.net 任何人都可以用 C# 中的签名浮点数来解释这种奇怪的行为吗

发布于03月24日

以下是带注释的示例:

class Program
{
    // first version of structure
    public struct D1
    {
        public double d;
        public int f;
    }

    // during some changes in code then we got D2 from D1
    // Field f type became double while it was int before
    public struct D2 
    {
        public double d;
        public double f;
    }

    static void Main(string[] args)
    {
        // Scenario with the first version
        D1 a = new D1();
        D1 b = new D1();
        a.f = b.f = 1;
        a.d = 0.0;
        b.d = -0.0;
        bool r1 = a.Equals(b); // gives true, all is ok

        // The same scenario with the new one
        D2 c = new D2();
        D2 d = new D2();
        c.f = d.f = 1;
        c.d = 0.0;
        d.d = -0.0;
        bool r2 = c.Equals(d); // false! this is not the expected result        
    }
}

那么，你怎么看？

推荐答案

错误位于下面两行System.ValueType行中:(我进入了参考源)

if (CanCompareBits(this)) 
    return FastEqualsCheck(thisObj, obj);

(两种方法都是[MethodImpl(MethodImplOptions.InternalCall)])

当所有字段都是8字节宽时，CanCompareBits错误地返回TRUE，从而导致对两个不同但语义相同的值进行逐位比较.

当至少有一个字段不是8字节宽时，CanCompareBits返回false，代码继续使用反射在字段上循环，并 for each 值调用Equals，这将正确地将-0.0视为等于0.0.

以下是来自SSCLI的CanCompareBits美元的来源:

FCIMPL1(FC_BOOL_RET, ValueTypeHelper::CanCompareBits, Object* obj)
{
    WRAPPER_CONTRACT;
    STATIC_CONTRACT_SO_TOLERANT;

    _ASSERTE(obj != NULL);
    MethodTable* mt = obj->GetMethodTable();
    FC_RETURN_BOOL(!mt->ContainsPointers() && !mt->IsNotTightlyPacked());
}
FCIMPLEND