6: Data and Data Types in Visual Basic .NET

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6: Data and Data Types
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For an example that is more in tune with Visual Basic, take that number 76 we were discussing earlier. It s easy enough to convert it to binary representation, as in 01001100. The .NET Framework includes a few data types that do this conversion automatically, varying only by the number of binary digits (bits) they can handle. In the computer world, 76 also represents a letter of the alphabet the capital letter L. That s because there s a data type that establishes a dictionary between binary values and alphabetic (and other) characters. Windows programs have long used ASCII (American Standard Code for Information Interchange) as its number-to-character dictionary. This 8-bit system documents how to convert the numbers 0 through 255 into all the various characters used in English, including punctuation and other miscellaneous characters. Another dictionary, Unicode, uses 16 bits of data to handle around 65,000 different characters. .NET uses Unicode for its character and string data types. Another rule-bearing data type is Boolean, which uses a single bit to represent either True (a bit value of 1) or False (0). Negative integers, floating-point and fixed-point decimal values, and dates and times round out the kinds of basic data most often managed by computers and their applications. More complex data structures can be built up from these basic types.
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All data types in .NET are implemented as classes within the System namespace. One such data type is System.Byte, which implements an 8-bit integer value, just like we discussed earlier. It holds integer values from 0 to 255. These values are always stored using 8 bits of binary data, but they magically appear in decimal form whenever you ask them to be presented. The .NET Framework includes 15 core interpretive data types: 8 for integers, 3 for decimal numbers, 2 for character data, a combined data type for dates and times, and a Boolean data type.
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Integer Data Types
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Based on the number of available data types (8 out of the 15 core types), you would think that most programmers worked with integers all day long and you d be right. Whether it s actual user data or loop counters or status codes or the storage method for enumerated data types, integers show up everywhere in .NET code. The range of values for an integer data type depends directly on the number of binary digits managed by that data type; the more digits, the bigger the range. Also, half of the integer data types store both positive and negative values (called signed integers), whereas the other half support only positive numbers ( unsigned ). Table 6-1 lists the eight integer data types included with .NET, and their associated ranges.
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Table 6-1. Integer data types in .NET .NET data type
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System.Byte System.SByte System.Int16 System.UInt16 System.Int32 System.UInt32 System.Int64 System.UInt64
Bits 8 8 16 16 32 32 64 64
Style Unsigned Signed Signed Unsigned Signed Unsigned Signed Unsigned
Range of values 0 to 255 128 to 127 32,768 to 32,767 0 to 65,535 2,147,483,648 to 2,147,483,647 0 to 4,294,967,295 9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 0 to 18,446,744,073,709,551,615
Looking at these types another way, Table 6-2 shows the relationship between the types and their number of bits and range style.
Table 6-2. Bits and signed status for integer .NET data types 8-bits Signed Unsigned
SByte Byte
16-bits
Int16 UInt16
32-bits
Int32 UInt32
64-bits
Int64 UInt64
Decimal Data Types
Once upon a time, life was happy. Strangers said hello when they met you on the street. Succulent fruit burst forth from the trees. In short, God was in His heaven, and everything was right with the world and then along came fractions. At first, they
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6: Data and Data Types
didn t seem that bad, since so many of them could be easily converted into a plain numeric form by inserting a decimal point in the number: 1/2 became 0.5; 1/4 became the longer yet smaller 0.25; 1/3 became 0.333333333333333333333333333333333333 3333333333333333333333333333333333333333333333333333333333333333333 3333333333333333333333333333333333333333333333333333333333333333333... hey, what s going on here I can t write all those 3s. The book would be 2,000 pages, or more. Eventually people discovered that in many cases, it just wasn t worth the bother of writing out all the 3s, so they just stopped at some point, as in 0.33333333. It wasn t perfectly accurate, but it was good enough. This is what life is like for computer-based decimal values. You can have perfect accuracy up to a point. After that, you have to settle for good enough. The .NET Framework includes three decimal data types. Two of them accept limited accuracy in exchange for a large range of values. The third has perfect accuracy, but its range is more limited. Table 6-3 documents these three types.
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