C# - Issues with boxing / unboxing / typecasting ints. I don't understand

boxing, c#, c#-4.0, casting, unboxing

Solution

Unboxing checks the exact type as explained in the documentation.

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.

As you can see the first step is to check that the object instance matches the target type.

Also quote from the documentation:

For the unboxing of value types to succeed at run time, the item being unboxed must be a reference to an object that was previously created by boxing an instance of that value type. Attempting to unbox null causes a NullReferenceException. Attempting to unbox a reference to an incompatible value type causes an InvalidCastException.

So to fix this error make sure that the type matches before attempting to unbox:

object thirdTest = Convert.ToInt16(0);
short thirdtest2 = (short)thirdTest;

Problem

I'm having a hard time understanding this. Consider the following example: ``` protected void Page_Load(object sender, EventArgs e) { // No surprise that this works Int16 firstTest = Convert.ToInt16(0); int firstTest2 = (int)firstTest; // This also works object secondTest = 0; int secondTest2 = (int)secondTest; // But this fails! object thirdTest = Convert.ToInt16(0); int thirdtest2 = (int)thirdTest; // It blows up on this line. } ``` The specific error that I get at runtime is `Specified cast is not valid.` If I QuickWatch `(int)thirdTest` in Visual Studio, I get a value of `Cannot unbox 'thirdTest' as a 'int'`. What the heck is going on here?

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