C# – Can anyone explain this strange behavior with signed floats in C#

cfloating pointnet

Here is the example with comments:

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        
    }
}

So, what do you think about this?

Best Answer

The bug is in the following two lines of System.ValueType: (I stepped into the reference source)

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

(Both methods are [MethodImpl(MethodImplOptions.InternalCall)])

When all of the fields are 8 bytes wide, CanCompareBits mistakenly returns true, resulting in a bitwise comparison of two different, but semantically identical, values.

When at least one field is not 8 bytes wide, CanCompareBits returns false, and the code proceeds to use reflection to loop over the fields and call Equals for each value, which correctly treats -0.0 as equal to 0.0.

Here is the source for CanCompareBits from SSCLI:

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

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For earlier versions of .NET,

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C# – How to detect if this dictionary key exists in C#

You can use ContainsKey:

if (dict.ContainsKey(key)) { ... }

or TryGetValue:

dict.TryGetValue(key, out value);

Update: according to a comment the actual class here is not an IDictionary but a PhysicalAddressDictionary, so the methods are Contains and TryGetValue but they work in the same way.

Example usage:

PhysicalAddressEntry entry;
PhysicalAddressKey key = c.PhysicalAddresses[PhysicalAddressKey.Home].Street;
if (c.PhysicalAddresses.TryGetValue(key, out entry))
{
    row["HomeStreet"] = entry;
}

Update 2: here is the working code (compiled by question asker)

PhysicalAddressEntry entry;
PhysicalAddressKey key = PhysicalAddressKey.Home;
if (c.PhysicalAddresses.TryGetValue(key, out entry))
{
    if (entry.Street != null)
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Best Answer

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