Aktivera och förstå NRT

<!-- In .csproj — enable for the whole project -->
<PropertyGroup>
  <Nullable>enable</Nullable>
</PropertyGroup>

// Or per-file:
#nullable enable
// ... code with NRT warnings
#nullable disable
// ... code without NRT warnings

#nullable enable

string nonNull = "Hello";    // OK
string? maybeNull = null;    // OK — explicitly nullable

nonNull = null;              // WARNING: CS8600
Console.WriteLine(maybeNull.Length); // WARNING: CS8602 — may be null

// Fix:
if (maybeNull is not null)
    Console.WriteLine(maybeNull.Length); // safe

string? name = GetName();

// After null check, name is treated as non-null:
if (name is not null)
    Console.WriteLine(name.Length);  // no warning

// Same with early return:
if (name is null) return;
Console.WriteLine(name.Length);     // no warning — null was returned

// Pattern matching:
if (name is string s)
    Console.WriteLine(s.ToUpper()); // no warning

#nullable enable

public class Order
{
    public string CustomerName { get; set; } // CS8618: not initialized
    public string? TrackingNumber { get; set; } // OK — nullable

    public Order(string name)
    {
        CustomerName = name; // now initialized — CS8618 gone
    }
}

string? s = GetValue();
Console.WriteLine(s.Length); // CS8602: s might be null

// Option 1: Initialize in declaration
public string Name { get; set; } = "";

// Option 2: Require via constructor
public class Product
{
    public string Name { get; }
    public Product(string name) => Name = name;
}

// Option 3: null-forgiving for ORM entities
public string Name { get; set; } = null!;
// null! tells compiler "trust me, this will be set by EF Core"

// Without constraint: T could be nullable
public T? Find<T>(int id) { ... }

// With notnull: T must be a non-nullable type
public T FindRequired<T>(int id) where T : notnull
{
    var result = InternalFind<T>(id);
    return result ?? throw new KeyNotFoundException();
}

// T? is meaningful only when T is known to be non-nullable
public T? FindOrDefault<T>(int id) where T : class { ... }

public interface IRepository<T>
{
    T? FindById(int id);       // may return null
    T GetOrThrow(int id);      // never null
}

// Implementation:
public class ProductRepo : IRepository<Product>
{
    public Product? FindById(int id)  => _db.Find(id);
    public Product  GetOrThrow(int id) =>
        _db.Find(id) ?? throw new KeyNotFoundException();
}

// Nullable value type (runtime Nullable<int>)
int? age = null;
bool hasValue = age.HasValue;
int value  = age.GetValueOrDefault();

// Nullable reference type (compile-time annotation only)
string? name = null;
// string? has NO runtime wrapper — it's just a compile-time hint
// null check is still needed at runtime
if (name is not null)
    Console.WriteLine(name.ToUpper());

// Use ! when you know the value cannot be null
var user = _db.Users.FirstOrDefault(u => u.Email == email);
var name = user!.Name; // user is guaranteed by business logic

// EF Core navigation properties set by the framework:
public class Order
{
    public Customer Customer { get; set; } = null!; // set by EF Core
}

#nullable enable

public class WeatherResponse
{
    public string City { get; set; } = "";     // always present
    public double Temperature { get; set; }     // always present
    public string? Description { get; set; }    // optional field
    public string? AlertMessage { get; set; }   // only when there's an alert
}

// Consumer:
var weather = await api.GetWeatherAsync("London");
Console.WriteLine(weather.City);            // safe
Console.WriteLine(weather.Description?.ToUpper() ?? "N/A"); // safe null-handling