Resolved 'Use of possibly unassigned field' when passing struct to method.

[benjm-ma]

Hey Guys,

Here is the code I am trying to compile:

using System;

class Exercise {

    static void Main()
    {
        Point p;
        p.x = 40.0;
        p.y = 45.0;

        if (NormalizePoint(out p)) {
            Console.WriteLine ($"x normalized to {p.x}\ny normalized to {p.y}");
        } else {
            Console.WriteLine ($"Points cannot be normalized");
        }
    }

    public struct Point
    {
        public double x;
        public double y;

        public Point (double _x, double _y)
        {
            x = _x;
            y = _y;
        }
    }

    public static bool NormalizePoint (out Point p)
    {
        if (p.x < 1d || p.y < 1d) return false;
        if (p.x > 100d || p.y > 100d) return false;

        p = new Point (p.x / 100d, p.y / 100d);
        return true;
    }
}

When attempting to compile using mcs (Mono C# compiler version 6.8.0.123) I get the following errors:

struct.cs(32,10): error CS0170: Use of possibly unassigned field `x' struct.cs(32,22): error CS0170: Use of possibly unassigned field `y' struct.cs(30,48): error CS0177: The out parameter `p' must be assigned to before control leaves the current method

I've omitted some of the above errors as it repeats.

To eliminate the last compilation error there I extended the body of the if statements within NormalizePoint to assign p to itself as the struct's values should not change.
This gets a warning from the compiler so I know I'm going about this the wrong way. What would be the better way to achieve what I'm trying to do here in regards to
using an out variable within a function which may not modify the variable under certain conditions?

For the first two methods, I understand what the compiler is telling me, I just don't see how it applies to the code (maybe I don't understand). I don't really know what to do
to resolve this error as I am under the impression that the struct and it's fields are initialized within Main before it is passed to NormalizePoint.

Any help and suggestions would be greatly appreciated.

Thank you

 

[jmcilhinney]

The out keyword is for parameters used ONLY to pass data out of a method. I'm not 100% sure but I think that out parameters may even be implicitly set to the default value for their type. If you want to use a parameter to pass data in and out then you use ref rather than out.

 

[NoUserHere]

Hello @benjm-ma, welcome to the forums.

For some revised reading and edification, please read : C# - Using in, out, and Ref with Parameters | Pluralsight

 

[benjm-ma]

Thank you for the link @Sheepings ! I will give that a read over.

 

[NoUserHere]

It so nice to see a young dev using structs, as they are so undervalued and under used in functional programming. However... I don't have time to dig into this :
if (p.x < 1d || p.y < 1d) => You can't do that. Your struct has no default value. And Out is not correct in that context.
Adding p = default; before the if statement is an improvement, but also not correct in the way of what you're doing. Your struct has no default value.
Instead, notice how changing your method to : public static bool NormalizePoint (ref Point p) removes the errors?
You have quite a bit of refactoring to do.

 

[benjm-ma]

@Sheepings reading through the article you posted lead me to the solution you provided in your second post there. Now I understand why the two compiler errors were directly related to how the struct was being passed into the method.

Thank you

 

[NoUserHere]

You're are most welcome. If you need any other help, you know where we are. Always happy to help.