How to get InvalidCastException from Array.ConstrainedCopy

Question

Here is the sample code for the discussion (consider Reptile "is a" Animal and Mammal "is a" Animal too)

Animal[] reptiles = new Reptile[] 
    { new Reptile("lizard"), new Reptile("snake") };

Animal[] animals = new Animal[]
    { new Reptile("alligator"), new Mammal("dolphin") };

try
{
  Array.ConstrainedCopy(animals, 0, reptiles, 0, 2);
}
catch (ArrayTypeMismatchException atme)
{
  Console.WriteLine('[' + String.Join<Animal>(", ", reptiles) + ']');
}

When I run this code I get a ArrayTypeMismatchException, with as comment

Array.ConstrainedCopy will only work on array types that are provably compatible, without any form of boxing, unboxing, widening, or casting of each array element. Change the array types (i.e., copy a Derived[] to a Base[]), or use a mitigation strategy in the CER for Array.Copy's less powerful reliability contract, such as cloning the array or throwing away the potentially corrupt destination array.

However when I look at MSDN I see this method also throws an InvalidCastException. The condition for throwing an InvalidCastException is:

At least one element in sourceArray cannot be cast to the type of destinationArray.

So I am stumped, how do you get an InvalidCastException out of this method, if as it states there can never be any casting of an array element?

That is an excellent question. Sadly the method is ultimately implemented in the CLR it looks like so its not possible to use something like reflector to answer this question. Hopefully somebody who knows this method will see this and answer... — Chris
– Chris, Commented Sep 18, 2013 at 7:20
My suspicion is that the underlying method that gets called has a parameter called reliable which I assume relates to the guarantees made. I suspect the underlying method can throw the InvalidCastException but not when the reliable parameter is true as in this case. reliable is false when using the basic copy command (so the same underlying method but with the boolean different). The basic copy will throw the InvalidCastException as you'd expect if you used that instead. — Chris
– Chris, Commented Sep 18, 2013 at 7:30
Is Animal an interface where Mammal and Reptile is implemented from or is Animal a base class that extends both Mammal and Reptile? — JNYRanger
– JNYRanger, Commented Sep 26, 2013 at 1:29
@JNYRanger hi, in this particular example Mammal extends Animal directly and Reptile extends Animal directly. But please don't focus on this detail, at this point ANY information leading to the capture, dead or alive, of an InvalidCastException from from this method will be appreciated. — Mishax
– Mishax, Commented Sep 26, 2013 at 7:25

Michael Liu · Accepted Answer · 2013-09-26 16:04:01Z

Without access to the actual native implementation of Array.Copy, the best we can probably do is examine the Shared Source CLI. Here are the relevant lines of code from clr\src\vm\comsystem.cpp:

FCIMPL6(void, SystemNative::ArrayCopy, ArrayBase* m_pSrc, INT32 m_iSrcIndex, ArrayBase* m_pDst, INT32 m_iDstIndex, INT32 m_iLength, CLR_BOOL reliable)
{
    // ...

    r = CanAssignArrayTypeNoGC(gc.pSrc, gc.pDst);

    if (r == AssignWrongType) {
        // [Throw ArrayTypeMismatchException]
    }

    if (r == AssignWillWork) {
        // [Copy the array using memmove, which won't throw any exception]
        return;
    }
    else if (reliable) {
        // [Throw ArrayTypeMismatchException]
    }

    // [Handle other cases]
}

When Array.ConstrainedCopy calls SystemNative::ArrayCopy with the reliable parameter set to TRUE, either the array is copied using memmove or ArrayTypeMismatchException is thrown. In neither case will InvalidCastException be thrown.

user541686 · Accepted Answer · 2013-09-20 08:21:56Z

3

Honestly, I think it's just a copy-paste typo; they just forgot to remove it from the list of exceptions.

answered Sep 20, 2013 at 8:21

user541686

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Alexander Simonov · Accepted Answer · 2013-09-24 20:54:18Z

3

From MSDN (Remarks section):

The sourceArray type must be the same as or derived from the destinationArray type; otherwise, an ArrayTypeMismatchException is thrown.

In your example the animals array type is not the same as or derived from the reptiles array type (an Animal is not a Reptile). That's why an ArrayTypeMismatchExcetion is thrown.

Based on the above conditions and the exception message from your example it can be concluded that there is no way to get an InvalidCastException when calling Array.ConstrainedCopy method. It's a bug in documentation.

answered Sep 24, 2013 at 20:54

Alexander Simonov

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Vitaliy · Accepted Answer · 2013-09-26 14:03:40Z

3

Actual type of first array in heap is Reptile[].

Animal[] reptiles = new Reptile[] { ... };
//IL_0002:  newarr     Reptile

Second array:

Animal[] animals = new Animal[] { ... };
// IL_0025:  newarr     Animal

There is no cast from Reptile[] to Animal[]. So it's correct behavior of the Array.ConstrainedCopy() method.

This code will work correctly:

    Animal[] reptiles = new Animal[] { new Reptile("lizard"), new Reptile("snake") };

    Animal[] animals = new Animal[] { new Reptile("alligator"), new Mammal("dolphin") };

answered Sep 26, 2013 at 14:03

Vitaliy

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Mohsen Heydari · Accepted Answer · 2013-09-27 14:13:35Z

ConstrainedCopy has the same implementation as Array.Copy except for a ReliabilityContractAttribute.
If we open ConstrainedCopy in IL Disassembler, we see that all it does is load its arguments onto the stack and pass them to Array.Copy.
As the exception states, Array.ConstrainedCopy throws exceptions in certain cases when Array.Copy does not. ConstrainedCopy validates arrays before it copies them.

enter image description here

Example: The regular Array.Copy method will silently copy a byte array to an int array. The Array.ConstrainedCopy method instead throws an exception. This can improve reliability.

class Program
{
    static void Main()
    {
    byte[] original = new byte[10];
    original[0] = 1;

    int[] destination = new int[10];

    // This will work if you call Array.Copy instead.
    Array.ConstrainedCopy(original, 0, destination, 0, original.Length);
    }
}

BTW:
Array.ConstrainedCopy does not allow the widening conversion to take place.

Conclusion:
The Array.ConstrainedCopy method does not allow certain copies. It is more discriminating than Array.Copy. It also throws exceptions. Often, ConstrainedCopy is not necessary.

About the the question:
System.InvalidCastException: Is one of the exceptions that is throw-able by calling Array.Copy, since Array.ConstrainedCopy is calling Array.Copy, It is correct to document the spcification of Array.ConstrainedCopy throwing System.InvalidCastException but due to priority of validation we will never see InvalidCastExceptionbut.

BartoszKP · Accepted Answer · 2013-09-22 19:07:52Z

2

The following code throws InvalidCastException on Mono 2.10.2.0 (whereas the code from question doesn't). The explanation from the documentation doesn't fit this case though.

Animal[] reptiles = new Reptile[] 
    { new Reptile("lizard"), new Reptile("snake") };

object[] animals = new object[]
    { new Reptile("alligator"), new Mammal("dolphin") };

try
{
    Array.ConstrainedCopy(animals, 0, reptiles, 0, 2);
}
catch (ArrayTypeMismatchException atme)
{
    //Console.WriteLine('[' + String.Join<Animal>(", ", reptiles) + ']');
}

edited Sep 22, 2013 at 19:07

answered Sep 22, 2013 at 18:53

BartoszKP

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2 Comments

user743382 Over a year ago

Honestly, this seems like a bug in Mono. The whole point of Array.ConstrainedCopy is that either the entire array is copied, or nothing is copied. Looking at the arrays after Array.ConstrainedCopy throws an exception shows that the copying of the first element is not reverted. Interesting example though.

BartoszKP Over a year ago

I agree, this behaviour doesn't make any sense really :)

Acrotygma · Accepted Answer · 2013-09-25 14:07:23Z

I took a sneak peek into the Array class where I saw that Array.ContrainedCopy() just calls Array.Copy() inside without any type validation where as the CLR method Array.Copy() actually can throw a InvalidCastException:

(From MSDN Array.Copy

When copying from a reference-type or value-type array to an Object array, an Object is created to hold each value or reference and then copied. When copying from an Object array to a reference-type or value-type array and the assignment is not possible, an InvalidCastException is thrown.

I assume Array.Copy() just elevates the InvalidCastException to Array.ConstrainedCopy(), so it technically is possible that Array.ConstrainedCopy() throws a InvalidCastException altough it shouldn't per definition.

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