Question

Not much to say,
This is the issue:

    public static T WriteIfNotNull<T>(ManagementObject retObject, string parameter)
    {
        return retObject[parameter] != null ? (T)retObject[parameter] : default(T);
    }

It breaks when T is an int and retObject[parameter] is an object. The odd part (for me at least) is that if T is not T and is actually a specified int it does work.

It says it can’t be casted to T (even that in this case T is int).

@Edit: @dtb here it goes:

Specified cast is not valid. 
(T)(object)retObject[parameter] Cannot unbox 'retObject[parameter]' as a 'T'    int         

And the values:

parameter: "ProcessId"

retObject[parameter]: 4

default(T): 0

@@Edit: Here’s the call too

pr.ProcessId = Util.WriteIfNotNull<int>(retObject, "ProcessId");

Answer 1

It seems to me that what’s happening is that retObject[parameter] isn’t actually an int but is another numeric type (uint for process IDs typically), in which case you’d want to use Convert to actually change the value:

return retObject[parameter] != null 
    ? (T)Convert.ChangeType(retObject[parameter], typeof(T)) 
    : default(T);

The reason a simple cast doesn’t work here is because casting from object to int only works if it’s actually a boxed int. You can’t do a downcast and conversion cast in one operation because there is not a conversion cast from object to int. Thus casting away from object requires you to cast to the exact type or a valid super-type, interface, etc of the exact type.

So, to convert a boxed uint to an int we’d have to either unbox as an int first (problematic) or use one of the Convert methods to change the type (as shown above).

Convert.ChangeType() works for converting between compatible types even if they are boxed. For example, if retObject[parameter] returns a long then this will allow it to be converted to a T of int.

I have a blog post on this here with more detail, hopefully this makes sense. The gist, again, is that casting from object is a downcast (no conversion casts are defined for object) thus you must cast it to it’s precise type (or a valid sub-type).