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Why can't I cast boxed int to nullable decimal?

Answer 1

+12 A:

A boxed int can only be unboxed to an int. This, however, is legal:

object zero = 0;
decimal? dec = (decimal?)(int)zero;

See MSDN or the ECMA 334 C# spec for details. The key here is the following:

Unboxing is an explicit conversion from the type object to a value type or from an interface type to a value type that implements the interface. An unboxing operation consists of:

Checking the object instance to make sure that it is a boxed value of the given value type.
Copying the value from the instance into the value-type variable.

Edit: This linked article is worth pulling out of the comments. Thanks Rob Kennedy!

Jason 2009-07-09 13:37:30

Ok, but why such interim casting is necessary?

Przemaas 2009-07-09 13:39:58

The first cast (int) unboxes the boxed int to a value type of type int (this is a legal operation). The second cast (decimal?) casts the resulting int to decimal?.

Jason 2009-07-09 13:41:53

Microsoft (for good reason) never wrote a implicit cast overload for object to decimal.

Michael Meadows 2009-07-09 13:42:56

@Michael Meadows: This is legal: object zero = 0m; decimal? dec = (decimal)zero; Note the 'm' explicitly telling the compiler that the literal "0" is to be interpreted as a decimal.

Jason 2009-07-09 13:44:28

@Jason, a decimal is an object, so the first example is polymorphism. The second example is explicit casting. What I was saying was to answer @Przemaas' comment. You must explicitly cast the "zero" variable to int first, because you cannot implicitly cast an object to int, and since it's not an INullable<decimal>, the explicit cast directly to that type fails.

Michael Meadows 2009-07-09 13:48:28

@Michael Meadows: Either I misread your comment, or you deleted it and changed "explicit" to "implicit." I initially read your comment to state that there was no explicit cast from object to decimal. Apologies if I misread.

Jason 2009-07-09 13:50:10

Eric Lippert goes into quite some detail about why this works the way it does. The summary: Unboxing to anything but the original type would involve *a lot* of generated code for all the different possibilities since the compiler doesn't know what the original boxed type is. http://blogs.msdn.com/ericlippert/archive/2009/03/19/representation-and-identity.aspx

Rob Kennedy 2009-07-09 14:15:01

@Rob : I was looking for this post but wasn't able to find it... thanks ;)

Thomas Levesque 2009-07-09 14:25:11

Answer 2

A:

See this article http://msdn.microsoft.com/en-us/magazine/cc301569.aspx

Specifically "The common language runtime first ensures that the reference type variable is not null and that it refers to an object that is a boxed value of the desired value type. If either test fails, then an InvalidCastException exception is generated."

I think you are failing on the object of that value. I think the coversion to int works because that 0 literal will convert to an int and then an int converts to decimal.

If you do this it works

 decimal? test=0;
 object zero = test;
 decimal? dec = (decimal?)zero;

But I think the "0" in your snippet is not a "decimal" type.

I am still not positive cause this gets the same exception.

        int test=0;
     object zero = test;
     decimal? dec = (decimal?)zero;

Maestro1024 2009-07-09 14:03:23

My problem originally comes from below method:public TResult Convert(object value){try { return (TResult)value; }catch (InvalidCastException) { return default(TResult); }}This method returns null when called:Convert<decimal?>(0);I cann't make any assumptions about TResult and I don't know what exactly comes as an argument. That is why I cannot use Convert.To...() goodies.

Przemaas 2009-07-09 14:32:37

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Why can't I cast boxed int to nullable decimal?

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