Question

Why does this work:

ComboBox cb = sender as ComboBox;
int validSelection = Convert.ToInt32(cb.Tag);
if (cb.SelectedIndex != validSelection) {
    cb.SelectedIndex = validSelection;
}

…but this:

ComboBox cb = sender as ComboBox;
int validSelection = (int)cb.Tag; // <-- fails
if (cb.SelectedIndex != validSelection) {
    cb.SelectedIndex = validSelection;
}

…errors out with “Object not set to a reference of an object”?

Answer 1

That’s because a conversion is not the same thing as a cast.

cb.Tag is probably a string, not an int, and one is not directly convertible to the other. Convert.ToInt32() actually parses the string and creates a new int with the converted value.

Casts only try to interpret an instance of a type as an instance of another type.