C# – Issues with boxing / unboxing / typecasting ints. I don’t understand

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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;

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