Multiple Font Style Combinations in vb.net

Question

If I want to create a font with multiple style combinations, like bold AND underline, I have to place the 'or' statement between it, like in the example below:

  lblArt.Font = New Font("Tahoma", 18, FontStyle.Bold Or FontStyle.Underline)

If you place bold 'and' underline, it won't work, and you only get 1 of the 2 (like how the or statement should be working), while that would be the logically way to do it. What is the reason behind this?

Answer 1

Boolean logic works a bit differently than the way we use the terms in English. What's happening here is that the enumerated FontStyle values are actually bit flags, and in order to manipulate bit flags, you use bitwise operations.

To combine two bit flags, you OR them together. An OR operation combines the two values. So imagine that FontStyle.Bold was 2 and FontStyle.Underline was 4. When you OR them together, you get 6—you've combined them together. In Boolean logic, you can think of an OR operation as returning "true" (i.e., setting that bit in the result) if either of the bits in the two operands are set, and "false" if neither of the bits in the two operands are set.

You can write a truth table for such an operation as follows:

| A | B | A OR B |
|---|---|--------|
| 0 | 0 |   0    |
| 0 | 1 |   1    |
| 1 | 0 |   1    |
| 1 | 1 |   1    |

Notice that the results more closely mirror what we, in informal English, would call "and". If either one has it set, then the result has it set, too.

In contrast to OR, a bitwise AND operation only returns "true" (i.e., sets that bit in the result) if both of the bits in the two operands are set. Otherwise, the result is "false". Again, a truth table can be written:

| A | B | A AND B |
|---|---|---------|
| 0 | 0 |    0    |
| 0 | 1 |    0    |
| 1 | 0 |    0    |
| 1 | 1 |    1    |

Assuming again that FontStyle.Bold has the value 2 and FontStyle.Underline has the value 4, if you AND them together, you get 0. This is because the values effectively cancel each other out. The net result is that you don't get any font styles—precisely why it doesn't work when you write FontStyle.Bold And FontStyle.Underline.

In VB, a bitwise OR operation is performed using the Or operator. The And operator performs a bitwise AND operation. So in order to do a bitwise inclusion of values, which is how you combine bit flags, you use the Or operator.