Why does this conversion doesn't work?

问题:

following code behaves strange (at least for me):

int testValue = 1234;

this.ConversionTest( testValue );

private void ConversionTest( object value )
{
    long val_1 = (long) (int) value; // works
    long val_2 = (long) value;       // InvalidCastException
}

I don't understand why the direct (explicit) cast to long doesn't work. Can someone explain this behaviour?

Thanks

回答1:

The value parameter of your ConversionTest method is typed as object; this means that any value types -- for example, int -- passed to the method will be boxed.

Boxed values can only be unboxed to exactly the same type:

• When you do (long)(int)value you're first unboxing value to an int (its original type) and then converting that int to a long.
• When you do (long)value you're attempting to unbox the boxed int to a long, which is illegal.