturn]
public static void ThrowNotFound(int id)
    => throw new KeyNotFoundException($"Entity {id} not found");

// Usage — no null warning after the call:
var order = _db.Orders.Find(id);
if (order is null) ThrowNotFound(id);

Console.WriteLine(order.Id); // no CS8602 — compiler knows ThrowNotFound threw

MemberNotNull: 필드에 대한 사후 조건

public class DataLoader
{
    private string? _data;

    [MemberNotNull(nameof(_data))]
    private void EnsureLoaded()
    {
        if (_data is null)
            _data = LoadFromFile();
    }

    public string GetData()
    {
        EnsureLoaded();
        return _data; // no CS8603 — compiler knows _data is set
    }
}

주석 조합하기

public static class Guard
{
    [return: NotNull]
    public static T NotNull<T>(
        [NotNull][AllowNull] T? value,
        [CallerArgumentExpression(nameof(value))] string? name = null)
        where T : class
    {
        ArgumentNullException.ThrowIfNull(value, name);
        return value;
    }
}

// Usage:
var product = Guard.NotNull(await _repo.FindAsync(id));
Console.WriteLine(product.Name); // no warning

null 연산자: ?. ?? ??=

string? name = GetName();

// ?. safe navigation
int? len = name?.Length;
string? upper = name?.ToUpper().Trim();

// ?? default value
string display = name ?? "Anonymous";

// ??= lazy init
name ??= "Default";

// Chaining
string result = user?.Profile?.DisplayName ?? user?.Name ?? "Guest";

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복습: Nullability 주석