I have DAL where I convert database null value to their equivalent representation in C#. For example:
NULL for Numeric = 0
NULL for String = String.Empty
NULL for DateTime = "1/1/0001" (i.e. DateTime.MinValue)
The problem, for date, lies in the presentation layer, especially in GridViews. You cannot show 1/1/01 to users.
What I used to do is check if myDate.Year=1 or myDate.Year < AcceptedDate and display empty string, but seems to be extra effort unlike other types
Please am open to better approach. Thanks.
Use Nullable datatype to store null value.
DateTime? value = null;
int? myNullableInt = 1;
value = DateTime.Now;
How to check whether variable has value or null
if (value!=null)
String value can store null, so there is no diffrent datatype for string to store null.
string var;
if (var == null)
or
if (String.IsNullOrEmpty(var))
You can also use DateTime.MinValue constant.
http://msdn.microsoft.com/en-us/library/system.datetime.minvalue.aspx
Your conditions would be:
if (myDate == DateTime.MinValue)
You can use Nullable DateTime, so you will return DateTime? instead of DateTime from your DAL. This way you can check if returned value is null.
DateTime? dateTime = null;
As the others mention, you could use a System::Nullable<DateTime>.
The other approach I've seen is to use a standard DateTime and just use a special value such as DateTime.MinValue. This is useful if you need to honor an existing interface's types and can't change the DateTime to a Nullable<DateTime>.
You can either use a Nullable DateTime as the others suggested, or use this trick:
(To prevent non valid defaults.)
// If dateTime has not been initialize, initialize to Now
// (or to any other legal inital values)
dateTime = ((dateTime != new DateTime()) ? dateTime : DateTime.Now);
This trick is useful if you have to use a non-nullable DateTime and want to provide a default if none. (E.g. you have a non-nullable DateTime column in a DB and want to set the value only if row is new.)
I don't think you have much choice but to make the check like you have been and display accordingly. A nullable type might make things easier for you. Depending on your data, even the numeric should be treated this way. DBNull != 0.
Related
if (File.Exists(settingsFile))
{
string[] lines = File.ReadAllLines(settingsFile);
if (lines.Length > 0)
{
trackBarHours.Value = Convert.ToInt32(optionsfile.GetKey("trackbarhours"));
trackBarMinutes.Value = Convert.ToInt32(optionsfile.GetKey("trackbarminutes"));
trackBarSeconds.Value = Convert.ToInt32(optionsfile.GetKey("trackbarseconds"));
savedMilliseconds = Convert.ToInt32(optionsfile.GetKey("milliseconds"));
dateTimePicker1.Value = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue"));
richTextBox1.Text = optionsfile.GetKey("result");
}
}
because the key "timetargetvalue" is not yet created in the settingsFile because i didn't saved it yet for the first time the value of the key of "timetargetvalue" is '01/01/0001 00:00:00'
in that case that there is no yet the key hwo can i handle the datetime exception ?
dateTimePicker1 is a DateTimePicker control.
the exception is on the line :
dateTimePicker1.Value = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue"));
System.ArgumentOutOfRangeException: 'Value of '01/01/0001 00:00:00' is not valid for 'Value'. 'Value' should be between 'MinDate' and 'MaxDate'.
Parameter name: Value'
what should i check against of so it will not throw the exception ?
DateTimePicker.Value must be above DateTimePicker.MinimumDateTime, which is 'January 1, 1753'.
When you haven't set the timetargetvalue, it will resolve to '01/01/0001 00:00:00', as you have seen, which is too early.
So you need to check the value before assigning it to DateTimePicker.Value.
You can do it like this:
DateTime tempDateTime = Convert.ToDateTime(optionsfile.GetKey("timetargetvalue");
dateTimePicker1.Value = tempDateTime >= DateTimePicker.MinimumDateTime ? tempDateTime : DateTimePicker.MinimumDateTime;
When dealing with a Struct such as DateTime that does not have any value we need to consider that this is not a class and can not be set to null. It must always have some value. (see https://learn.microsoft.com/en-us/dotnet/api/system.datetime?view=net-7.0)
The exception mentions in a round about way that the range of acceptable values is between dateTimePicker1.MinDate and dateTimePicker1.MaxDate so one option is to check if your value is within this range. But it's unlikely to be the best option. (see https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.datetimepicker.mindate?view=windowsdesktop-6.0)
I'm pretty sure that DateTime default value is equal to that of DateTime.Min but if you really wanted to check if the value is default then I would suggest comparing it to default(DateTime) would be better.
This pretty much covers the use of DateTime and value defaults when null is not an option. Which brings up a possibly more desirable option. Encapsulation.
We could instead encapsulate the DateTime struct into a Nullable class. The encapsulating class will be nullable and will also be able to present the encapsulated value through a property called Value. (see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-value-types)
There are two ways to declare such a Nullable class, both of which compile to the same thing:
Nullable<DateTime> myNullableDate = null;
DateTime? anotherNullableDate = null;
Since the DateTime is encapsulated in a Nullable object we can start using a null reference check. We can also call a method on Nullable called HasValue which returns a bool (True if it has a value).
EDIT: I notice that you're not doing any checks before trying to parse the DateTime and then directly setting it into the DateTimePicker.Value which can accept a null value. (although setting null won't clear a previously set value).
As such perhaps what you might want to do is handle the scenario a bit better and then use a DateTime.TryParse() instead. (see https://learn.microsoft.com/en-us/dotnet/api/system.datetime.tryparse?view=net-7.0)
e.g. (not the most optimized code, but I think it's easier to follow along in a more verbose form)
private DateTime? LoadDateFromOptions(string key)
{
var rawValue = optionsfile.GetKey(key);
if (!string.IsNullOrEmpty(rawValue))
{
return null;
}
DateTime dateValue;
bool isSuccess = DateTime.TryParse(rawValue, out dateValue);
if (isSuccess)
{
return dateValue;
}
else
{
return null;
}
}
and then instead of having that exception you can load the value optionally a bit more like this:
var timeTarget = LoadDateFromOptions("timetargetvalue");
if (timeTarget != null)
{
dateTimePicker1.Value = timeTarget;
}
I have two variables of type DateTime and DateTime?
DateTime StartDateFromDb;
DateTime? StartDateFromFilter;
if(StartDateFromDb.Date == StartDateFromFilter.Date);
While comparing, .Date is not allowingfor StartDateFromFilter of type allow null
Thanks in advance
Use the Value property available as
if(StartDateFromFilter.HasValue && StartDateFromDb.Date == StartDateFromFilter.Value.Date)
PS: Better to add a null value check. StartDateFromFilter must have a value.(HasValue is true when DateTime? type variable is not null)
For any nullable type , you can use value property.
StartDateFromFilter.Value.Date
In your case , this should work fine
if(StartDateFromDb.Date == StartDateFromFilter.Value.Date)
//// in this case .Date is not allowingfor StartDateFromFilter
I would like to check if the decimal number is NULL or it has some value, since the value is assigned from database in class object:
public decimal myDecimal{ get; set; }
and then I have
myDecimal = Convert.ToDecimal(rdrSelect[23].ToString());
I am trying:
if (rdrSelect[23] != DBNull.Value)
{
myDecimal = Convert.ToDecimal(rdrSelect[23].ToString());
}
But I am getting this:
the result of the expression is always 'true' since a value of type
'decimal' is never equal to null
How can I check if that decimal number has some value?
A decimal will always have some default value. If you need to have a nullable type decimal, you can use decimal?. Then you can do myDecimal.HasValue
you can use this code
if (DecimalVariable.Equals(null))
{
//something statements
}
decimal is a value type in .NET. And value types can't be null. But if you use nullable type for your decimal, then you can check your decimal is null or not. Like myDecimal?
Nullable types are instances of the System.Nullable struct. A nullable
type can represent the normal range of values for its underlying value
type, plus an additional null value.
if (myDecimal.HasValue)
But I think in your database, if this column contains nullable values, then it shouldn't be type of decimal.
I've ran across this problem recently while trying to retrieve a null decimal from a DataTable object from db and I haven't seen this answer here. I find this easier and shorter:
var value = rdrSelect.Field<decimal?>("ColumnName") ?? 0;
This was useful in my case since i didn't have a nullable decimal in the model, but needed a quick check against one. If the db value happens to be null, it'll just assign the default value.
Assuming you are reading from a data row, what you want is:
if ( !rdrSelect.IsNull(23) )
{
//handle parsing
}
Decimal is a value type, so if you wish to check whether it has a value other than the value it was initialised with (zero) you can use the condition myDecimal != default(decimal).
Otherwise you should possibly consider the use of a nullable (decimal?) type and the use a condition such as myNullableDecimal.HasValue
If you're pulling this value directly from a SQL Database and the value is null in there, it will actually be the DBNull object rather than null. Either place a check prior to your conversion & use a default value in the event of DBNull, or replace your null check afterwards with a check on rdrSelect[23] for DBNull.
You can also create a handy utility functions to handle values from DB in cases like these.
Ex. Below is the function which gives you Nullable Decimal from object type.
public static decimal? ToNullableDecimal(object val)
{
if (val is DBNull ||
val == null)
{
return null;
}
if (val is string &&
((string)val).Length == 0)
{
return null;
}
return Convert.ToDecimal(val);
}
Q:
I want to check the DateTime against null value to empty the cell in my report if the datetime is null.but i don't know how to do this :it appears like this 1/1/0001 if it was null.and i want it to be empty cell.
This is the datatype in my dataset :
and this is the expression value of my column :
=FormatDateTime(Fields!D_DateTime.Value,2)
As I told you in my comment, you should check if your date is DateTime.MinValue (the minimum value a date can assume, which is exactly 01/01/0001).
if (your_date_property == DateTime.MinValue)
{
// Do what you need
}
=IIf(FormatDateTime(Fields!D_DateTime.Value,2)=CDate("1/1/0001"),"",FormatDateTime(Fields!D_DateTime.Value,2))
Thanks a lot ,i think this fixes my problem.
As datetime is a struct rather than class i.e. a value type rather than a reference type; it must be initialized with some value. It cannot have null values.
Hence to check the default value you should check the equality with DateTime.MinValue
i.e.
if(D_DateTime.Value == DateTime.MinValue)
{
//write code here for default value handling
}
Change the type of the field in the dataset (rd:TypeName) to System.Nullable (Of System.DateTime). Then you can simply test =Fields!D_DateTime.Value Is Nothing.
Like #Marco suggested you can check for MinValue. And if you want to pass NULL to the nullable parameter, you can use the following code for reportviewer parameter.
Vb.Net
Dim rpFrom As New ReportParameter("FromDate", New String() {Nothing}, False)
C#
ReportParameter rpFrom = new ReportParameter("FromDate", new string[] { null }, false);
I'm trying to reuse the same code I've always used but now it is encountering an error.
I'm looping through various user tables, and in there I do this:
DateTime dcdt = (DateTime)u.DateCreated;
DateTime lldt = (DateTime)u.LastLogon;
userRow["DateCreated"] = dcdt.ToShortDateString();
inside the loop. I get the error:
System.InvalidOperationException: Nullable object must have a value.
The error highlights "lldt" line, instead of "dcdt" which comes first. That is strange in and of itself. Both these fields in the database "allow nulls" is checked. And they both could be null or neither might be null.
The two values are both listed as DateTime? types through intellisense.
I don't understand why ASP.NET refuses to allow me to output blank for null dates. If it is empty/null, then logic would suggest that ASP.NET should just print nothing.
How else am I suppose to output null dates? I tried adding if statements to prevent trying to cast null DateTimes, but it doesn't help, it makes no sense.
As you've said, the data type of u.LastLogon is DateTime?. This means that it may or may not have a value. By casting to DateTime, you are requiring it to have a value. In this case, it does not.
Depending on what you're trying to do with it, you may want to check the HasValue property:
userRow["LastLogon"] = u.LastLogin.HasValue ?
(object) u.LastLogin.ToShortDateString() : DBNull.Value;
If your database LastLogon column is of DateTime type, then you should be able to do:
userRow["LastLogon"] = u.LastLogin.HasValue ?
(object) u.LastLogin.Value : DBNull.Value;
You need to do something like the following in your data access code:
DataTable dt = ExecuteSomeQuery() ;
object value = dt.Rows[0]["nullable_datetime_column"] ;
DateTime? instance = value != null && value is DateTime ? (DateTime?)value : (DateTime?)null ) ;
If the column returned is NULL, it will be returned as a System.DBNull, otherwise it will be returned as an instance of DateTime (or whatever the appropriate mapped type is — int, string, etc). Consequently, you need to check the type of object returned from the query before trying to cast it.
Looks like you are trying to call a method (dcdt.ToShortDateString()) on a DateTime? which doesn't have a value (it is, indeed, null). Try this:
dcdt.HasValue ? dcdt.ToShortDateString() : String.Empty;
EDIT (Just re-read the question): Also, don't try to convert to DateTime. Preserve the nullable.
EDIT #2 (based on comments):
Try this:
if (dcdt.HasValue)
{ userRow["DateCreated"] = dcdt.ToShortDateString(); }
else
{ userRow = DbNull.Value }
I saw that Dexter asked how he should go about it. Well, I would create an extension.
static class DateTimeExtensions
{
public static string ToString(this DateTime? dateTime, string format)
{
return dateTime.HasValue ? dateTime.Value.ToString(format) : String.Empty;
}
}
And then you can do:
DateTime? dt = null;
DateTime? dt2 = DateTime.Now;
Console.WriteLine(dt.ToString("dd-MM-yy"));
Console.WriteLine(dt2.ToString("dd-MM-yy"));
Note that I can call extension method on a nullable type if the object is null.
The problem is .NET null is not the same as SQL NULL. SQL Null is System.DBNull. So it is a [non-null] value in .NET.
Short answer
DateTime? dateTime = u.LastLogon?.ToShortDateString()