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How to validate date format in C#

I want to check if a given string is a valid .net date format.
so i have the following function that checks if the date format is correct
public void IsValidDateFormat(string dateFormat)
{
var flag = true;
try
{
DateTime.ParseExact(DateTime.Now.ToString(),
dateFormat,
System.Globalization.CultureInfo.InvariantCulture);
}
catch (Exception ex)
{
flag = false;
}
return flag;
}
However, The method is not working as expected. For a valid date format also it returns false.
dateFormat = ,, => returns false =>Passed
dateFormat = someinvalidformat => returns false =>Passed
dateFormat = MM/dd/yyyy => returns false => Not Passed
So MM/dd/yyyy is valid dateformat. But method returns false.
Is there a better way to check if given date format is valid .Net date format?
Update 1
I understand why method fails for MM/dd/yyyy or for other valid date formats. I am not asking why it fails. MM/dd/yyyy is just common valid date time format i am using here for example.
I am looking for a way to check if the given date format is valid .Net date format. So it could be any .Net date format.

Since the format returned by DateTime.ToString does not match your format (it includes the time part), ParseExact fails.
Validate the format by using public string ToString(string format, System.IFormatProvider provider) instead
public bool IsValidDateFormat(string dateFormat)
{
try {
string s = DateTime.Now.ToString(dateFormat, CultureInfo.InvariantCulture);
DateTime.Parse(s, CultureInfo.InvariantCulture);
return true;
} catch {
return false;
}
}
Note that date/time formats that may seem not to be valid, can in fact be valid, as some non-format characters are just outputted as is. E.g.
DateTime.Now.ToString("abcdefg", CultureInfo.InvariantCulture)
results in "abc27e6A.D.". So it is in fact a valid date/time format, even if it does not make a lot of sense. You can enhance the quality of the test by trying to parse the resulting date string. This will eliminate a lot of nonsensical formats.
This test ...
Console.WriteLine(IsValidDateFormat(",,"));
Console.WriteLine(IsValidDateFormat("a"));
Console.WriteLine(IsValidDateFormat("MM/dd/yyyy hh:mm:ss"));
Console.WriteLine(IsValidDateFormat("MM/dd/yyyy"));
Console.WriteLine(IsValidDateFormat("abcdefg"));
Console.ReadKey();
... prints
False
False
True
True
False

why not TryParse() or exact version? https://learn.microsoft.com/es-es/dotnet/api/system.datetime.tryparse?view=netframework-4.8
returns true if system can parse.
It does "the same" your method does.

Detect the date format C#

I have some automated C# windows services to upload text to the database. The 'text' is generated by a third party application where we don't have any control.
My issue is that the text contains a column for date.
The default date format is DD/MM/YY. But some times we get MM/YY/DD
Is there any tricky way to identify or convert MM/YY/DD to DD/MM/YY. The data might only contain date for three, four days. So I plan to check if the date is in tolerance with three or four days, it will be accepted. Other wise manually correct it.
For example,
14/08/17 is accepted
08/17/14 is not accepted. Logic should convert this to 14/08/17
Any ideas ?

You could try and parse with the good format, if it goes ok there is no problem, it will return the date. If it goes wrong, you tryparse with the 'secondary format'. If everything goes ok, it will return the date.
Note that if the parsed date it's more than 3 days ahead, it won't count as a valid date and will return null.
if(DateTime.TryParseExact(input, "dd/MM/yy", CultureInfo.InvariantCulture, DateTimeStyles.None, out dateValue)
{
int daysBetween = (dateValue-DateTime.Now).Days
if(daysBetween < 4)
{
return dateValue
}
}
if(DateTime.TryParseExact(input, "MM/yy/dd", CultureInfo.InvariantCulture, DateTimeStyles.None, out dateValue)
{
int daysBetween = (dateValue-DateTime.Now).Days
if(daysBetween < 4)
{
return dateValue;
}
}
return null
If null gets returned, you'll have an invalid date, if not, that will be the parsed date

Timespan calculate difference

I got 2 textboxes, and want to calculate the difference of the time between those 2.
And i got it working, but it´s barely, at least i think i just made it working with some "hacks".
It will also fail if something is not correct, and will throw an exception, which i would like to prevent (probably with an IF, to just ignore if it´s not correct).
Here is the code:
private void button1_Click(object sender, EventArgs e)
{
try
{
var first = TimeSpan.ParseExact(First.Text.Replace("-", ":").Remove(First.Text.LastIndexOf("-"), 1).Insert(First.Text.Length - 4, "."), "g", CultureInfo.InvariantCulture);
var last = TimeSpan.ParseExact(Last.Text.Replace("-", ":").Remove(First.Text.LastIndexOf("-"), 1).Insert(First.Text.Length - 4, "."), "g", CultureInfo.InvariantCulture);
var difference = first - last;
CalcDiff.Text = difference.TotalSeconds.ToString("F3");
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}
AS you can see, it´s many "replace" and all kinds of stuff, just to make it work.
The formatting i am providing is this:
2013-05-22 12-07-55-324
And for that matter, i only care about the time, which is: 12-07-55-324
So, i am telling it to replace - with :, to just make itwork, and stuff like that.
So, is there a way to improve that. And can i make some kind of Condition, so i can´t just, press the button, and it will fail if it´s not correct (for example the formatting is wrong, or it´s nothing there, or Text instead).

You can use TimeOfDay of a DateTime which can be parsed via DateTime.ParseExact:
string dtString = "2013-05-22 12-07-55-324";
TimeSpan ts = DateTime.ParseExact(dtString, "yyyy-MM-dd HH-mm-ss-fff", CultureInfo.InvariantCulture).TimeOfDay;
DEMO
You can use DateTime.TryParseExact to check if the string can be parsed successfully:
DateTime firstDt;
DateTime lastDt;
if ( DateTime.TryParseExact(First.Text, "yyyy-MM-dd HH-mm-ss-fff", CultureInfo.InvariantCulture, DateTimeStyles.None, out firstDt )
&& DateTime.TryParseExact(Last.Text, "yyyy-MM-dd HH-mm-ss-fff", CultureInfo.InvariantCulture, DateTimeStyles.None, out lastDt ))
{
// successfully parsed both Datetimes
var difference = firstDt.TimeOfDay - lastDt.TimeOfDay;
CalcDiff.Text = difference.TotalSeconds.ToString("F3");
}
Here is a more readable approach since you are concerned about complexity of the if-statement:
DateTime firstDt, lastDt;
bool canParseFirst = DateTime.TryParseExact(First.Text, "yyyy-MM-dd HH-mm-ss-fff", CultureInfo.InvariantCulture, DateTimeStyles.None, out firstDt);
bool canParseLast = DateTime.TryParseExact(Last.Text, "yyyy-MM-dd HH-mm-ss-fff", CultureInfo.InvariantCulture, DateTimeStyles.None, out lastDt );
if(canParseFirst && canParseLast)
{
// successfully parsed both Datetimes
var difference = firstDt.TimeOfDay - lastDt.TimeOfDay;
CalcDiff.Text = difference.TotalSeconds.ToString("F3");
}
else
{
// inform the user
}

You should use DateTime instead, you'll still be able to get the difference in the two times pretty easily (just Google finding the difference between two DateTime objects), plus you can use TryParseExact to specify strange formats like you have there. PLUS that method will help alleviate those pesky exceptions you mentioned! Instead of throwing an exception, it will simply return true if it parsed successfully and false otherwise.

Valid date check with DateTime.TryParse method

I am using Datetime.TryParse method to check the valid datetime. the input date string would be any string data. but is returning false as the specify date in invalid.
DateTime fromDateValue;
if (DateTime.TryParse("15/07/2012", out fromDateValue))
{
//do for valid date
}
else
{
//do for in-valid date
}
Edit: I missed. I need to check the valid date with time as "15/07/2012 12:00:00".
Any suggestions are welcome.

You could use the TryParseExact method which allows you to pass a collection of possible formats that you want to support. The TryParse method is culture dependent so be very careful if you decide to use it.
So for example:
DateTime fromDateValue;
string s = "15/07/2012";
var formats = new[] { "dd/MM/yyyy", "yyyy-MM-dd" };
if (DateTime.TryParseExact(s, formats, CultureInfo.InvariantCulture, DateTimeStyles.None, out fromDateValue))
{
// do for valid date
}
else
{
// do for invalid date
}

You should be using TryParseExact as you seem to have the format fixed in your case.
Something like can also work for you
DateTime.ParseExact([yourdatehere],
new[] { "dd/MM/yyyy", "dd/M/yyyy" },
CultureInfo.InvariantCulture,
DateTimeStyles.None);

As the others said, you can use TryParseExact.
For more informations and the use with the time, you can check the MSDN Documentation

Detect format Date

Is there a way, a good way, to test if a string than I want to transform in DateTime is dd/MM/yyyy or MM/dd/yyyy ?
Thanks,

No, because it could be both. Is 11/10/2010 November 10th or October 11th?
Yes, in some cases (if one number is above 12) it will be unambiguous - but I think it's better to force one format or the other. If you just treat anything which can be done as MM/dd/yyyy that way, and move on to dd/MM/yyyy if it fails (or the other way round) then you'll get some very surprised users.
If this is part of a web application or something similar, I would try to make it completely unambiguous by using month names instead of numbers where possible.

No, but you could try both when parsing:
DateTime result;
if (DateTime.TryParseExact(
"05/10/2010",
new[] { "MM/dd/yyyy", "dd/MM/yyyy" },
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out result)
)
{
// successfully parsed date => use the result variable
}

This problem will exist until all accepts and uses the ISO way. I'm a Swedish programmer working a lot with American and English clients and it's surprisingly hard to get these clients to use the standardized date format.
ISO 8601 - Use it!

Take a look at DateTime.ParseExact and DateTime.TryParseExact.
string date1 = "24/12/2010";
CultureInfo provider = CultureInfo.InvariantCulture;
DateTime dt1 = new DateTime(1, 1, 1);
bool dt1Valid = false;
try
{
dt1 = DateTime.ParseExact(date1, "dd/MM/yyyy", provider);
dt1Valid = true;
}
catch
{
dt1Valid = false;
}