使用Visual Basic无法将具有小数点后12位的双精度/十进制值转换为字符串

Question

I am trying to convert a double with 12 decimal places to a string, but it only returns a string with 10 decimal places.

I have tried ToString , Format, FormatNumber, CStr and System.Convert.ToString . They all return 10 decimal places.

(Also same behavior with the decimal data type)

Example:

d = Date.Parse(sDate).ToOADate (value is 41261.001388888886)

d.ToString = 41261.0013888889

CStr(d) = 41261.0013888889

FormatNumber(d, 12) = 41,261.001388888900

In the case of Format and FormatNumber , when 12 places are specified, the value is rounded of to 10 digits.

This cropped up when I constructed a Filter for a DataView object.

Is there a way around this or is this a limitation in .Net ?

Answer 1

In looking at the MSDN page for the Decimal Type it States:

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.

So based on your example

Module Module1

    Sub Main()
        Dim d As Decimal
        d = 41261.001388888886D

        Console.WriteLine(d)
        Console.WriteLine(CStr(d))
        Console.WriteLine(FormatNumber(d, 12))
        Console.WriteLine(d.ToString("#0.000000000000"))
        Console.ReadLine()
    End Sub

End Module

Displays:

41261.001388888886
41261.001388888886
4,1261.001388888886
41261.001388888886

---

Give this a try it seems to work correctly

Dim d As Decimal = CDec(String.Format("{0:G20}", Date.Parse(CStr(Now)).ToOADate))

Answer 2

Double.ToString uses only 15 digit precision.

You should use d.ToString("R")

From MSDN :

The round-trip ("R") format specifier guarantees that a numeric value that is converted to a string will be parsed back into the same numeric value.

...

When a Single or Double value is formatted using this specifier, it is first tested using the general format, with 15 digits of precision for a Double and 7 digits of precision for a Single. If the value is successfully parsed back to the same numeric value, it is formatted using the general format specifier. If the value is not successfully parsed back to the same numeric value, it is formatted using 17 digits of precision for a Double and 9 digits of precision for a Single.