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ParseExact with format "dMMyyyy" not recognizing date

I was looking in the Microsoft doc's and I can't find any explanation why ParseExact doesn't understand my date.
Could somebody explain why this code throws an exception?
DateTime.ParseExact("6092019", "dMMyyyy", CultureInfo.InvariantCulture, DateTimeStyles.None)

From the docs: DateTime.ParseExact Method
If format is a custom format pattern that does not include date or
time separators (such as "yyyyMMddHHmm"), use the invariant culture
for the provider parameter and the widest form of each custom format
specifier. For example, if you want to specify hours in the format
pattern, specify the wider form, "HH", instead of the narrower form,
"H".
So in your case you probably should use approach suggested in John's answer - add "missing" zero and parse with wider date format "dd"

The problem here seems to be that d can be a one-digit or two-digit date, so the parser struggles to determine if "2102019" refers to the 2nd of November 2019, or the 21st of... and then it breaks. With delimiters, the parser is able to act more intelligently. It will happily parse "2-10-2019" using "d-MM-yyyy".
My suggested solution to your problem is to pad the string, and change your format string:
string dateToParse = "6092019";
string paddedDateToParse = dateToParse?.PadLeft(8, '0'); // 06092019
DateTime parsedDate = DateTime.ParseExact(paddedDateToParse, "ddMMyyyy", CultureInfo.InvariantCulture, DateTimeStyles.None);
Try it online

Transform between datetime formats

I am facing a problem in which I need to transform dates in a given input format into a target one. Is there any standard way to do this in C#?
As an example say we have yyyy.MM.dd as the source format and the target format is MM/dd/yyy (current culture).
The problem arises since I am using a parsing strategy that gives priority to the current culture and then if it fails it tries to parse from a list of known formats. Now say we have two equivalent dates one in the source culture above (2015.12.9) and the other in the current culture (9/12/2015). Then if we attempt to parse this two dates the month will be 12 for the first case and in the second will be 9, so we have an inconsistency (they were supposed to mean be the same exact date).
I believe that if existing it should be something as
DateTime.Convert(2015.12.9, 'yyyy/MM/dd', CultureInfo.CurrentCulture).
Any ideas?
EDIT:
Thank you all for your ideas and suggestions, however the interpretation most of you gave to my question was not quite right. What most of you have answered is a direct parse in the given format and then a conversion to the CurrentCulture.
DateTime.ParseExact("2015.12.9", "yyyy.MM.dd", CultureInfo.CurrentCulture)
This will still return 12 as month, although it is in the CurrentCulture format. My question thus was, is there any standard way to transform the date in yyyy.MM.d to the format MM/dd/yyy so that the month is now in the correct place and THEN parsed it in the target culture. Such function is likely to be unexisting.

DateTime.ParseExact is what you are looking for:
DateTime parsedDate = DateTime.ParseExact("2015.12.9", "yyyy.MM.d", CultureInfo.InvariantCulture);
Or eventualy DateTime.TryParseExact if you're not confident with input string.

I know it's late but I try to explain little bit deep if you let me..
I am facing a problem in which I need to transform dates in any format
to a target one.
There no such a thing as dates in any format. A DateTime does not have any implicit format. It just has date and time values. Looks like you have a string which formatted as date and you want to convert another string with different format.
Is there any standard way to do this in C#?
Yes. You can parse your string with DateTime.ParseExact or DateTime.TryParseExact first with specific format to DateTime and then generate it's string representation with a different format.
As an example say we have yyyy.MM.dd as the source format and the
target format is MM/dd/yyy (current culture).
I didn't understand what is the meaning of current culture in this sentences and I assume you want yyyy not yyy, but you can generate it as I described above like;
string source = "2015.12.9";
DateTime dt = DateTime.ParseExact(source, "yyyy.MM.d", CultureInfo.InvariantCulture);
string target = dt.ToString("MM/dd/yyyy", CultureInfo.InvariantCulture); // 12/09/201
The problem arises since I am using a parsing strategy that gives
priority to the current culture and then if it fails it tries to parse
from a list of known formats.
Since you didn't show any parsing strategy and there is no DateTime.Convert method in .NET Framework, I couldn't any comment.
Now say we have two equivalent dates one in the source culture above
(2015.12.9) and the other in the current culture (9/12/2015). Then if
we attempt to parse this two dates the month will be 12 and in the
second will be 9, so we have an inconsistency.
Again.. You don't have DateTime's. You have strings. And those formatted strings can't belong on any culture. Sure all cultures might parse or generate different string representations with the same format format a format does not belong any culture.
I assume you have 2 different string which different formatted and you wanna parse the input no matter which one it comes. In such a case, you can use DateTime.TryParseExact overload that takes string array for all possible formats as a parameter. Then generate it's string representation with MM/dd/yyy format and a culture that has / as a DateSeparator like InvariantCulture.
string s = "2015.12.9"; // or 9/12/2015
string[] formats = { "yyyy.MM.d", "d/MM/yyyy" };
DateTime dt;
if (DateTime.TryParseExact(s, formats, CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
Console.WriteLine(dt.ToString("MM/dd/yyyy", CultureInfo.InvariantCulture));
}

The Simple and Best way to do it is Using .ToString() Method
See this code:
DateTime x =DateTime.Now;
To Convert This Just Write like This:
x.ToString("yyyyMMdd")//20151210
x.ToString("yyyy/MM/dd)//2015/12/10
x.ToString("yyyy/MMM/dd)//2015/DEC/10 //Careful About M type should be capital for month .
Hope helpful

Standard date format vs custom date format

I provide an application where the user is able to format any date by any given format string.
The problem I discovered is the following:
Let's say, I want to format the date as single month index. This is, for today "6".
But:
DateTime.Today.ToString("dd.MM.yyyy") => 26.06.2014<br/>
DateTime.Today.ToString("M") => **26 Juni**<br/>
DateTime.Today.ToString("dM") => 266
Using msdn I found out, that "M" is - if used singular - a standard format specifier, but if used with other chars, its a custom format specifier (http://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.110).aspx; http://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx)
Question: How to force interpreting the given format string always as custom format specifier?

In your second case, you need to specify that the M is a custom format specifier by providing the % sign:
Console.WriteLine(DateTime.Today.ToString("%M"));
Prints:
6
According to MSDN: Custom Date and Time Format Strings and MSDN: Using Single Custom Format Specifiers, the % sign:
Defines the following character as a custom format specifier.
I guess if the user can specify the string, you should always escape it with %.

"Input string was not in a correct format" error when trying to display hour portion with 'h'

I am trying to display a datetime field using the hour portion only without leading zeros for the single digit hours as in: return string.Format("{0:h}", MyDateTimefield) but I get a "Input string was not in a correct format" error. Why?
return string.Format("{0:hh}", MyDateTimefield) works. Looking for the correct format and not a workaround.

From the pertinent docs:
If the "h" format specifier is used without other custom format specifiers, it is interpreted as a standard date and time format specifier and throws a FormatException. For more information about using a single format specifier, see Using Single Custom Format Specifiers later in this topic.
Following that link gets you to:
To use any of the custom date and time format specifiers as the only specifier in a format string […], include a space before or after the specifier, or include a percent ("%") format specifier before the single custom date and time specifier.

From this link using h is the correct format for hour without leading 0. Very interesting.. the following all seem to work:
return string.Format("{0: h}", MyDateTimefield)
return string.Format("{0:h }", MyDateTimefield)
return string.Format("{0:h:m}", MyDateTimefield)
But as soon as you put in return string.Format("{0:h}", MyDateTimefield) it throws an exception.
As for why, I'm not sure. If you're okay with a space the first 2 lines should work.

parse iCalendar date using c#

Is there a direct way to parse an iCalendar date to .net using c#?
An iCalendar date looks like this:
2009-08-11T10:00+05:0000
I need to parse it to display it in a friendly format... thanks

string strDate = "2009-08-11T10:00+05:0000";
DateTimeFormatInfo dtfi = new DateTimeFormatInfo();
dtfi.FullDateTimePattern = "yyyy-MM-ddTHH:mmzzz";
DateTime dt = DateTime.Parse(c.Substring(0, c.Length-2), dtfi);
zzz is for time zone, but is only recognized when expressed like this: +xx:xx.
I tested with your example, removing the last 2 0's then parsing with a custom DateTimeFormatInfo works.

You can use DateTime.Parse() to parse everything before the +. I do not know the iCalendar format specification but I assume after the + is the hours/minutes to add to the date before the +. So you could then use AddHours() and AddMinutes() to add the required bits to the DateTime returned by DateTime.Parse().
This requires a bit of string parsing but with a bit of regex you should be fine...

Since this is not a standard format string, but you know the exact format, you can use DateTime.ParseExact and specify a custom format string, like this:
DateTime.ParseExact(d, "yyyy-MM-ddTHH:mmzzz00", CultureInfo.InvariantCulture);
The 'zzz' specifier represents the hours and minutes offset from UTC, and the two concluding zeros are just literals to match format with which you're dealing.