variable decimal places in .Net string formatters?

Question

Fixed decimal places is easy

String.Format(\"{0:F1}\", 654.321);

gives

654.3

How do I feed the number of

Answer 1

I use an interpolated string approach similar to Wolfgang's answer, but a bit more compact and readable (IMHO):

using System.Globalization;
using NF = NumberFormatInfo;

...

decimal size = 123.456789;  
string unit = "MB";
int fracDigs = 3;

// Some may consider this example a bit verbose, but you have the text, 
// value, and format spec in close proximity of each other. Also, I believe 
// that this inline, natural reading order representation allows for easier 
// readability/scanning. There is no need to correlate formats, indexes, and
// params to figure out which values go where in the format string.
string s = $"size:{size.ToString("N",new NF{NumberDecimalDigits=fracDigs})} {unit}";

Answer 2

use

string.Format("{0:F2}", 654.321);

Output will be

654.32

Answer 3

The string to format doesn't have to be a constant.

int numberOfDecimalPlaces = 2;
string formatString = String.Concat("{0:F", numberOfDecimalPlaces, "}");
String.Format(formatString, 654.321);

Answer 4

Use the custom numeric format string Link

var value = 654.321;
var s = value.ToString("0.##");

Answer 5

Another option is using interpolated strings like this:

int prec = 2;
string.Format($"{{0:F{prec}}}", 654.321);

Still a mess, but yet more convenient IMHO. Notice that string interpolation replaces double braces, like {{, with a single brace.

Answer 6

I used two interpolated strings (a variant of Michael's answer):

double temperatureValue = 23.456;
int numberOfDecimalPlaces = 2;

string temperature = $"{temperatureValue.ToString($"F{numberOfDecimalPlaces}")} \u00B0C";