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C# Datarow item DateTime format being ignored

I am filling a DataTable using the results of a SQL SP. One of the columns is a DateTime type. When I try to format the DateTime value, it seems to be ignored.
For example:
foreach (DataRow row in sourceTable.Rows)
{
row["DateOfActivity"] = Convert.ToString(((DateTime)row["DateOfActivity"]).ToString("yyyy-MM-dd"));
Console.WriteLine(row["DateOfActivity"]);
}
Results in:
2/17/2016 12:00:00 AM
How can I make sure the DateTime format is retained?

In a databable, columns have specific types. When you assign back to row["DateOfActivity"], you're assigning to a DateTime value. Writing that back to the console will give you the default .ToString() call.
The solution is to wait to format the output until you actually show it to the user. Get rid of the first line inside the loop completely and have the second line just look like this:
Console.WriteLine(row["DateOfActivity"].ToString("yyyy-MM-dd"));

The type definition of the row["DateOfActivity"] column is not present in your question, however I am going to assume it is defined as a DateTime type. This type definition stores the raw date/time value, without a format.
From MSDN:
Internally, all DateTime values are represented as the number of ticks (the number of 100-nanosecond intervals) that have elapsed since 12:00:00 midnight, January 1, 0001. The actual DateTime value is independent of the way in which that value appears when displayed in a user interface element or when written to a file. The appearance of a DateTime value is the result of a formatting operation. Formatting is the process of converting a value to its string representation.
Because the appearance of date and time values is dependent on such factors as culture, international standards, application requirements, and personal preference, the DateTime structure offers a great deal of flexibility in formatting date and time values through the overloads of its ToString method. The default DateTime.ToString() method returns the string representation of a date and time value using the current culture's short date and long time pattern. The following example uses the default DateTime.ToString() method to display the date and time using the short date and long time pattern for the en-US culture, the current culture on the computer on which the example was run.
You should be formatting the value of a DateTime type when you output it. If you want to store a formatted version of this value in your DataTable you will need to define the column as a string and then store the formatted value.
For more information see this link on the DateTime type.

A Datetime without time without string.format

I have a DataTable coming from a stored procedure which I'm writing to an excel file. There's a column with a DateTime datatype, and looking at the values in there, they're just generic dates with the time stamp.
I've tried using the DateTime.Date property, but that still gives me a time stamp. Further, I've tried to create a new DateTime object using the year,month,day constructor but it still adds a time stamp:
DateTime newDate = DateTime(oldDate.Year, oldDate.Month, oldDate.Day);
I'm trying to keep the column datatype to DateTime but remove the time stamp, so this rules out ToString("") formatting. Is there another way?

A DateTime always contains a date and a time portion. If you use the Date property it returns a new DateTime where the time is 0:00:00 so midnight at the same day. You want a string representation of the datetime without the time.
You can use DateTime.ToString:
string result = oldDate.ToString("d"); // uses current culture's date format
or
string result = oldDate.ToShortDateString(); // same as above
or
string result = oldDate.ToString("MM-dd-yyyy"); // custom format
Edit: "so this rules out ToString("") formatting. Is there another way?"
No, because of the reason mentioned above.
It's important to separate the data from how it is displayed. If you need to display it without time use the code above, you can store the original DateTime variable for future processing, select it again from database or use DateTime.Parse/DateTime.ParseExcact to get a DateTime from the string.

The 'problem' is that there is no Date struct in .NET, you only have a DateTime struct. That will always contain both date and time. You can only format it as date.
Or, you could of course write your own struct containing only the date part, or give up and use string.Format to format it as a date (possibly using the short date string d).

If you mean time part with timestamp, a DateTime instance always have both date and time part. DateTime.Date property just sets the time value set to midnight.
You can get it's string representation if you want only it's Date part. You can get standard date and time format or custom date and time format with DateTime.ToString() method.
Usually, you can use ShortDatePattern to get only string representation of Date part which uses standard "d" format of your CurrentCulture.
DateTime.Now.ToString("d");
There is a proposal for System.Date and System.Time types for .NET Framework in dotnet/corefx on GitHub page by the way.
Proposal: System.Date type

You should use DateTime.Now.ToShortDateString();

DateTime.TryParse - What was actually parsed?

I'm struggling a little bit in C# with DateTime.TryParse().
Essentially, given a string I need to extract the year and/or month and day in the current display culture. Sometimes I only get a year, or a month, or all three. Depending on what I get, I have a different control flow.
So far, I managed to parse a variety of strings into a DateTime; that isn't my problem.
My problem is that I wish to know WHAT was actually parsed (i.e. did I get a month or a year, or both).
The uninitialized DateTime defaults to 01/01/0001, and I cannot set everything to an invalid date, such as 99/99/9999 and then see what was filled.
I was thinking maybe I need to do regex, but the DateTime class provides that parsing for multiple cultures, which is very important in this project.
I've tried searching for this, but maybe I'm not using the right terms, because surely someone else must have had this issue before.
Update:
Here's some sample code of what I've got:
string strIn = Console.ReadLine();
DateTimeStyles enStyles = DateTimeStyles.AllowInnerWhite | DateTimeStyles.AllowLeadingWhite | DateTimeStyles.AllowTrailingWhite | DateTimeStyles.AssumeLocal;
bFound = DateTime.TryParse(strIn, CultureInfo.CreateSpecificCulture("en-US"), enStyles, out cDT);
Now, bFound will be true if something was parsed successfully. However, I need to know which parts of the date were parsed successfully

I dont understand you but are you looking for a specified format for your datetime?
string dateAndTimeFormat = "yyyy.MM.dd HH:mm:ss:fff"; // example of format
string dateAndTime = yourdatetimevalue;
DateTime toDateTime = DateTime.ParseExact(dateTime, dateTimeFormat, System.Globalization.CultureInfo.InvariantCulture)
How formats are used:
http://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.71).aspx
EDIT 1
The tryparse returns true or false. False if it fails. Maybee that can be usefull?
Otherwise you can set the culture before the tryparse, if you are able to do so.
DateTime.TryParse(dateString, culture, styles, out dateResult)
Check the examples here :http://msdn.microsoft.com/en-us/library/9h21f14e.aspx
Under remarks:
"This method tries to ignore unrecognized data, if possible, and fills in missing month, day, and year information with the current date. If s contains only a date and no time, this method assumes the time is 12:00 midnight. If s includes a date component with a two-digit year, it is converted to a year in the current culture's current calendar based on the value of the Calendar.TwoDigitYearMax property. Any leading, inner, or trailing white space character in s is ignored. The date and time can be bracketed with a pair of leading and trailing NUMBER SIGN characters ('#', U+0023), and can be trailed with one or more NULL characters (U+0000)."
Hope some of that helps.

The DateTime.TryParse() returns value only on success.
So for below code example the variable dt is initialized to 01/01/0001 00:00:00 when declared.
When TryParse tries to extract date from string(MM/DD/YYYY format), and if it failes, then dt variable is having value 01/01/0001 00:00:00. Otherwise dt will contain the actual extracted datetime value (as in 2).
1)
DateTime dt;
DateTime.TryParse("23/15/2013", out dt);
// dt contains "01/01/0001 00:00:00"`
2)
`DateTime dt;
DateTime.TryParse("23/12/2013 6:25", out dt);
// dt contains "23/12/2013 06:25:00"`
There is no need to check WHAT was actually parsed.
Datetime value will be parsed if it's valid otherwise default datetime value will be returned.

C# Date Conversion From Euro Time To en-US Time Needed For Date Calculations

I have a date that is entered through the system (from a database) as dd/mm/yy I need to programmatically convert the date to en-US format to mm/dd/yyyy so that I can do some date calculations within the code. The code that I have so far is:
String myJames = "25/04/13" // Date String comes in as non-US date
String myJames2 = System.DateTime.Today.ToString(myJames); // I think the problem is here
DateTime d1 = Convert.ToDateTime(myJames2);
DateTime d2 = DateTime.Now;
TimeSpan t = d2 - d1;
double NrOfDays = t.TotalDays;
I know this is not completely correct, especially in the first few lines. Any help getting the dates into one en-US format for effective comparisons would be greatly appreciated.

Just to check I understand your question. You have a date as a string and you want to convert that string into a datetime so you can use it in a calculation? And your problem is that the string isn't in the format that the locale the code is running in would use?
In which case use DateTime.ParseExact.
DateTime d1 = DateTime.ParseExact(myJames,"dd/MM/yy");
This line of code would replace your line declaring and assigning d1. The line assigning to myJames2 can be removed as it isn't needed.

Everytime you convert from or to a string, culturesettings are involved.
So.. if you are converting a DateTime to string, and your culture is en-US, it will automatically converted to: MM/dd/YYYY.
This is also true for converting back. If you convert a string back to a DateTime, the culturesettings are used to see what format the string is in.
Teh culture settings are always: Thread.CurrentThread.CurrentCulture.
Most conversion functions allow to override the format (like "MM/dd/yyyy") and/or the culture. So you can create your own culture and use this during conversions.
You say the database uses dd/MM/yy, but normaly a DateTime in a database is not formatted, it is just a binary value. Or is it stored as a text? If it is stored as a text, than you should ALWAYS convert it to a DateTime using the correct culture or format.

SQL datetime to C# string and back to SQL datetime

I have a webservice method that gets data from sql of the format
2012-11-18 11:21:03 when i save it to C# string it becomes this format: 18.11.2012 11:21:03
How do i change it back to the SQL format 2012-11-18 11:21:03 ?

Parse it into a dateTime again
DateTime myTime = DateTime.Parse(myString);
and back into a proper to string
myTime.ToString("yyyy-MM-dd HH:mm:ss");
Or just read it into a datetime and cut out the middleman.

You can get the universally sortable string format (which looks like the one used by SQL server) by using the format string "u" like this:
var dateTimeString = String.Format("{0:u}", yourDateTime);

Simply run the below code,
var newDateTime = oldDateTime.Date.ToString("yyyy-MM-dd HH:mm:ss");
Its just converting it back to the SQL Format DATETIME

Trouble with Dates as strings is they are ambiguous and the formats can vary based on where you are in the world, or even local machine settings. You might assume a date string is yyyy-mm-dd but what if it is actually yyyy-dd-mm? Some dates will appear to work and some will be invalid.
In other words is 2013-02-10 the 10th of February or is it the 2nd of October? If it is just a string you have no way of knowing for sure what was intended.
Your best bet as suggested by #Haedrian is to store in a DateTime C# object, not a string. That way it is never ambiguous and you have access to various date specific functions. If you must store as a string you can convert back to a date as above or use
DateTime.TryParse(datestring, out dateVariable);
which won't throw an exception for an invalid format. Depends if you want exceptions!
Also I would suggest if you must use strings to use a 3 character month in strings, which again eliminates the ambiguity, e.g.
"dd-MMM-yy hh:mm tt"