[Mono-dev] InvalidCastException - which makes no sense

[Miguel de Icaza]

Hello,

What happens is this.   The value returned from ExecuteScalar is boxed.
This basically means that you have a wrapper that indicates the type of the
boxed object, as well as the contents of the boxed object.

The reality is that: (Int64) (scalar) and (Int32) (Int64) (scalar) perform
different operations on this context.

This is what actually happens on the second case:

tmp = Unbox-the-value-which-i-know-is-a-64-bit-value (scalar)
tmp32 = explicit-cast-64-to-32 tmp

They happen to have the same syntax, but they are different operations.

When you tried doing "(Int32) scalar", you were telling the compiler
"unbox-the-value-which-i-know-is-32-bit-value", but when the runtime went
to unbox this, it noticed it was 64, so it told you "InvalidCastException"




On Fri, Oct 24, 2014 at 5:23 PM, Edward Ned Harvey (mono) <
edward.harvey.mono at clevertrove.com> wrote:

>  This is a fun one.  I'd love it if anyone could explain this to me.
>
>
>
> using (var command = new SqliteCommand(@"SELECT COUNT(*) FROM someTable
> WHERE someColumn = @someValue ", dbConn))
>
> {
>
>     command.Parameters.Add(new SqliteParameter("someValue", "foobar"));
>
>     object scalar = command.ExecuteScalar();        // object returned has
> type Int64
>
>     Int64 count64 = (Int64)(scalar);                              // works
> fine.  Value is 0
>
>     Int32 count32 = (Int32)(Int64)(scalar);                // works fine.
> Value is 0
>
>     Int32 count = (Int32)(scalar);                   // throws
> InvalidCastException
>
>     ...
>
> }
>
>
>
> In the above, scalar is obviously an object, but the object returned by
> ExecuteScalar() is of type Int64.  The value is 0.
>
>
>
> This works fine:
>
> Int64 count64 = (Int64)(scalar);
>
>
>
> This works fine:
>
> Int32 count32 = (Int32)(Int64)(scalar);
>
>
>
> This throws InvalidCastException:
>
> Int32 count = (Int32)(scalar);
>
>
>
> The best I can gather, the object needs to be cast to Int64 before it can
> be cast to Int32.  Which makes no sense to me, but that's the way it is.
>
> _______________________________________________
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> Mono-devel-list at lists.ximian.com
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>
>
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