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}";