n n n in VB.NET

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must not be an existing Visual Basic keyword (normally called reserved keywords); must begin with an alphabetic character or an underscore (_); must contain only alphabetic characters, decimal digits, and underscores; must contain at least one alphabetic character or decimal digit it if begins with an underscore; must not be more than 16 383 characters long
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Some examples of suitable identi ers in programs are:
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CustomerName _date A24 Data_Connection_String
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While the following are invalid:
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Date 24A Database'Location _ (this is a reserved keyword) (begins with a digit) (contains a punctuation symbol) (underscore only not allowed)
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32 Variables
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How Visual Basic indicates a syntax error (here, an invalid variable name)
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Fortunately, Visual Basic will let you know if a name you apply to something does not meet these rules, as shown in Figure 32 In a Code window, once a line of code is completed (by pressing Enter), Visual Basic will mark any words or parts of the line that constitute an error by underlining with a blue wavy line (Figure 32) If you oat the mouse cursor (an I-bar) over the marked text, a pop-up hint will be displayed to describe the problem This will happen for any type of violation of the rules of the Visual Basic language (syntax errors) Since variables are used for storing values, and we must sometimes make direct assignments of values to variables in code, we will often have to enter these values directly in code VB requires us to ensure that these values are easy to tell from the other bits of text in program code identi ers, and other parts of statements Due to the rules for identi ers listed above, this turns out to be quite simple, as shown in Table 32
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Table 32 Rules for literal values in code
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Type Numeric (Byte, Short, Integer, Long, Single, Double) Char String Date
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Rule
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Simply state the value directly
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Example
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126 100000 25 X , (comma) What, me worry ABCDEFGHIJ 01/01/2003 25/12/2003 12:30:00
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State the value in double quotes State the value in double quotes Enclose in quotes to spell out the date as a string (year part must have all 4 digits) If omitted, time part will be 00:00:00
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3 n The Visual Basic NET Language
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Exercise 33
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Indicate which of the following are legal variable names, and state why the ones that are not are illegal: a) My Date Of Birth b) My-Date-Of-Birth c) My_Date_Of_Birth d) 2ndAttempt e) AttemptNo2 f) Attempt2 g) _SystemDirectory h) __SystemFolder i) NET_Programmer_ID j) #_of_Attempts
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324 Initializing Variables
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A variable without a value is useless Whenever you declare a simple variable in a VB program, it will have a null value assigned to it For the simple types such as numbers, dates, etc, this will be the equivalent of a zero value; number variables start with a value of 0 (or 00 for one of the types that supports fractional values), strings with a value of two double quotes side-by-side signi es the empty string, or no text), dates with a value of 01/01/0001 00:00:00 (a date value is entered in a program by enclosing it in double quotes, like a string, and incorporates a time part) and Booleans with a value of False More complex variable types are de ned as classes, and for these a null value means no object We will discuss the rami cations of this later These values are assigned automatically by VB (or, more correctly, by the Common Type System) to ensure that a new variable will at least take on some sensible and valid value However, it is often the case that when we declare a variable, the next thing we will do is assign a speci c value to it Creating a new variable for a speci c purpose is therefore a matter of declaring it (which gives it an identi er and assigns a speci c type) and then initializing it, which assigns a desired initial value This can be done using two lines of code:
Dim myName As String myName = "Alistair"
These individual statements can be one after the other as shown, or a lot of other program statements can come between the rst and the second Sometimes this can lead to unclear programming, where you have to search through the subsequent code to nd what value a variable has been initialized to Visual Basic allows us to perform this compound task in a single line, as follows
Dim myName As String = "Alistair"