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Surprising int.ToString output

I have been working on a project, and found an interesting problem:
2.ToString("TE"+"000"); // output = TE000
2.ToString("TR"+"000"); // output = TR002
I also have tried with several strings other than "TE" but all have the same correct output.
Out of curiosity, I am wondering how come this could have happened?

Simply based on Microsoft's documentation, Custom Numeric Format Strings, your strings "TE000" and "TR000" are both custom format strings, but clearly they are parsed differently.
2.ToString("TE000") is just a bug in the formatter; it's going down a buggy path because of the unescaped "E". So it's unexpectedly assuming the whole thing is a literal.
2.ToString("TR000") is being interpreted as an implied "TR" literal plus 3 zero-filled digits for an integer value; therefore, you get "TR002".
If you truly want TE and TR verbatim, the expressions 2.ToString("\"TE\"000") and 2.ToString("\"TR\"000") will accomplish that for you by specifying TE and TR as explicit literals, instead of letting the formatter guess if they are valid format specifiers (and getting it wrong).

The ToString needs to PARSE the format string and understand what to do with it.
Let's take a look to the following examples:
2.ToString("TE000"); //output TE000
2.ToString("E000"); //output 2E+000
2.ToString("0TE000); //output 2TE000
2.ToString("T"); //throws exception
2.ToString("TT"); //output TT
This shows that if the ToString parser can understand at least part of the format, it will assume that the rest is just extra characters to print with it. If the format is invalid for the given number (like when you use a DateTime string format on a number), it will throw an exception. If it can not make sense of the format, it will return the format string itself as the result.

You cannot use a numeric format to achieve a custom format, instead use something like this:
int i = 2;
String.Format("TE{0:X3}", i);

See Custom Numeric Format Strings. The E means the exponent part of the scientific notation of the number. Since 2 is 2E000 in exponential notation, that might explain it.

decimal/money in formatted string

I can't format decimal in custom formatted string
0.656 => 0.67;
23.656 => 23.67;
5105.54 => 5 105.54;
1234567,89 => 1 234 567,89
I found several posts:
c# - Using String Format to show decimal upto 2 places or simple integer
c# - Converting Decimal to string with non-default format
but when try to use them getting several problem
for example:
on value
0.656 i'm getting ".656" or ".66"
23.656 => " 23.656" or " 23.66"
Car someone recommend links where I can find formatstring rules?

I don't think you actually want to convert 0.656 to 0.67, cause it is just wrong. I guess you mean it should display as 0.66
Use
YourNumber.ToString("0.##");
If you really want to have spaces (which again i think it is wrong):
YourNumber.ToString("#,##0.##").Replace("."," ")

Give this a good read:
Custom Numeric Format Strings
You can use String.Format or ToString() overloades to achieve your goal.

If you want to format a number as a currency value, use this
var d = 0.656;
Console.WriteLine("{0:C}", d); // prints "€ 0,66", in my case
Make sure your localization settings give you the correct currency symbol and decimal character, I.E. a point or a comma.

Formatting strings into groups of 3 digits by periods

I know I can format strings using the String.Format() method. Is it possible to format like this?
Example:
string: 1568
formatted: 1.568
string: 168794521
formatted: 168.794.521
string: 987
formatted: 987
Sorry that I can't make myself more clear.

You can format a number that way, but not a string. For example, if you have an integer value, you can use:
int value = 168794521;
string formatted = value.ToString("N0");
With the proper culture, this will format as shown.
If you are using a string, you would need to convert it. You could also explicitly provide a culture to guarantee "." as a thousands separator:
int value = Int32.Parse("168794521");
string formatted = value.ToString("N0", new CultureInfo("de-DE"));

string someNumericValue = "168794521";
int number = int.Parse(someNumericValue); // error checking might be appropriate
value.ToString("0,0", CultureInfo.CreateSpecificCulture("el-GR"));
This will put points in for thousand specifiers.
It's possible that if you want this, your culture may already do this.

Yes, you can do that. SteveX has written a great blog post on string formatting. You could also look at this blog post, and the MSDN documentation.
You probably want to look at the bottom of this documentation in the "More Resources" section for more info about different types of standard format strings.
Here is the relavant part from the SteveX blog on formatting numbers:
Currency {0:c}
Decimal (Whole number) {0:d}
Scientific {0:e}
Fixed point {0:f}
General {0:g}
Number with commas for thousands {0:n}

DotNet DateTime.ToString strange results

Why does:
DateTime.Now.ToString("M")
not return the month number? Instead it returns the full month name with the day on it.
Apparently, this is because "M" is also a standard code for the MonthDayPattern. I don't want this...I want to get the month number using "M". Is there a way to turn this off?

According to MSDN, you can use either "%M", "M " or " M" (note: the last two will also include the space in the result) to force M being parsed as the number of month format.

What's happening here is a conflict between standard DateTime format strings and custom format specifiers. The value "M" is ambiguous in that it is both a standard and custom format specifier. The DateTime implementation will choose a standard formatter over a customer formatter in the case of a conflict, hence it is winning here.
The easiest way to remove the ambiguity is to prefix the M with the % char. This char is way of saying the following should be interpreted as a custom formatter
DateTime.Now.ToString("%M");

Why not use
DateTime.Now.Month?

You can also use System.DateTime.Now.Month.ToString(); to accomplish the same thing

You can put an empty string literal in the format to make it a composite format:
DateTime.Now.ToString("''M")

It's worth mentioning that the % prefix is required for any single-character format string when using the DateTime.ToString(string) method, even if that string does not represent one of the built-in format string patterns; I came across this issue when attempting to retrieve the current hour. For example, the code snippet:
DateTime.Now.ToString("h")
will throw a FormatException. Changing the above to:
DateTime.Now.ToString("%h")
gives the current date's hour.
I can only assume the method is looking at the format string's length and deciding whether it represents a built-in or custom format string.

Custom Numeric Format Strings: Dynamic Decimal Point

I am trying to format a double in C# such that it uses the thousand separator, and adds digits upto 4 decimal places.
This is straight forward except that I dont want to have the decimal point if it is an integer. Is there a way to do this using the custom numeric format strings rather than an if statement of tenary operator?
Currently I have:
string output = dbl.ToString(dbl == (int)dbl ? "#,##0" : "#,##0.####");
Thanks

I believe your second format string of "#,##0.##" should be exactly what you want -- the # format character is a placeholder that will NOT display zeros.
If you had "#,###.00" then you would get trailing zeros.
test code:
double d = 45.00;
Console.Writeline(d.ToString("#,##0.##"));
Gives output of "45". Setting d to 45.45 gives output "45.45", which sounds like what you're after.
So you had the answer after all! ;)
Incidentally, there's a handy cheat-sheet for format strings (amongst other handy cheat-sheets) at http://john-sheehan.com/blog/net-cheat-sheets/

No, there is not any built-in format string for this. Your current solution is the best way to accomplish this.
MSDN lists both the standard numeric format strings and custom numeric format strings, so you should be able to see for yourself that none directly matches your needs.