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Decimal Data Type (Visual Basic)

  • บทความ

Holds signed 128-bit (16-byte) values representing 96-bit (12-byte) integer numbers scaled by a variable power of 10. The scaling factor specifies the number of digits to the right of the decimal point; it ranges from 0 through 28. With a scale of 0 (no decimal places), the largest possible value is +/-79,228,162,514,264,337,593,543,950,335 (+/-7.9228162514264337593543950335E+28). With 28 decimal places, the largest value is +/-7.9228162514264337593543950335, and the smallest nonzero value is +/-0.0000000000000000000000000001 (+/-1E-28).

Remarks

The Decimal data type provides the greatest number of significant digits for a number. It supports up to 29 significant digits and can represent values in excess of 7.9228 x 10^28. It is particularly suitable for calculations, such as financial, that require a large number of digits but cannot tolerate rounding errors.

The default value of Decimal is 0.

Programming Tips

  • Precision. Decimal is not a floating-point data type. The Decimal structure holds a binary integer value, together with a sign bit and an integer scaling factor that specifies what portion of the value is a decimal fraction. Because of this, Decimal numbers have a more precise representation in memory than floating-point types (Single and Double).

  • Performance. The Decimal data type is the slowest of all the numeric types. You should weigh the importance of precision against performance before choosing a data type.

  • Widening. The Decimal data type can be converted to Single or Double without encountering a System.OverflowException error. However, this conversion may result in loss of precision, as Single and Double prioritize accommodating larger values over preserving precision.

Dim decimalValue As Decimal = 1234567890123456789012345D
Dim doubleValue As Double = CDbl(decimalValue)

Console.WriteLine("Decimal value: " & decimalValue)
Console.WriteLine("Double value: " & doubleValue)

The output of the preceding example shows that the Decimal value retains full precision, while the Double value loses precision due to the limitations of the Double data type.

 Decimal value: 1234567890123456789012345
 Double value: 1.23456789012346E+24

  • Trailing Zeros. Visual Basic does not store trailing zeros in a Decimal literal. However, a Decimal variable preserves any trailing zeros acquired computationally. The following example illustrates this.

    Dim d1, d2, d3, d4 As Decimal
d1 = 2.375D
d2 = 1.625D
d3 = d1 + d2
d4 = 4.000D
MsgBox("d1 = " & CStr(d1) & ", d2 = " & CStr(d2) &
      ", d3 = " & CStr(d3) & ", d4 = " & CStr(d4))

    The output of MsgBox in the preceding example is as follows:

    d1 = 2.375, d2 = 1.625, d3 = 4.000, d4 = 4

  • Type Characters. Appending the literal type character D to a literal forces it to the Decimal data type. Appending the identifier type character @ to any identifier forces it to Decimal.

  • Framework Type. The corresponding type in the .NET Framework is the System.Decimal structure.

Range

You might need to use the D type character to assign a large value to a Decimal variable or constant. This requirement is because the compiler interprets a literal as Long unless a literal type character follows the literal, as the following example shows.

Dim bigDec1 As Decimal = 9223372036854775807   ' No overflow.
Dim bigDec2 As Decimal = 9223372036854775808   ' Overflow.
Dim bigDec3 As Decimal = 9223372036854775808D  ' No overflow.

The declaration for bigDec1 doesn't produce an overflow because the value that's assigned to it falls within the range for Long. The Long value can be assigned to the Decimal variable.

The declaration for bigDec2 generates an overflow error because the value that's assigned to it is too large for Long. Because the numeric literal can't first be interpreted as a Long, it can't be assigned to the Decimal variable.

For bigDec3, the literal type character D solves the problem by forcing the compiler to interpret the literal as a Decimal instead of as a Long.

See also