C#•14mo ago

❔ Optional function parameter

I have this function public void abc(Entity entity, List<string> directions, Position occupied = null but I get

CS1750: A type of value <null> cannot be used as a default parameter because there are no standard conversions to type Position

17 Replies

Thinker•14mo ago

Is Position a struct? In which case you could do Position? occupied = null

CaedenOP•14mo ago

Yes

Thinker•14mo ago

yeah structs can't be null unless you have a ? at the end which indicates that it's nullable

CaedenOP•14mo ago

Cannot convert Position? to Position

Thinker•14mo ago

Show your actual code

CaedenOP•14mo ago

The error above came from Position? x = null; as a test Crap it works if both types are ? Thabks

MODiX•14mo ago

Thinker

REPL Result: Success

record struct Position(int X, int Y);

Position? x = null;

record struct Position(int X, int Y);

Position? x = null;

Compile: 523.168ms | Execution: 74.427ms | React with ❌ to remove this embed.

Thinker•14mo ago

works fine

CaedenOP•14mo ago

I have the parameter Position? which works I have an if statement to check if it is null, and in the else I assume its not null and use the properties. C# raises error CS1061: Position? does not contain a definition for …

Thinker•14mo ago

yeah you have to check that the value isn't null and then you can access .Value which is the actual value

MODiX•14mo ago

Thinker

REPL Result: Success

record struct Position(int X, int Y);

Position? x = new(1, 2);

if (x is not null)
{
    Console.WriteLine($"X is {x.Value.X}");
}

record struct Position(int X, int Y);

Position? x = new(1, 2);

if (x is not null)
{
    Console.WriteLine($"X is {x.Value.X}");
}

Console Output

X is 1

X is 1

Compile: 642.851ms | Execution: 86.451ms | React with ❌ to remove this embed.

CaedenOP•14mo ago

It works but how come? What does the ? do?

Thinker•14mo ago

Makes the type nullable Structs cannot be null by themselves, so there's a special type called Nullable<T> which wraps around other structs and makes them able to be null. Nullable<T> looks a bit like this:

public readonly struct Nullable<T>
{
    public bool HasValue { get; }
    
    public T Value { get; }
}

public readonly struct Nullable<T>
{
    public bool HasValue { get; }
    
    public T Value { get; }
}

It has two properties, HasValue which indicates whether the value is null or not, and Value which is the actual wrapped value in case it's not null. Position? is just syntactic sugar for Nullable<Position> So if you want a struct which can be null then you put a ? at the end of it. Now you can access the value using .Value, however you first need to make sure that the value actually exists, otherwise you'll get an exception.

CaedenOP•14mo ago

Ah very nice Would you wrap a class in nullable

Thinker•14mo ago

No because reference types can already be null ? for reference types (classes, interfaces, etc.) is just a compiler annotation which marks them as possibly null

CaedenOP•14mo ago

Okay, thank you awesome explanation above

Accord•14mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Optional function parameter