Develop Reference

Why does float.Parse("123,45") not throw an exception

... but returns 12345?
The doc for Single.Parse says:
Exceptions
...
FormatException
s does not represent a numeric value.
...
For my understanding "123,45" doesn't represent a proper numeric value (in countries that use comma as thousands separator).
The system's CultureInfo has:
CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator == "."
CultureInfo.CurrentCulture.NumberFormat.NumberGroupSeparator == ","
CultureInfo.CurrentCulture.NumberFormat.NumberGroupSizes == [3]
Apparently the comma is simply ignored and this leads to even more irritating results: "123,45.67" or "1,23,45.67"–which look utterly wrong–become 12345.67.
Supplementary question
I don't get what this sentence in the doc is supposed to mean and whether this is relevant for this case:
If a separator is encountered in the s parameter during a parse operation, and the applicable currency or number decimal and group separators are the same, the parse operation assumes that the separator is a decimal separator rather than a group separator.

In the default and US culture, the comma (,) is legal as a separator between groups. Think of larger numbers like this:
987,654,321
That it's in the wrong place for a group doesn't really matter; the parser isn't that smart. It just ignores the separator.
For the supplemental question, some cultures use commas as the decimal separator, rather than a group separator. This part of the documentation clarifies what will happen if the group separator and decimal separator are somehow set to the same character.

As Joel said, "the parser isn't that smart". The source code is available, so here's the proof.
The code for Single.Parse ends up calling Number.ParseNumber.
Interestingly, Number.ParseNumber is given a NumberFormatInfo object, which does have a NumberGroupSizes property, which defines "the number of digits in each group to the left of the decimal".
However, you'll notice that on line 851, where it checks for the group separator, it doesn't bother to reference the NumberGroupSizes property to check if the group separator is in an expected position. In fact Number.ParseNumber never uses the NumberGroupSizes property.
NumberFormatInfo.NumberGroupSizes is only ever used when converting a number to a string.

How to convert 2.3449 to Decimal

I am trying to convert 2.3449 to Decimal but It converts like 23449,0
I am converting it like below
string temp = "2.3449";
decimal value_ = Convert.ToDecimal(temp);
if I replace the DOT with COLON, it converts it perfectly. But, I dont want to Replace the DOT with COLON in string. What is the good way of converting without replacing.

Your culture treats . as the thousands separator, rather than decimal separator.
You can always use a specific culture:
var val = decimal.Parse(temp, CultureInfo.InvariantCulture);
The same goes for ToString - if you want to print the number with . as the decimal separator, just use the appropriate culture. The local culture (the default) is usually the best bet for anything user-facing, though. Invariant culture is perfect for persistence :)

c# - How do I round a decimal value to 2 decimal places. with , every 1000

So always add a comma and have it to two decimal places, "F" nearly works but can't find the right solution
decimal = 1000.5
test.Text = decimal.ToString("F")
I've also tried:
String.Format("{0:#,###.##}", decimal);
I want to display as the string as 1,000.50

Try:
String.Format("{0:#,###.00}", decimalNumber);
See: Custom Numeric Format Strings
0 - Zero placeholder Replaces the zero with the corresponding digit if
one is present; otherwise, zero appears in the result string. More
information: The "0" Custom Specifier.
It is not going to round the numbers, it is just string formatting.
For culture insensitive formatting do:
String.Format(CultureInfo.InvariantCulture, "{0:#,###.00}", decimalNmber);

String.Format(CultureInfo.InvariantCulture, "{0:0,0.00}", decimal)
For more options see this link.
And here you can test this online.

Why string.Format produces such result?

I have a line like the following in my code:
string buffer = string.Format(CultureInfo.InvariantCulture, "{0:N4}", 1008.0);
Why does buffer contain 1,008.0 (note comma) after executing this line?
Yes, I do guess that it's caused by my regional settings. The question is why they affect the result in this case?
EDIT:
Ok, I understand that it's completely my fault. It seems like I should have used F format specifier.

The InvariantCulture is loosely based on en-US which uses , as a thousands (group) separator.
Your result is what I would expect.
I also point you to the details of the N numeric format specifier:
The numeric ("N") format specifier converts a number to a string of the form "-d,ddd,ddd.ddd…", where "-" indicates a negative number symbol if required, "d" indicates a digit (0-9), "," indicates a group separator, and "." indicates a decimal point symbol.

You're using the invariant culture; your culture is irrelevant to this. For this, the N4 format means
-d,ddd,ddd,ddd...
That is, possible leading negative sign indicator and commas between thousands groups. For details see: http://msdn.microsoft.com/en-us/library/dwhawy9k#NFormatString
You can look at
NegativeSign
NumberNegativePattern
NumberGroupSizes
NumberGroupSeparator
NumberDecimalSeparator
NumberDecimalDigits
for the invariant culture. If you do, you'll see:
-
1
{ 3 }
,
.
2

You are getting the comma because of "{0:N4}"
n ----- Number with commas for thousands ----- {0:n}
Source:
You will get the comma even without specifying InvariantCulture
Console.WriteLine(string.Format("{0:n4}", 1008.0));

Putting Commas in a Decimal

IF i have the following:
MyString.ValueType = typeof(System.Decimal);
how can i make this have an output of decimals with commas? In other words, instead of seeing 1234.5, i'd like to see 1,234.5

Use:
String output = MyString.ValueType.ToString("N");
The "N" format specifier will put in the thousands separators (,). For details, see Decimal.ToString(string).
Edit:
The above will use your current culture settings, so the thousands separators will depend on the current locale. However, if you want it to always use comma, and period for the decimal separator, you can do:
String output = MyString.ValueType.ToString("N", CultureInfo.InvariantCulture);
That will force it to use the InvariantCulture, which uses comma for thousands and period for decimal separation, which means you'll always see "1,234.5".