Nullability analysis with the field keyword.md

Nullability analysis with the field keyword

Properties that use the field keyword will not have warnings such as

For properties that have a get accessor with a body and which use field, analyze using this pseudocode, providing the same warnings as we would with accessors as local functions.

void Prop()
{
   T? field = default;

   // If there is an initializer:
   field = ...init-expr...

   // If there is a constructor assignment:
   set(...non-null-value...);

   while (true)
   {
      if (...non-constant-value...)
          get()
      else
          // If there truly is a setter on the property.  Otherwise, this is elided.
          set(...non-null-value...);
   }

   return;

   T get() { ...get-body... }
   void set(T value) { ...set-body... }
}

public C()
{
   T? field = default;

   // If there is an initializer:
   field = ...init-expr...

   ...ctor-statements, replacing property accessor calls with get/set local function calls...

   while (true)
   {
      if (...non-constant-value...)
          get()
      else
          // If there truly is a setter on the property.  Otherwise, this is elided.
          set(...non-null-value...);
   }

   return;

   T get() { ...get-body... }
   void set(T value) { ...set-body... }
}

Example:

class C : ViewModel
{
    // No warnings
    public C()
    {
        AutoProp = "";
        AutoGetterProp = "";
    }

    // Squiggle on 'C': Non-nullable property AutoProp must contain a non-null value when exiting the constructor.
    public C(int p) { }

    // No warnings
    public string AutoProp { get; set; }
    // Squiggle on 'get': Possible null reference return when the `C(int p)` constructor is used
    public string AutoGetterProp { get; set => Set(ref field, value); }
    // Squiggle on 'field': Possible null reference return when the `C(int p)` constructor is used
    public string ManualFieldProp { get => field; set => Set(ref field, value); }
    // No warnings
    public string InitializedProp { get => field; set => Set(ref field, value); } = "";
    // No warnings
    public required string RequiredProp { get => field; set => Set(ref field, value); }
}

abstract class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;
    protected bool Set<T>(ref T field, T value, [CallerMemberName] string? propertyName = null);
}

Scenarios to consider

public string IntermediateStateProp { get => field; set => _myOldField = value; }

public string ConditionalProp { get => field; set { if (cond) field = value; } }