c# 4.0 - C# - Issues with boxing / unboxing / typecasting ints. I don't understand

Question

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?

Answer 1

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;