[Mono-dev] InvalidCastException - which makes no sense

[David Nelson]

Eric Lippert has a good write up about this behavior:

http://blogs.msdn.com/b/ericlippert/archive/2009/03/19/representation-and-identity.aspx

> On Oct 24, 2014, at 5:28 PM, Miguel de Icaza <miguel at xamarin.com> wrote:
> 
> 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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> 
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