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DateTime.ParseExact not working in C#, Date format Conversion not working

Parse Time not working as I want to convert "13-06-2019 00:00:00"(dd-MM-yyyy HH:mm:ss) to "06-13-2019 00:00:00"(MM-dd-yyyy HH:mm:ss)
tried with Convert.toDateTime() and DateTime.ParseExact()
IFormatProvider culture = new CultureInfo("en-US");
var a = DateTime.ParseExact(a, "MM-dd-yyyy hh:mm:ss", CultureInfo.InvariantCulture);
var b = DateTime.ParseExact(a, "yyyy-MM-dd HH:mm:ss", null);
var c = DateTime.ParseExact(a, "yyyy-MM-dd HH:mm:ss", culture);
Nothing working in it

DateTime structure uses Gregorian calendar under the hood and there is no 13th month in that calendar.
So, parsing 13 with MM specifier is wrong. I strongly suspect that you try to use dd-MM-yyyy HH:mm:ss format instead.
string a = "13-06-2019 00:00:00";
DateTime b = DateTime.ParseExact(a, "dd-MM-yyyy HH:mm:ss", CultureInfo.InvariantCulture);
Here a demonstration
Your second and third examples also don't work since their formats are completely different than your string. When you parse your string with ParseExact method, your strings and your format should match exactly.
Also I want to mention that, both hh and HH specifiers would work in my code example. But as a general format consideration, using dd-MM-yyyy HH:mm:ss format is much more common and reliable than the other option.

The format parameter of DateTime.ParseExact(date,format,culture) is the source format of the date string to be converted and the return value is type date which you can convert back to string as per the desired format.
var a = "13-06-2019 00:00:00";
IFormatProvider culture = new CultureInfo("en-US");
DateTime b = DateTime.ParseExact(a, "dd-MM-yyyy hh:mm:ss", CultureInfo.InvariantCulture);
Console.WriteLine($"{b:MM-dd-yyyy HH:mm:ss}");

Why doesn't DateTime.ParseExact work for this example outside of the US?

I have a string:
var stringDate = "8/19/2016 5:00:00 PM"
and I try using:
var dateTime = DateTime.ParseExact(stringDate,"M/d/yyyy h:mm:ss tt", null);
or
var dateTime = DateTime.ParseExact(stringDate, "M/d/yyyy h:mm:ss tt",
CultureInfo.Invariant);
or
var dateTime = DateTime.ParseExact(stringDate,"M/d/yyyy h:mm:ss tt",
CultureInfo.GetCultureInfo("en-us"));
It works fine if the machine is in the US but and for all 3 options I get the following error when running from a machine in London:
String was not recognized as a valid DateTime
what am I missing here?

As far as I can see, you need to use h specifier instead of hh specifier since your hour part does not have leading zero for single digit values.
var stringDate = "8/19/2016 5:00:00 PM";
var dateTime = DateTime.ParseExact(stringDate, "M/d/yyyy h:mm:ss tt",
CultureInfo.InvariantCulture);
Also I would suggest to use specific culture settings instead of null for any case. In your example, since null means CurrentCulture settings for DateTime parsing methods, your CurrentCulture settings;
May not use / as a DateSeparator or
May not use : as a TimeSeparator or
May not use GregorianCalendar as a Calendar property or
May not have AM or PM as a PMDesignator or AMDesignator properties.
For those, your ParseExact method throws exception even if your string and format perfectly matches.

How to find exact format of a date string?

I need to know exact date format that will perse the string 16-Aug-78 12:00:00 AM. I have tried various string like "ss-MMM-yy hh:mm:ss". Is there any way for finding it to converting to any general format. I am using CultureInfo.InvariantCulture class.

Your string format is wrong. It has to match your string format exactly. You can use dd-MMM-yy hh:mm:ss tt format instead.
Here an example in LINQPad.
string s = "16-Aug-78 12:00:00 AM";
var date = DateTime.ParseExact(s, "dd-MMM-yy hh:mm:ss tt",
CultureInfo.InvariantCulture);
date.Dump();
Custom Date and Time Format Strings
Or, since dd-MMM-yy hh:mm:ss tt is a standart date and time format for InvariantCulture, you can directly DateTime.Parse method like;
string s = "16-Aug-78 12:00:00 AM";
var date = DateTime.Parse(s, CultureInfo.InvariantCulture);
date.Dump();
Here a demonstration.
Is there any way for finding it to converting to any general format?
There is no way to get format of a string except you create your own formatting. Only you can know what is your string format exactly, computer can't.
For example; 01/02/2014 can be 1 February 2014 or 2 January 2014 depends on which custom format you can parse it.

Try as below
var dstring = "16-Aug-78 12:00:00 AM";
DateTime result;
var matchingCulture = CultureInfo.GetCultures(CultureTypes.AllCultures).FirstOrDefault(ci => DateTime.TryParse(dstring, ci, DateTimeStyles.None, out result))

You have wrong format string, you are using ss for day it should be dd. This article Custom Date and Time Format Strings explains what you need for custom format.
Use
"dd-MMM-yy hh:mm:ss tt"
Instead of
"ss-MMM-yy hh:mm:ss"
You can use DateTime.ParseExact to parse the string.
DateTime dt = DateTime.ParseExact("16-Aug-78 12:00:00 AM", "dd-MMM-yy hh:mm:ss tt", CultureInfo.InvariantCulture, DateTimeStyles.None);

You can try with DateTime.TryParseExact():
string strDate = "16-Aug-78 12:00:00 AM";
DateTime datDate;
if(DateTime.TryParseExact(strDate , new string[] {"dd-MMM-yy hh:mm:ss tt" },
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None, out datDate))
{
Console.WriteLine(datDate);
}
else
{
Console.WriteLine("Error in datetime format");
}

Convert timestamp string to DateTime object in c#

I have timestamp strings in the following format 5/1/2012 3:38:27 PM. How do I convert it to a DateTime object in c#

var date = DateTime.ParseExact("5/1/2012 3:38:27 PM",
"M/d/yyyy h:mm:ss tt",
CultureInfo.InvariantCulture);

You input string looks like in en-us format, which is M/d/yyyy h/mm/ss tt. You have to use proper CultureInfo instance while parsing:
var ci = System.Globalization.CultureInfo.GetCultureInfo("en-us");
var value = DateTime.Parse("5/1/2012 3:38:27 PM", ci);
or
var ci = new System.Globalization.CultureInfo("en-us");

Try to use DateTime.ParseExact method like;
string s = "5/1/2012 3:38:27 PM";
DateTime date = DateTime.ParseExact(s, "M/d/yyyy h:mm:ss tt", CultureInfo.InvariantCulture);
Console.WriteLine(date);
Converts the specified string representation of a date and time to its
DateTime equivalent using the specified format and culture-specific
format information. The format of the string representation must match
the specified format exactly.
Output will be;
01.05.2012 15:38:27
Be aware, this output can change based which Culture you used. Since my Culture is tr-TR, the date operator is . our culture.
Here is a DEMO.

Try the DateTime.ParseExact method

http://www.codeproject.com/Articles/14743/Easy-String-to-DateTime-DateTime-to-String-and-For
this maybe helps you. There you can find a detailled explanation of the ParseExact parameters.

Display datetime into MM/dd/yyyy HH:mm format c#

In database datetime is being stored in MM-dd-yyyy HH:mm:ss fromat.
However, I want to display datetime in "MM/dd/yyyy HH:mm" format.
I tried it by using String.Format().
txtCampaignStartDate.Text = String.Format("{0:MM/dd/yyyy
HH:mm}",appCampaignModel.CampaignStartDateTime);
Here appCampaignModel.CampaignStartDateTime is DateTime object having value in "MM-dd-yyyy HH:mm" format.
I want to display in "MM/dd/yyyy HH:mm" format.
Can anyone help me for this?

The slashes in this format MM/dd/yyyy HH:mm mean: "replace me with the actual separator of the current culture". You have to use CultureInfo.InvariantCulture explicitely:
txtCampaignStartDate.Text = appCampaignModel.CampaignStartDateTime.ToString("MM/dd/yyyy HH:mm", CultureInfo.InvariantCulture);
The "/" Custom Format Specifier
In database datetime is being stored in MM-dd-yyyy HH:mm:ss fromat.
Don't store datetimes with a format which means that you store them as (n)varchar. Use datetime instead.

txtCampaignStartDate.Text = appCampaignModel.CampaignStartDateTime
.ToString().Replace("-","/");
or
String.Format("{0:MM/dd/yyyy HH:mm}", dt);
Check : String Format for DateTime [C#]
or
String date = dt.ToString("MM/dd/yyyy HH:mm", DateTimeFormatInfo.InvariantInfo);
DateTime.ToString Method (String, IFormatProvider)

Why not just a simple replace?
txtCampaignStartDate.Text = BadString.Replace("-","/");

try using an added culture info:
CultureInfo ci = new CultureInfo("en-US");
txtCampaignStartDate.Text =
appCampaignModel.CampaignStartDateTime.ToString("MM/dd/yyyy HH:mm", ci);
p.s. the culture info used is an example.

To display your date in format you specified:
txtCampaignStartDate.Text = appCampaignModel.CampaignStartDateTime.ToString("g",DateTimeFormatInfo.InvariantInfo)
And for more date time formats you can go through this link.

Since you already have a DateTime object, just ToString it how you want:
appCampaignModel.CampaignStartDateTime.ToString("MM/dd/yyyy HH:mm");