Non-nullable reference type: why is my object considered nullable by the compiler?

Question

I\'ve a project on which I enabled the new Nullable reference type feature

 enable
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Answer 1

Check out the specification on nullable reference types. It states that var infers an annotated type for reference types.

The part under the heading nullable implicitly typed local variables reads:

var infers an annotated type for reference types. For instance, in var s = ""; the var is inferred as string?.

Answer 2

In the original implementation, foo would have been inferred as a Foo.

However, people complained that this got in the way of things like:

string? GetThing() => ...

var result = "";
if (condition)
{
    result = GetThing();
}

If result is inferred as a string, then the result = GetThing() line causes a warning: GetThing() returns a string?, and there's a warning if you try and assign a string? to a string.

The solution was to infer result as a string?, but the compiler knows that it's currently not null (its "flow state" is "NotNull").

This means that:

string? GetThing() => ...

var result = "";

// No warning, as the compiler knows that result isn't null
int l1 = result.Length; 

if (condition)
{
    result = GetThing();
}

// Warning: the compiler knows 'result' might have been re-assigned
int l2 = result.Length; 

---

For other examples of the flow state at work, see things like:

string? result = GetString();
if (result == null)
    throw new Exception();

// No warning: the compiler knows that result can't be null here: if it was,
// the exception above would have been thrown
int l1 = result.Length;

string? result = GetString();

// Warning: result might be null
int l1 = result.Length; 

// No warning: the compiler knows that result can't be null here: if it was,
// the line above would have thrown
int l2 = result.Length; 

string result = "hello";
if (result == null)
    Console.WriteLine("NULL!");

// Warning: because we checked for null above, the compiler assumes that we
// know something that it doesn't, and so result might be null.
int l1 = result.Length;