Non-nullable reference type default parameters in C# 8

Question

I was working to upgrade some code to C# 8.0 and came across an interesting pattern with default parameters and non-nullable reference types. I'm working if it's clear enough to the caller.

Given this pre C# 8.0 code,

Task FooAsync(
   Bar bar = null,
   CancellationToken ct = default)
{
    bar = bar ?? new Bar(); // some sensible default
}

the bar parameter is flagged as one to potentially make nullable (Bar? bar = null). This would indicate to the caller that they can pass null.

However, if I want to encourage the caller not to pass null for the parameter, I could do something like this in C# 8.0:

Task FooAsync(
   Bar bar = null!,
   CancellationToken ct = default)
{
    bar ?? = new Bar(); // some sensible default
}

Is this a sensible approach to avoid passing null into the parameter? I know the caller can still do: FooAsync(bar: null!) but that obviously sticks out.

Answer 1

My take on this is that the function signature should not lie. So if it can handle a null passed in, as it can, it should use

Bar? bar = null

Another approach could be to overload the function with another which only takes a single parameter. Something like ...

Task FooAsync(
   Bar bar,
   CancellationToken ct = default)
{
    ...
}

Task FooAsync(
   CancellationToken ct = default)
{
    return FooAsync(new Bar(), ct);
}