Proposal: Nullable var

[wojciechsura]

One of the most useful additions to C# language is var keyword, which simplifies declaration of variables, as long as compiler is able to deduce the type automatically.

I propose adding an option to quickly and automatically declare nullable type based on non-nullable one. For instance:

Stack<(IReadOnlyList<BasePlanItem> folder, int index)> stack = new();
stack.Push(plan, -1);

var? current = stack.Peek();
// Instead of:
// (IReadOnlyList<BasePlanItem> folder, int index)? current = stack.Peek();

This is a small quality-of-life improvement, which may simplify lifes of developers, especially after introducing nullable reference types.

[theunrepentantgeek]

Why would you need current to be a wider type than the return type of stack.Peek()?

Either stack.Peek() already returns (IReadOnlyList<BasePlanItem> folder, int index) (in which case your variable can never be null) or it already returns ((IReadOnlyList<BasePlanItem> folder, int index)? and you don't need var?.

(And, yes, I know that nullable reference types don't exist at runtime - but attributes on the implementation of Peek() have the same effect of declaring whether the return type is nullable or not.)

[HaloFour]

@theunrepentantgeek

In this case the return value is a value type (since it's a tuple), but your point stands, it's impossible for current to be null so why would one want to treat it as a nullable-value-type?

[svick]

I think this would only be useful in cases where:

1. The type is not a reference type. For those, var is already nullable, with the null state being tracked, so that in the common case where null doesn't get involved later on, it behaves as if it the variable was not nullable.
2. You initially assign a non-nullable value, but want to assign null or a nullable value to the same variable afterwards.
3. The type name is sufficiently long.

As such, to me this seems like it wouldn't be useful that often. And in cases where it would be useful, you could use an extension method instead:

public static T? AsNullable<T>(this T value) where T : struct 
    => value;

var current = stack.Peek().AsNullable();