CODEWITHSUNDEEP
c#.netnumber-formatting
ChaseMedallion
ChaseMedallion

Reputation: 21764

Do multiple leading '#' signs in format strings do anything?

In some legacy .NET code I've come across a number of custom numeric format strings like this:

###,##0.00

What is the difference between this and:

#,#0.00

?

EDIT:

Here are some example inputs I've tried, all of which yield the same result for both masks: 1000000, 1000, 100, 10, 1.456, -30000, 0.002.

EDIT:

@Sahuagin suggested that these masks could be the same because of how the culture is set to group to three digits. However, even using this I can't demonstrate a difference:

var culture = new CultureInfo("en-US");
culture.NumberFormat.NumberGroupSizes = new[] { 1, 2, 3, 4 };
culture.NumberFormat.NumberGroupSizes.Dump();

1234567890.ToString("#,#0.00", culture).Dump(); // 1234,567,89,0.00
1234567890.ToString("###,#0.00", culture).Dump(); // 1234,567,89,0.00

More generally, I understand that # is an "optional" digit which won't create leading or trailing zeroes. However, it seems like just a single # before the decimal point is enough to get all leading digits. The MSDN docs seem to differentiate between # and ## but the explanation doesn't make much sense to me and I haven't found an example where it makes a difference.

Upvotes: 2

Views: 174

Answers (1)

Dave Cousineau
Dave Cousineau

Reputation: 13198

# indicates a place where a digit will appear, if one exists in the number. Absent any such symbols, leading digits automatically appear (though you must have at least either one # or one 0, or no number will appear), but they are still useful as placeholders in some kinds of formats, for example a telephone number:

var value = 1234567890;
Console.WriteLine("{0:###-###-####}", value);
// outputs 123-456-7890

In your example of #,#0.00, I think that only manages to still format correctly (with groups of three) because , is a special grouping symbol, and the culture info is set to group digits in threes. Without that, you would get something like 123-45.67, if you used a - instead of a , for example.

Here is more specific information about , from MSDN:

The "," character serves as both a group separator and a number scaling specifier.

Group separator: If one or more commas are specified between two digit placeholders (0 or #) that format the integral digits of a number, a group separator character is inserted between each number group in the integral part of the output.

The NumberGroupSeparator and NumberGroupSizes properties of the current NumberFormatInfo object determine the character used as the number group separator and the size of each number group. For example, if the string "#,#" and the invariant culture are used to format the number 1000, the output is "1,000".

Number scaling specifier: If one or more commas are specified immediately to the left of the explicit or implicit decimal point, the number to be formatted is divided by 1000 for each comma. For example, if the string "0,," is used to format the number 100 million, the output is "100".

So in your first example of ###,##0.00, it could probably be reduced to #,0.00, if desired, although #,##0.00 is what I usually use since it is much more clear.

Upvotes: 3

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