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ToString format for fixed length of output - mixture of decimal and integer

I'm writing some code to display a number for a report. The number can range from 1. something to thousands, so the amount of precision I need to display depends on the value.
I would like to be able to pass something in .ToString() which will give me at least 3 digits - a mixture of the integer part and the decimal part.
Ex:
1.2345 -> "1.23"
21.552 -> "21.5"
19232.12 -> "19232"
Using 000 as a format doesn't work, since it doesn't show any decimals, neither does 0.000 - which shows too many decimals when the whole part is larger than 10.

You could write an extension method for this:
public static string ToCustomString(this double d, int minDigits = 3)
{
// Get the number of digits of the integer part of the number.
int intDigits = (int)Math.Floor(Math.Log10(d) + 1);
// Calculate the decimal places to be used.
int decimalPlaces = Math.Max(0, minDigits - intDigits);
return d.ToString($"0.{new string('0', decimalPlaces)}");
}
Usage:
Console.WriteLine(1.2345.ToCustomString()); // 1.23
Console.WriteLine(21.552.ToCustomString()); // 21.6
Console.WriteLine(19232.12.ToCustomString()); // 19232
Console.WriteLine(1.2345.ToCustomString(minDigits:4)); // 1.235
Try it online.

I don't think this can be done with ToString() alone.
Instead, start by formatting the number with 2 trailing digits, then truncate as necessary:
static string FormatNumber3Digits(double n)
{
// format number with two trailing decimals
var numberString = n.ToString("0.00");
if(numberString.Length > 5)
// if resulting string is longer than 5 chars it means we have 3 or more digits occur before the decimal separator
numberString = numberString.Remove(numberString.Length - 3);
else if(numberString.Length == 5)
// if it's exactly 5 we just need to cut off the last digit to get NN.N
numberString = numberString.Remove(numberString.Length - 1);
return numberString;
}

Here's a regex, that will give you three digits of any number (if there's no decimal point, then all digits are matched):
#"^(?:\d\.\d{1,2}|\d{2}\.\d|[^.]+)"
Explanation:
^ match from start of string
either
\d\.\d{1,2} a digit followed by a dot followed by 1 or 2 digits
or
\d{2}\.\d 2 digits followed by a dot and 1 digit
or
[^.]+ any number of digits not up to a dot.
First divide your number and then call ToString() before the regex.

Simple way to implement this just write
ToString("f2") for two decimal number just change this fnumber to get your required number of decimal values with integer values also.

String Format for up to 2 decimal places or simple integer and with thousand separator - C#

Here is a string format which formats a number up to 2 decimal places and if there are no decimal places then it will show the simple integer itself:
String.Format("{0:0.##}", 123.4567); // "123.46"
String.Format("{0:0.##}", 123.0); // "123"
so {0:0.##} will do the job. But I need to have a thousand separator on integer part. so something like 1234.456 should be like 1,234.45. I know {0:n0} will do a thousand separators but how to combine it with the decimal place formatter?

String.Format("{0:#,##0.##}", 123.0);

C# Formatting numbers with spaces delimiting thousands

I'm trying to format a decimal with the following requirements:
Thousands are separated by spaces " "
Decimal point is delimited by a
comma "," (this is achieved by using the appropriate culture, in this
case Croatian)
There are two digits after the decimal point.
So far I got this:
String.Format(new CultureInfo("hr-HR"), "{0:# ##0.00}", input)
This works well if the number has 4 or more digits before the decimal point. For example the value 5500.5 gives me "5 500,50" and -5500.5 gives me "-5 500,50", which is what I want.
But if the number has fewer digits, I get a white space in front of the number. For example 500.5 gives me " 500,50" instead of "500,50". And with a negative number, the space is put between the minus sign and the digits: -500.5 gives me "- 500,50". So I can't simply trim the result. How can I achieve what I need?

You can use a custom NumberFormatInfo to format numbers.
var nfi = new NumberFormatInfo();
nfi.NumberGroupSeparator = " "; // set the group separator to a space
nfi.NumberDecimalSeparator = ","; // set decimal separator to comma
And then format the number using
number.ToString("N2", nfi); // numeric format with 2 decimal digits
This gives you
(5500.5).ToString("N2", nfi) // "5 500,50"
(-500.5).ToString("N2", nfi) // "-500,50"
(-5500.5).ToString("N2", nfi) // "-5 500,50"

The simplest approach may be to just check if input is over a 1000 / less than -1000 before formatting it this way.
if(input >= 1000 || input <= -1000){
// use String.Format(new CultureInfo("hr-HR"), "{0:# ##0.00}", input)
}else{
// just use input.ToString()?
}
There's probably a way to coerce string.format into doing what you want, but why overcomplicate the issue?

C#: Exponential Format Specifier

I have a double number:
element.MaxAllowableConcLimitPpm = 0.077724795640326971;
I need to display it as
7.7725e-2
when I try to use it:
element.MaxAllowableConcLimitPpm.ToString("e4", CultureInfo.InvariantCulture)
it returns
7.7725e-002
How to say that mantissa should have one symbol instead of 3 ?
Format like this:
.ToString("0.0000e0")
returns
5.0000e2
instead of
5.0000e+2

You have to use a custom numeric format string - standard numeric format strings always have at least three digits in the exponent.
Example with a custom string:
using System;
public class Test
{
static void Main()
{
double value = 0.077724795640326971;
Console.WriteLine(value.ToString("0.0000e+0")); // 7.7725e-2
}
}
From the documentation for standard numeric format strings (emphasis mine):
The case of the format specifier indicates whether to prefix the exponent with an "E" or an "e". The exponent always consists of a plus or minus sign and a minimum of three digits. The exponent is padded with zeros to meet this minimum, if required.

How to add thousands separator to string value which may contain floating points

I have a string series of values which may or may not contain a floating point number. I want to add the thousands separator to this numeric string. I want to have the value with thousands separators and floating point number only when it's there. How can I do this?
Examples:
Input: 23456.78
Output: 23,456.78
Input: 23456
Output: 23,456

Try parsing to decimal (or double) and then format back to the required representation ("#,#.##########" format string in your case):
String input = "23456.78";
// 23,456.78
String output = decimal
.Parse(input, CultureInfo.InvariantCulture)
.ToString("#,#.##########", CultureInfo.InvariantCulture);