C＃中的接收器中的有效返回值变为null

Question

I'm just getting into C#, so forgive me if this is a basic question. I'm building a WPF application using .NET 4.0 and VS 2010.

I have a class in which I'm storing a List of objects; the objects have a Point object as a field. One of the method returns the object in list at a specific location:

public Marker markerAtLocation(Point location) {
    foreach (Marker nextMarker in Markers)
        if (nextMarker.Location().Equals(location))
            return nextMarker;
    return null;
}

Setting breakpoints and using Console.WriteLine I've confirmed that nextMarker is valid at the return. However, in the receiver the object is always null :

Marker top = markerAtLocation(new Point(location.X, location.Y + 1));
if (top == null)
    Console.WriteLine("Top is null");
else
    Console.WriteLine("Top is " + top.ToString());
Marker bottom = markerAtLocation(new Point(location.X, location.Y - 1));
if ((bottom != null) && (bottom.player == otherPlayerType()) && (top != null) && (top.player == otherPlayerType()))
    return true;

I don't know what's going wrong here…

Note that I'd initially thought it was an issue with .NET's Point struct using double values. I know in my app the location values will always be integers so I didn't use the .NET Point and created my own:

class Point {
    public int X, Y;

    public Point(int X, int Y) {
        this.X = X;
        this.Y = Y;
    }

    public bool Equals(Point anotherPoint) {
        return ((anotherPoint.X == X) && (anotherPoint.Y == Y));
    }
}

I'd appreciate any help!

Edit: In response to pbirkoff:

class Grid {
    public List<Marker> Markers;

    public Grid() {
        Markers = new List<Marker>();
    }

    public Grid(List<Marker> markers) {
        this.Markers = markers;
    }

    public void addMarker(Marker newMarker) {
        Markers.Add(newMarker);
    }

I've tried the solutions below to the best of my ability but none of them have helped. I'm including a link to the project itself as well as the problem I'm attempting to solve.

Project: http://dl.dropbox.com/u/7828009/ACSLPetteia.zip
Problem: http://wcipeg.com/etc/ACSL/VOL%2030/ACSL%20Petteia_sr_3.doc

Thanks in advance!

Answer 1

I implemented your code as succinctly as I could, making some mocks where appropriate (code below), and I cannot get it to fail. What am I missing in my code sample:

class Program
{
    static void Main(string[] args)
    {
        LoadMarkers();
        var location = new Point(45, 45);
        Marker top = markerAtLocation(new Point(location.X, location.Y + 1));
        if (top == null)
            Console.WriteLine("Top is null");
        else
            Console.WriteLine("Top is " + top.ToString());
        Marker bottom = markerAtLocation(new Point(location.X, location.Y - 1));

    }
    public static List<Marker> Markers = new List<Marker>();
    private static void LoadMarkers()
    {
        for (var q = 0; q < 50; q++)
            for (var w = 0; w < 50; w++)
                Markers.Add(new Marker(q, w));
    }
    public static Marker markerAtLocation(Point location)
    {
        foreach (Marker nextMarker in Markers)
            if (nextMarker.Location().Equals(location))
                return nextMarker;
        return null;
    }
}
class Marker
{
    private Point _loc;
    public Marker(int x, int y) { _loc = new Point(x, y); }
    public Point Location() { return _loc; }
}
class Point
{
    public int X, Y;
    public Point(int X, int Y)
    {
        this.X = X;
        this.Y = Y;
    }
    public bool Equals(Point anotherPoint)
    {
        return ((anotherPoint.X == X) && (anotherPoint.Y == Y));
    }
}

Answer 2

Could be related to curly braces (much more readable when you use them IMHO), though that's probably not very likely. What if you declare a local variable to the function as a holder and return the stored value of that variable? Does this solve the problem or does it still exhibit the same behavior? For example:

public Marker markerAtLocation(Point location) {
    Marker returnMarker = null;
    foreach (Marker nextMarker in Markers) {
       if (nextMarker.Location().Equals(location)) {
           returnMarker = nextMarker;
           break;
       } // end if
    } // end foreach
    return returnMarker;
} // end function

Answer 3

I'd first start by changing the names of your classes. Namespaces would normally take care of any conflicts, but the WPF point, being defined in System.Windows is most likely going to cause issues when trying to create your Point in a WPF Window class, which I'm guessing is where your receiver code is located. The reason I say this is that by default you get a "using System.Windows;" line at the top of the auto-generated Window classes VS creates.

Since everything derives from object, you'll want to override the bool Equals(object o) function as well, since this will help you catch the issue where it's comparing against a System.Windows.Point instead of your Point.

I think if you change the name of your class to IntPoint, you might find that the "new Point" in your receive code might not get refactored to "new IntPoint" since it could be resolving to System.Windows.Point.

Same goes for using Grid as a name. I try to stick to the convention of not using type names that match .NET type names to avoid odd conflicts.

So to sum up, change your class to the following:

class IntPoint {
    public int X { get; set; }
    public int Y { get; set; }

    public IntPoint(int X, int Y) {
        this.X = X;
        this.Y = Y;
    }

    public override bool Equals(object obj)
    {
        if (obj is IntPoint) return Equals(obj as IntPoint);
        return base.Equals(obj);
    }

    public bool Equals(IntPoint anotherPoint) {
        return ((anotherPoint.X == X) && (anotherPoint.Y == Y));
    }
}

I also changed your public fields to public properties using the shorthand default get/set syntax. I won't go into the details of properties vs fields, but it'd be good for you to search the web for .net properties vs public fields. Part of the reasoning has to do with how Binding works. .Net allows you to bind to properties, but if you tried to bind to one of the fields, I believe it will fail to find them.

Post back if you make these changes and it's still not working.

Answer 4

Are you like 100% positive that markerAtLocation is returning a non null Marker. In debug you see that line called?? No you don't see it call that line because the way you have it structured debug does not have a line to stop on to see it "return nextMarker(). You have 3 levels of logic going on here with no {}. Are you sure your code is running like you expect?

    foreach (Marker nextMarker in Markers)
    if (nextMarker.Location().Equals(location))
        return nextMarker;
        return null;

What I suspect is happening the loop only goes to the second line. On the last and only the last it proceeds to return nextMarker and it is true only if the match was on the last line. Clean up your logic - it cannot hurt.

    foreach (Marker nextMarker in Markers)
    {
        if (nextMarker.Location().Equals(location))
        {
            return nextMarker;
        }
    }
    return null;

Answer 5

I think that we'd need more information (as Jon requested) to ascertain exactly what is wrong here.

Perhaps you're using a static function and refering to non-static items such as your public class members. This would still compile and run although the non-static variables wouldn't have been initialised or set before they are used.

Always best to implement public class variables as properties eg public List<Marker> Markers{get;set;} instead of public List<Marker> Markers . The same would go for X and Y in your Point class.

My gut feeling is that some of your class instances are no longer in scope or because they are public variables it is possible that they have been overwritten - try defining them volatile - by the time that you come to refer to them.