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Painting Quick Response Code in Visual Basic .NET True

In certain cases, you can treat numbers as Boolean values. I ll talk about it more later on, but for now just know that False equates to zero (0), and True equates to everything else (although generally, 1 is used for everything else ).

Literal data values are all well and good, but they are useful only once, and then they re gone. Each time you want to use a literal value, you must retype it. It s as though the data values are stored in disposable cups instead of fine china teacups. And besides, only programmers enter literal values, not users, so they are of limited use in managing user data.

Variables are not simply disposable cups; they are reusable. You can keep putting the same type of tea over and over into the teacup. A string variable teacup can hold a string for reuse over and over. For instance, in this block of code, response holds the various strings assigned to it:

01 02 03 04 05 06 07 08 response = "A" MsgBox("Give me an 'A'!") MsgBox(response) MsgBox("Give me another 'A'!") MsgBox(response) MsgBox("What's that spell ") response = StrDup(2, "A") MsgBox(response)

The variable response is assigned twice with two different strings: an A (line 01) and then AA (line 07). It keeps whatever value was last assigned to it; both lines 03 and 05 display A in a message box window. And you don t have to assign just literal strings to it; anything that generates a string can assign its result to response. Line 07 uses a built-in Visual Basic function, StrDup, to return the two-character string AA and assign it to response. Using variables is a two-step process. First you must declare the variable, and then you assign a value to it. The Dim statement takes care of the declaration part; it lets you indicate both the name and the type of a variable. Its basic syntax is pretty straightforward:

where response is the name of the variable and String is its type. Assignment occurs using the = assignment operator:

response = "The answer"

A single variable can have new values assigned to it over and over again. For those times when you want your variable to have some specific value immediately upon declaration, you can combine declaration and assignment into a single statement:

Dim response As String = "The answer"

Of course, you re not limited to just a single declaration; you can create as many variables as you need in your code. Each one normally uses its own Dim statement:

You also can combine these into a single statement, although I think it s just plain ugly:

See, I told you it was ugly. This is just the start of what s possible with the Dim statement. I ll get into more details as the chapter progresses.

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I talked about value types and reference types in 1. Value type variables store an actual value; the tea in a value type teacup is the content itself. All of the literal data values I mentioned previously, except for Strings, are value types. Reference type variables store a reference to the actual data, data found somewhere else in memory. When you look into a reference type teacup, you have to read the tea leaves at the bottom to determine where the real data resides. Either reference types have data or they don t. In the absence of data, a reference type has a value of Nothing, a Visual Basic keyword that indicates no data. Value types are never Nothing; they always contain some value, possibly the default value for that type (such as zero for numeric types). A special nullable type does let you assign Nothing to a value type, allowing you to implement the same is there any data here at all logic that exists with reference types. I ll talk about nullable types in 6.