我嘗試覆蓋 != 運算符：public class Box{ public Box() { } public Box(double height, double width) { Height = height; Width = width; } public double Height { get; set; } public double Width { get; set; } public override int GetHashCode() { unchecked { return (Height.GetHashCode() * 397) ^ Width.GetHashCode(); } } public override bool Equals(object obj) { if (ReferenceEquals(null, obj)) return false; if (ReferenceEquals(this, obj)) return true; return obj.GetType() == GetType() && Equals((Box)obj); } protected bool Equals(Box other) { return Math.Abs(Height - other.Height) + Math.Abs(Width - other.Width) < 0.001; } public static bool operator ==(Box left, Box right) { if (ReferenceEquals(null, left)) return false; if (ReferenceEquals(null, right)) return false; return left.Equals(right); } public static bool operator !=(Box left, Box right) { var t = !(left == right); return t; }}public class BetterBox:Box{}并嘗試使用 != 運算符var box = new Box();var betterBox = box as BetterBox;if(betterBox!=null){ --do-something}在這種情況下 != returntrue并且代碼進入if. 這里有什么問題？為什么會發(fā)生？在 mdsn 我看到相同的代碼：https ://docs.microsoft.com/en-us/previous-versions/dotnet/netframework-4.0/336aedhh%28v%3dvs.100%29
2 回答
慕姐8265434
TA貢獻1813條經(jīng)驗 獲得超2個贊
這是Box使用相等檢查的完整實現(xiàn)（實現(xiàn)IEquatable<Box>）。以下是測試結果：
a b a==b a!=b a.Equals(b) b.Equals(a)
null null True False
[30×10] null False True False
[30×10] [30×10] True False True True
null [30×10] False True False
我已經(jīng)實現(xiàn)了Equals(Box), Equals(object), GetHashCode(), operator ==,operator !=和ToString().
public class Box : IEquatable<Box>
{
// Place values in constants
public const double SizeTolerance = 0.001;
public double Width { get; set; }
public double Height { get; set; }
public static bool operator ==(Box left, Box right)
{
if(!ReferenceEquals(left, null))
{
// consider that left.Equals(null) should return false
return left.Equals(right);
}
return ReferenceEquals(left, right);
}
public static bool operator !=(Box left, Box right)
{
return !(left==right);
}
#region IEquatable Members
/// <summary>
/// Equality overrides from <see cref="System.Object"/>
/// </summary>
/// <param name="obj">The object to compare this with</param>
/// <returns>False if object is a different type, otherwise it calls <code>Equals(Box)</code></returns>
public override bool Equals(object obj)
{
if(obj is Box other)
{
return Equals(other);
}
return false;
}
/// <summary>
/// Checks for equality among <see cref="Box"/> classes
/// </summary>
/// <param name="other">The other <see cref="Box"/> to compare it to</param>
/// <returns>True if equal</returns>
public bool Equals(Box other)
{
if(ReferenceEquals(other, null))
{
return false;
}
return Math.Abs(Width-other.Width)<SizeTolerance
&& Math.Abs(Height-other.Height)<SizeTolerance;
}
/// <summary>
/// Calculates the hash code for the <see cref="Box"/>
/// </summary>
/// <returns>The int hash value</returns>
public override int GetHashCode()
{
unchecked
{
int hc = 17;
hc = 23*hc + Width.GetHashCode();
hc = 23*hc + Height.GetHashCode();
return hc;
}
}
#endregion
public override string ToString()
{
return $"[{Width}×{Height}]";
}
}
以及測試它的代碼：
static void Main(string[] args)
{
Debug.WriteLine($"{"a",8} {"b",8} {"a==b",8} {"a!=b",8} {"a.Equals(b)",14} {"b.Equals(a)",14}");
Box a = null;
Box b = null;
Debug.WriteLine($"{a?.ToString()??"null",8} {b?.ToString()??"null",8} {a==b,8} {a!=b,8} {a?.Equals(b),14} {b?.Equals(a),14}");
a = new Box() { Height = 10, Width = 30 };
Debug.WriteLine($"{a?.ToString()??"null",8} {b?.ToString()??"null",8} {a==b,8} {a!=b,8} {a?.Equals(b),14} {b?.Equals(a),14}");
b = new Box() { Height = 10, Width = 30 };
Debug.WriteLine($"{a?.ToString()??"null",8} {b?.ToString()??"null",8} {a==b,8} {a!=b,8} {a?.Equals(b),14} {b?.Equals(a),14}");
a = null;
Debug.WriteLine($"{a?.ToString()??"null",8} {b?.ToString()??"null",8} {a==b,8} {a!=b,8} {a?.Equals(b),14} {b?.Equals(a),14}");
}
}
千巷貓影
TA貢獻1829條經(jīng)驗 獲得超7個贊
您對==運算符的實現(xiàn)是錯誤的。檢查空值時需要考慮兩個操作數(shù)。目前，如果left為 null，則返回 false，忽略right操作數(shù)的值。如果他們兩個都是null它應該返回true。
public static bool operator ==(Box left, Box right)
{
var isLeftNull = ReferenceEquals(null, left);
var isRightNull = ReferenceEquals(null, right);
if (isLeftNull && isRightNull)
{
return true;
}
if (isLeftNull || isRightNull)
{
return false;
}
return left.Equals(right);
}
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