c# - 字典键存在时找不到

Question

为什么我的字典在我刚插入时说一个键不存在？与我的Equals 方法或双重比较有关吗？

代码如下:

// Test: I know the dictionary contains nCoord but its saying the key doesn't exist
Dictionary<UTMCoordinate, int> planes = new Dictionary<UTMCoordinate, int>();
UTMCoordinate nCoord    = new UTMCoordinate(337394.136407966, 6263820.40182064, 0, 56, UTMCoordinate.Hemisphere.H_SOUTHERN);
planes[nCoord]          = 1;

bool exists = planes.ContainsKey(nCoord);  // always returns false

我对 UTMCoordinate 的实现如下:

public class UTMCoordinate
{
    public enum                 Hemisphere {H_NOTHERN, H_SOUTHERN};
    public const double         DIF_TOLERANCE    = 0.0005;
    public double               x               { get; set; }
    public double               y               { get; set; }
    public double               elev            { get; set; }
    public uint                 UTMZone         { get; set; }
    public Hemisphere           hemisphere      { get; set; }

    public UTMCoordinate(double x, double y, double elev=double.MinValue, uint utmZone=uint.MinValue, Hemisphere hemisphere=Hemisphere.H_SOUTHERN) {
        this.x          = x;
        this.y          = y;
        this.elev       = elev;
        this.UTMZone    = utmZone;
        this.hemisphere = hemisphere;
    }

    public override int GetHashCode() {
        unchecked // Overflow is fine, just wrap
        {
            int hash = 17;
            // Suitable nullity checks etc, of course :)
            hash = hash * 23 + x.GetHashCode();
            hash = hash * 23 + y.GetHashCode();
            hash = hash * 23 + elev.GetHashCode();
            hash = hash * 23 + UTMZone.GetHashCode();
            hash = hash * 23 + hemisphere.GetHashCode();
            return hash;
        }
    }

    public override bool Equals(object obj)
    {
        UTMCoordinate other = obj as UTMCoordinate;
        if (other == null)
            return false;

        return double.Equals(x, other.x) && double.Equals(y, other.y) && double.Equals(elev, other.elev) && uint.Equals(UTMZone, other.UTMZone) && double.Equals(hemisphere, other.hemisphere);
    }
}

编辑 根据 Daniel A. White 的建议，我使用了不同的双重比较方法。不幸的是它仍然没有识别 key :

public override bool Equals(object obj)
{
    //return base.Equals (obj);
    UTMCoordinate other = obj as UTMCoordinate;
    if (other == null)
        return false;

    //return double.Equals(x, other.x) && double.Equals(y, other.y) && double.Equals(elev, other.elev) && uint.Equals(UTMZone, other.UTMZone) && double.Equals(hemisphere, other.hemisphere);
    return Math.Abs (x-other.x) <= DIF_TOLERANCE && Math.Abs (y-other.y) <= DIF_TOLERANCE && Math.Abs (elev-other.elev) <= DIF_TOLERANCE && uint.Equals(UTMZone, other.UTMZone) && hemisphere == other.hemisphere;
}

Answer 1

如果我从你的问题中提取你的代码:

Dictionary<UTMCoordinate, int> planes = new Dictionary<UTMCoordinate, int>();
UTMCoordinate nCoord    = new UTMCoordinate(337394.136407966, 6263820.40182064, 0, 56, UTMCoordinate.Hemisphere.H_SOUTHERN);
planes[nCoord]          = 1;

bool exists = planes.ContainsKey(nCoord);

我为 exists 得到的值是 true。

但是，如果我这样做:

nCoord.x = 1.0;
exists = planes.ContainsKey(nCoord);

exists 的值突然变为 false，即使该对象仍在字典中。这是因为 GetHashCode 的值已更改。它是 -1473667404，但在属性 x 赋值后它变成了 201352392。

字典使用 GetHashCode 的值来确定将键放入哪个桶，因此当哈希码更改时，字典可能会尝试在错误的桶中找到键，然后报告它没有t 包含 key 。

我怀疑您的代码中正在发生这种情况。

因此您需要更改您的对象，使其不可变。

public double               x               { get; private set; }
public double               y               { get; private set; }
public double               elev            { get; private set; }
public uint                 UTMZone         { get; private set; }
public Hemisphere           hemisphere      { get; private set; }

然后不要更改构造函数之外的任何值。