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c# convert yyyymmdd text to dd/MM/yyyy

I have some text in rows as yyyymmdd (eg: 20181211) which I need to convert to dd/MM/yyyy
I am using:
cashRow["BusinessDate"] = Convert.ToDateTime(row.Cells[ClosingDate.Index].Value.ToString()).ToString("dd/MM/yyyy");
I get the error "string was not recognized as a valid DateTime.
Any ideas where I am going wrong?

You'll need to use ParseExact (or TryParseExact) to parse the date:
var date = "20181217";
var parsedDate = DateTime.ParseExact(date, "yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture);
var formattedDate = parsedDate.ToString("dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture)
Here we tell ParseExact to expect our date in yyyyMMdd format, and then we tell it to format the parsed date in dd/MM/yyyy format.
Applying it to your code:
cashRow["BusinessDate"] = DateTime.ParseExact(row.Cells[ClosingDate.Index].Value.ToString(), "yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture).ToString("dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture);

You have to use ParseExact to convert your yyyyMMdd string to DateTime as follows and then everything is okay.
cashRow["BusinessDate"] = DateTime.ParseExact(row.Cells[ClosingDate.Index].Value.ToString(), "yyyyMMdd", CultureInfo.InvariantCulture).ToString("dd/MM/yyyy");

As you already know the exact format of your input yyyyMMdd (eg: 20181211), just supply it to DateTime.ParseExact() and then call ToString() on the returned DateTime object.
string YourOutput = DateTime.ParseExact(p_dates, "yyyyMMdd", System.Globalization.CultureInfo.InvariantCulture).ToString("dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture);

Try this:
cashRow["BusinessDate"] = row.Cells[ClosingDate.Index].Value.ToString("dd/MM/yyyy");

String was not recognized as a valid DateTime. Throws an Exception

I'm trying to convert current date to a specified format.
DateTime date = DateTime.ParseExact(DateTime.Now.ToString(), "yyyy-MM-dd HH:mm:ss.fff",
CultureInfo.InvariantCulture,
DateTimeStyles.None);
I'm receiving the following exception.
String was not recognized as a valid DateTime.
My local TimeZone is (UTC+10:00)Melbourne.
What am I doing wrong here?

Your code (even if it worked), would do nothing. It would simply serialize and deserialize the date. I believe you're looking for this:
string date = DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff");
It doesn't work because DateTime.Now.ToString() is giving a string like (I happen to be in the same timezone, and presumably have the same culture as you):
14/01/2016 3:54:01 PM
Which is of the format:
dd/MM/yyyy h:mm:ss tt
Which does not match the format you're using: yyyy-MM-dd HH:mm:ss.fff

Try this:
string fm = "yyyy-MM-dd HH:mm:ss.fff";
string str = DateTime.Now.ToString(fm, CultureInfo.InvariantCulture);
DateTime dt = DateTime.ParseExact(str, fm, CultureInfo.InvariantCulture);
EDIT:
A better way to achieve the date in the format would be like
DateTime now = DateTime.Now;
CultureInfo culture = new CultureInfo("en-AU"); //Melbourne
Thread.CurrentThread.CurrentCulture = culture;
Console.WriteLine(now.ToString("yyyy-MM-ddTHH:mm:ss.fff"));
IDEONE DEMO

How to convert date format to custome date format?

I have the following output in string:
24/05/15 11:40:50 AM
now I want to convert this string into -> 2015-05-24 11:40:50.000
I have tried below method but it gives me error :
String was not recognized as a valid DateTime.
DateTime.ParseExact("24/05/15 11:40:50 AM",
"yyyy-MM-dd HH:mm:ss",
CultureInfo.InvariantCulture);

From documentation;
Converts the specified string representation of a date and time to its
DateTime equivalent. The format of the string representation must
match a specified format exactly.
In your case, they are not.
First, you can parse it to DateTime with specific format and you can generate a string representation with a specific format from that DateTime. Like;
string s = "24/05/15 11:40:50 AM";
DateTime dt;
if(DateTime.TryParseExact(s, "dd/MM/yy hh:mm:ss tt", CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
Console.WriteLine(dt.ToString("yyyy-MM-dd hh:mm:ss.fff"));
}
Prints;
2015-05-24 11:40:50.000

Converting string to date time

I have a date which comes in a string like so:
09/25/2014 09:18:24
I need it like this (yyyy-mm-dd):
2014-09-25 09:18:24
The object that this date goes into is a nullable date.
Tried this does not work:
DateTime formattedDate;
bool result = DateTime.TryParseExact(modifiedDate, "yyyy-MM-dd",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out formattedDate);
Any clues?
Thanks in advance.

From DateTime.TryParseExact
Converts the specified string representation of a date and time to its
DateTime equivalent. The format of the string representation must
match a specified format exactly.
In your case, they are not. Use yyyy-MM-dd HH:mm:ss format instead.
string s = "2014-09-25 09:18:24";
DateTime dt;
if(DateTime.TryParseExact(s, "yyyy-MM-dd HH:mm:ss",
CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
Console.WriteLine(dt);
}
It is a little bit unclear but if your string is 09/25/2014 09:18:24, then you can use MM/dd/yyyy HH:mm:ss format instead. Just a tip, "/" custom format specifier has a special meaning as replace me with current culture or supplied culture date separator. That means, if your CurrentCulture or supplied culture's DateSeparator is not /, your parsing operation will fail even if your format and string matches exactly.
If you have already a DateTime and you want to format it, you can use DateTime.ToString(string) method like;
dt.ToString("yyyy-mm-dd", CultureInfo.InvariantCulture); // 2014-09-25
or
dt.ToString("yyyy-mm-dd HH:mm:ss", CultureInfo.InvariantCulture); // 2014-09-25 09:18:24
Remember, a DateTime does not have any implicit format. It just contains date and time values. String representations of them have formats.

In answer to your question, to convert it as you prefer, do it like this:
string originalDate = "09/25/2014 09:18:24";
DateTime formattedDate;
if (DateTime.TryParseExact(originalDate, "MM/dd/yyyy HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.None, out formattedDate))
{
string output = formattedDate.ToString("yyyy-mm-dd HH:mm:ss", CultureInfo.InvariantCulture);
}
And then output will have your desired format.

DateTime dateOf = Convert.ToDateTime("09/25/2014 09:18:24");
string myFormat = "yyyy-mm-dd";
string myDate = dateOf.ToString(myFormat); // output 2014-18-25
Datetime format

Issue in Converting Date Time c#

I am facing an issue when converting the datetime
var date = DateTime.Now;
txtdate.Text = date.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);
//I need to pass it later to as a DateTime variable. When i re-convert it gives me an error)
DateTime dtReconvert =Convert.toDateTime(txtdate.Text); //Error String was not recognized as a valid DateTim
When i set the datetime to something like "01/01/2013"
and convert it to Date Time it not giving me any error.

Use DateTime.ParseExact with the format "dd/MM/yyyy"
DateTime dtObject = DateTime.ParseExact(txtdate.Text,
"dd/MM/yyyy",
CultureInfo.InvariantCulture);

try this
DateTime.ParseExact(txtdate.Text, "MM/dd/yyyy", System.Globalization.CultureInfo.InvariantCulture)
or
Convert.ToDateTime(txtdate.Text, CultureInfo.InvariantCulture)
Microsoft have adopted MM/dd/yyyy as a culture-agnostic format, it's an ambiguous format which isn't something that I would want to build a large system on.