Generic arrays and casting to an object array

This question was posted recently on the CodeProject forums. The OP wanted to know why this code would not compile (I’ve simplified it):

private void FooInternal(object[] values)
{
}

private void Foo<T>(T[] values)
{
    FooInternal(values);
}

The error thrown is:

Error	2	Argument 1: cannot convert from 'T[]' to 'object[]'

The fix is to add a constraint:

private void Foo<T>(T[] values) where T:class
{
    FooInternal(values);
}

If you are wondering why that is needed, consider this code snippet:

struct S
{
}

static void Main(string[] args)
{
    object[] arr = new S[10];
}

That won’t compile either. The reason’s that each of those value type members need to be boxed into an object to get an object array, meaning it’s now a different array. Since that is not standard behavior for casts, the compiler will not automatically do that for you (which is a good thing). If you did it manually, this is what it’d look like:

object[] arr = new S[10].Cast<object>().ToArray();

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