8000 NullReferenceException when checking if NDArray is null · Issue #290 · SciSharp/NumSharp · GitHub
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Plankton555 opened this issue Jun 25, 2019 · 8 comments
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NullReferenceException when checking if NDArray is null #290

Plankton555 opened this issue Jun 25, 2019 · 8 comments
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@Plankton555
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Summary
It doesn't seem to be possible to use if (myArray == null) to check whether myArray is null, if it has been initialized to NDArray myArray = null;. This is because the == operator has been overridden without any additional checks.

Minimal example of bug

static void Main(string[] args)
{
    NDArray myArray = null;

    if (myArray == null) // NullReferenceException is thrown here
    {
        Debug.WriteLine("myArray is null");
    }
}

Expected behaviour
myArray == null should return true, not throw a NullReferenceException.
@henon
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henon commented Jun 25, 2019 •
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I recently had similar problems with my equality operator implementations in Numpy.NET and Torch.NET. Here is a very good solution for comparison: You need to use object.ReferenceEquals to check against null.

        public static bool operator ==(NDArray a, object b)
        {
            if (ReferenceEquals(a, b))
                return true;
            if (ReferenceEquals(a, null))
                return false;
            if (ReferenceEquals(b, null))
                return false;
            return a.Equals(b);
        }

        public static bool operator !=(NDArray a, object b)
        {
            if (ReferenceEquals(a, b))
                return false;
            if (ReferenceEquals(a, null))
                return true;
            if (ReferenceEquals(b, null))
                return true;
            return !a.Equals(b);
        }

The comparison logic for NDArrays that are not the same reference should reside in the Equals method

@Nucs
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Nucs commented Jun 25, 2019 •
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A known issue,
Use object.Equals(myArray, null) to perform such checks.

Edit: or @henon 's suggestion which works well too.

@Nucs Nucs added the bug Something isn't working label Jun 25, 2019
@Oceania2018
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@Plankton555 Try to use if (myArray is null).

@Plankton555
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@Oceania2018 Will try that.

My current work-around is to initialize to NDArray myArray = new NDArray(typeof(float[])); and check for if (measurementMemory.len == 0), but I want to change that to a less ugly solution asap :P

@Plankton555
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Yes, if (myArray is null) works as expected. Thanks for the work-around, @Oceania2018!

@Plankton555
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If it is a known issue (to quote Nucs), shouldn't it still be fixed? (regarding closing this ticket)

@Nucs
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Nucs commented Jun 26, 2019

@Plankton555, It is a known issue and its fix is complicated that'll require breaking changes. This is why it has been delayed.
It happens (as far as I recall) because of the implicit casting mechansim in NDArray.

I'll keep it open till we either document the proper use (myArray is null) or fix it.

@Nucs Nucs reopened this Jun 26, 2019
@Nucs Nucs mentioned this issue Jun 28, 2019
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Nucs added a commit that referenced this issue Aug 6, 2019
@Nucs
Unit test for #290
cfce6df
@Nucs
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Nucs commented Aug 6, 2019 •
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I've added a unit-test and calling (ndarray==null) works as expected.

@Nucs Nucs closed this as completed Aug 6, 2019
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@henon @Nucs @Plankton555 @Oceania2018
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