C# 6.0 null-safe member access/method/indexer call use cases

gistfile1.cs

nullness inspection
{
  a?.M();
  a.F(); // <= possible NRE
  a?.M(a != null); // <= expression always true
}

inspection "Redundant null-propagation"
{
  var a = new A();
  a?.M( // <= redundant ? warning
    a?.P); // <= redundant ? warning
}

inspection "Merge sequential null-checks"
{
  if (a != null && a.P != null && a.P.U != null)
  if (a != null && a.P.U != null)
  if (a != null && a.P.M(x) != null) // invocation at end
  if (_ && a?.P != null && a.P?.U != null)
  if (a?.P != null && a.P.U != null)
  if (a == null || a.P == null)
  if (s.HasValue && s.Value.P != null)
  if (d != null && d(x) != null)

  // =>

  if (a?.P?.U != null)
  if (a?.P.U != null) // short-circuiting
  if (a?.P.M(x) != null)
  if (_ && a?.P?.U != null)
  if (a?.P?.U != null)
  if (a?.P == null)
  if (s?.P != null) // value-type
  if (d?.Invoke(x) != null) // delegate 
}


inspection "Null-propagating method call(s)"
{
  var t = x.Y;
  if (t != null) { t.M(); }
  if (t == null) { } else { t.P.M(); }
  if (t != null) {
    var p = t.P as X;
    p.M();
    p?.U();
    if (p != null) { G(); }
  }
  if (t is X) { ((X) t).M(); }

  // =>

  var t = x.Y;
  t?.M();
  t?.P.M();          // short-circuiting
  var p = t?.P as X; // lifted variable + as-cast
  p?.M();            // lifted method call
  p?.U();
  if (p != null) { G(); } // lifted if
  (t as X)?.M();
}


inspection "Null-propagating method return"
{
  if (t == null) { return null; } else { return t.M(x).P; }

  // =>

  return t?.M(x).P;
}


inspection "Replace control instructions with null-propagation"
{
  for (;;) {
    if (a == null) continue;
    var b = a.B;
    if (b == null) continue;
    var d = b.C.D;
    d.M();
    // continue
  }

  // =>

  for (;;) {
    var b = a?.B
    var d = b?.C.D; // short-circuiting
    d?.M();
  }
}


inspection "Reduce nested null-checks with null-propagation"
{
  var a = GetA();
  if (a != null) {
    var b = a.B;
    if (b != null) {
      var d = b?.C.D;
      var e = d.E;
      if (e != null) e.M();
    }
  }

  // =>

  var a = GetA();
  var b = a?.B;
  var d = b?.C.D; // short-circuiting
  var e = d?.E; // lifted variable
  e?.M();
}


inspection "Replace conditional operator with null-propagation"
{
  var b = (a == null) ? null : a.B;
  var c = (a != null) ? a.B.C : null;
  var d = (a != null ? a.B.C : null).D();
  var e = s.HasValue ? s.Value.P : null;

  // =>
  
  var b = a?.B;
  var c = a?.B.C; // short-circuiting
  var d = (a?.B.C).D(); // parentheses required
  var e = s?.P; // value type
}


inspection "Replace if statement with null-propagation"
{
  if (p != null) { t = p.U.T; } else { t = null; }
  T t = null; if (p != null) { t = p.T; }
  
  // =>

  t = p?.U.T;
  T t = null; t = p?.T;
}


inspection "Replace conditional operator with null-propagation with ?? operator"
{
  // NOTE: .C is of ref/non-nullable type
  var c = (a == null) ? 42 : a.B.C;
  T c; if (a == null) { c = 42; } else { c = a.B.C; }

  // =>

  var c = a?.B.C ?? 42;
}


inspection "Replace null-checked access with null-propagation"
{
  if (t != null) t = t.T;

  // =>
  
  t = t?.T;
}


inspection "Merge sequential null-checks (using lifted nullable operators)"
{
  if (t != null && t.M())
  if (t != null && t.P == 42)

  // =>

  if (t?.M() == true)
  if (t?.P == 42)
}


// inside LINQ expressions
{
  xs.Where(x => x != null).Select(x => x.B.C).Where(x => x != null)
  xs.Where(x => x.A != null).Select(x => x.A.B.C).Where(x => x != null)
  from x in xs where x.A != null
               where x.A.B != null select x.A.B

  // =>

  xs.Select(x => x?.B.C).Where(x => x != null)
  xs.Select(x => x.A?.B.C).Where(x => x != null)
  from x in xs where x.A?.B != null select x.A.B
}


// user's .NotNull(x => x.Y)-like methods:
{
  T t = a.With(_ => _.B);
  // U With<T,U>(Func<T, U>) where T: class, where U: class;
  //   note: false positives. Think about names like 'NotNull'/'NotNull'/'IfNotNull'/'Try'
  // U? With<T,U>(Func<T?,U>) where T: struct, where U: struct;
  // U? With<T,U>(Func<T?,U?>) where T: struct, where U: struct;
  // U With<T,U>(Func<T,U>) where T: class;
  // + note: 'struct => class' and 'class => struct' overloads
  // + node: overloads with default values
  // + note: parameter '_' may not be used at all
  
  // =>
  
  T t = a?.B;
}


// the same, more complex example
{
  D d = a.NotNull(x => x.B.C).NotNull(x => x.D);

  // =>

  D d = a?.B.C?.D; // short-circuiting
}


// delegate invoke
{
  D d = o as D;
  d(); // possible NRE

  // =>

  D d = o as D;
  d?.Invoke();
}

Maybe: detect expression that will behave the same after transformation to nullable type

if (x is T && ((T) x).Value == 42)
// =>
if ((x as T)?.Value == 42) // lifted ==

Offer transformation via context action, not QF.

Maybe: ability to lift whole declarations (only as context action, not suggestion)

if (person != null) {
   var something = person.GetSomething(arg, 42);
   var component = something["argument"].Component;
   component.Foo(something);
}

=>

var something = person?.GetSomething(arg, 42);
var component = something?["argument"].Component;
component?.Foo(something /* only when T is not value type */);

TODO: reverse transformation!

var function = functionDeclaration.DeclaredElement;
if (function == null) return null;

return function.ReturnType;

=>

var function = functionDeclaration.DeclaredElement;
return function?.ReturnType;