C#•12mo ago

getting warning: CS8618 - Non-nullable variable must contain a non-null value when exiting construct

I guess, it's not a big deal since the code is getting compiled but I still want to understand why I'm getting this warning and how can I resolve this. I was using a generic struct

truct Rectangle<T> 
{
    T length;
    T width;

    public T Length{
        readonly get{ return length; }
        set{ length = value; }
    }
    public T Width{
        readonly get{ return width; }
        set{ width = value; }
    }

    public Rectangle(T L, T W) // <-- this is where I am getting the warning
    {
        Length = L;
        Width = W;
    }
}

truct Rectangle<T> 
{
    T length;
    T width;

    public T Length{
        readonly get{ return length; }
        set{ length = value; }
    }
    public T Width{
        readonly get{ return width; }
        set{ width = value; }
    }

    public Rectangle(T L, T W) // <-- this is where I am getting the warning
    {
        Length = L;
        Width = W;
    }
}

29 Replies

ero•12mo ago

With the code you've provided, what's stopping me from creating a Rectangle<Rectangle<List<Rectangle<StringBuilder>>>>?

Jibz•12mo ago

sanity?

ero•12mo ago

So nothing, basically That's the issue you're having I can pass any nullable class as T

Thinker•12mo ago

The actual error occurs because the compiler can't tell that Length and Width aren't null if their corresponding fields aren't null.

ero•12mo ago

This is obviously a glaring issue, and is if I do pass a nullable type, then it's your responsibility to make sure length and width are not null when the constructor exits

Thinker•12mo ago

What you can do is either ignore the warning because it doesn't really matter in this case, or add [NotNullWhenNotNull(nameof(length))] to Length, and the same for Width.

Jibz•12mo ago

so what can I do to stop you?

ero•12mo ago

This is because you explicitly specified those two to be non-null

Thinker•12mo ago

Maybe constraint T to INumber<T> if Ero thinks that's more sane

ero•12mo ago

Yup, exactly that

Jibz•12mo ago

INumber?

ero•12mo ago

You may also consider using auto-properies for Length and Width You may also consider renaming Length to Height

Thinker•12mo ago

Basically so that T can only be a number, because using string for length and height doesn't really make sense.

ero•12mo ago

public readonly record struct Rectangle<TUnit>(
  TUnit Width,
  TUnit Height) where TUnit : INumber<TUnit>;

public readonly record struct Rectangle<TUnit>(
  TUnit Width,
  TUnit Height) where TUnit : INumber<TUnit>;

Should get you where you want

Thinker•12mo ago

struct Rectangle<T> where T : INumber<T>
{
    public T Length { get; set; }
    public T Width { get; set; }

    public Rectangle(T length, T width)
    {
        Length = length;
        Width = width;
    }
}

struct Rectangle<T> where T : INumber<T>
{
    public T Length { get; set; }
    public T Width { get; set; }

    public Rectangle(T length, T width)
    {
        Length = length;
        Width = width;
    }
}

(if you prefer the full type over a record) Although the record will also give you equality methods, which might be something you want

Jibz•12mo ago

The type or namespace name 'INumber<>' could not be found

ero•12mo ago

Is anyone surprised Can you tell us what type of project you're making?

Thinker•12mo ago

INumber is in System.Numerics so you'll have to import that

ero•12mo ago

What tutorial are you following? Do you know what .net version your project is using?

Jibz•12mo ago

Yeah dotnet 7

ero•12mo ago

Ah, that's good then Importing System.Numerics should do it

Jibz•12mo ago

well.. this is frustrating. I'm using INumber as struct Rectangle <T> where T : INumber<T> but the warning doesn't go away 😓

ero•12mo ago

Try where T : unmanaged, INumber<T> Might need to turn them around I wonder if it's possible to just constrain it to notnull instead of unmanaged Though I think unmanaged is a better fit

Thinker•12mo ago

yeah that has nothing to do with the INumber<T> thing at all again this is the actual issue

Jibz•12mo ago

right, so this error can't be removed?

Thinker•12mo ago

If you were using auto-properties then it would go away That's the easiest way

Jibz•12mo ago

nvm. Yeah, its gone! thanks both of you!

Thinker•12mo ago

Accord•12mo 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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getting warning: CS8618 - Non-nullable variable must contain a non-null value when exiting construct