There is paradox in the exception description:
Nullable object must have a value (?!)
This is the problem:
I have a DateTimeExtended class,
that has
{
DateTime? MyDataTime;
int? otherdata;
}
and a constructor
DateTimeExtended(DateTimeExtended myNewDT)
{
this.MyDateTime = myNewDT.MyDateTime.Value;
this.otherdata = myNewDT.otherdata;
}
running this code
DateTimeExtended res = new DateTimeExtended(oldDTE);
throws an InvalidOperationException with the message:
Nullable object must have a value.
myNewDT.MyDateTime.Value - is valid and contain a regular DateTime object.
What is the meaning of this message and what am I doing wrong?
Note that oldDTE is not null. I've removed the Value from myNewDT.MyDateTime but the same exception is thrown due to a generated setter.
You should change the line this.MyDateTime = myNewDT.MyDateTime.Value; to just this.MyDateTime = myNewDT.MyDateTime;
The exception you were receiving was thrown in the .Value property of the Nullable DateTime, as it is required to return a DateTime (since that's what the contract for .Value states), but it can't do so because there's no DateTime to return, so it throws an exception.
In general, it is a bad idea to blindly call .Value on a nullable type, unless you have some prior knowledge that that variable MUST contain a value (i.e. through a .HasValue check).
EDIT
Here's the code for DateTimeExtended that does not throw an exception:
class DateTimeExtended
{
public DateTime? MyDateTime;
public int? otherdata;
public DateTimeExtended() { }
public DateTimeExtended(DateTimeExtended other)
{
this.MyDateTime = other.MyDateTime;
this.otherdata = other.otherdata;
}
}
I tested it like this:
DateTimeExtended dt1 = new DateTimeExtended();
DateTimeExtended dt2 = new DateTimeExtended(dt1);
Adding the .Value on other.MyDateTime causes an exception. Removing it gets rid of the exception. I think you're looking in the wrong place.
When using LINQ extension methods (e.g. Select, Where), the lambda function might be converted to SQL that might not behave identically to your C# code. For instance, C#'s short-circuit evaluated && and || are converted to SQL's eager AND and OR. This can cause problems when you're checking for null in your lambda.
Example:
MyEnum? type = null;
Entities.Table.Where(a => type == null ||
a.type == (int)type).ToArray(); // Exception: Nullable object must have a value
Try dropping the .value
DateTimeExtended(DateTimeExtended myNewDT)
{
this.MyDateTime = myNewDT.MyDateTime;
this.otherdata = myNewDT.otherdata;
}
Assign the members directly without the .Value part:
DateTimeExtended(DateTimeExtended myNewDT)
{
this.MyDateTime = myNewDT.MyDateTime;
this.otherdata = myNewDT.otherdata;
}
In this case oldDTE is null, so when you try to access oldDTE.Value the InvalidOperationException is thrown since there is no value. In your example you can simply do:
this.MyDateTime = newDT.MyDateTime;
Looks like oldDTE.MyDateTime was null, so constructor tried to take it's Value - which threw.
I got this message when trying to access values of a null valued object.
sName = myObj.Name;
this will produce error. First you should check if object not null
if(myObj != null)
sName = myObj.Name;
This works.
I got this solution and it is working for me
if (myNewDT.MyDateTime == null)
{
myNewDT.MyDateTime = DateTime.Now();
}
Related
Can anyone help me to figure out as to why I am getting this error message, when I didn't enter in any date or time when I click the submit button?
An exception of type 'System.InvalidOperationException' occurred in
mscorlib.dll but was not handled in user code Additional information:
Nullable object must have a value.
int result1 = this.startDate.SelectedDate.Value.Date.CompareTo(DateTime.Now.Date);
int result2 = this.endDate.SelectedDate.Value.Date.CompareTo(this.startDate.SelectedDate.Value.Date);
if (result1 >= 0 && result2 > 0)
{
//To Do navigate to sprint dashboard
// var userProjects = new UserStory();
//var sprintDashboardPage = new SprintDashboardPage(usersProjects);
//var mainWindow = this.GetWindow() as MainWindow;
//mainWindow.MainFrame.Navigate(sprintDashboardPage);
this.ErrorMessage = "Date is VALID";
this.CloseWindow();
}
else
{
this.ErrorMessage = "Choose Valid Date";
}
this.startDate.SelectedDate is a Nullable<DateTime>. This means you have to check whether this.startDate.SelectedDate has a value assigned before using it by doing:
if(this.startDate.SelectedDate.HasValue) {
this.startDate.SelectedDate.Value.Date.CompareTo(DateTime.Now.Date);
}
The following will also work:
if(this.startDate.SelectedDate != null) {
this.startDate.SelectedDate.Value.Date.CompareTo(DateTime.Now.Date);
}
Same applies when accessing this.endDate.SelectedDate.
In C#, DateTime is Value Type which means it must always have a value assigned to it. In other words, it can't be set to null. Its default value is DateTime.MinValue. Int32 (or int) is another example of a Value Type and it's default value is 0.
In order to make a Value Type support null, you can wrap it with Nullable<>. Doing so means that you have to explicitly check whether a value has been assigned before accessing the value, or an InvalidOperationException will be thrown.
The DateTime picker control in WPF returns a Nullable<DateTime> object so that you can know whether a user has actually selected a value in the control, as it will be set a non-null value.
You're calling the Value property on a Nullable<> without first checking its HasValue property returns true.
The exception is thrown in the .Value property of the DateTime? (nullable DateTime), as it is required to return a DateTime, but it can't because there's no DateTime to return.
It is a bad idea to access .Value property of Nullable type without checking it. You should consider adding null-checking code. ( e.g. with HasValue property )
I have a SqlParameter contained in a var coming back from a stored procedure and the value is either Null, an int or -1. How do cast the var to check if it is -1 in C#?
var p_eventID = new SqlParameter()
{
ParameterName = "#EventID",
Value = (eventID.HasValue
? (object)eventID.Value
: (object)System.Data.SqlTypes.SqlInt32.Null),
Direction = ParameterDirection.InputOutput
};
I have tried:
if ((int?)eventID.Value == -1)
But it gives me an invalid cast error.
SQL requests report NULLs to C# through DBNull objects, forcing the cast to fail.
You can do the other kind of cast to int? without triggering an error:
var eventIdInt = eventId as int?;
if (eventIdInt == -1) {
...
}
Rather than throwing an exception, the x as type style of cast would produce a null value when the cast fails.
Your approach when an error occurs, it will throw an Invalid Cast Exception. However, if you utilize the as int if it fails it will return a null. Which will be a bit more graceful, and forgiving.
So you could do the following:
if(eventId != null)
{
// Do Stuff.
}
So that returned value from your scalar, should either contain an actual int or a null.
So in order to solve why you receive the error with the (int) we would need to see how you implement the scalar and know the type of object it is actually returning before the cast.
I am returning a scalar value from a SQL Server 2008 database:
string reason = cmd.ExecuteScalar().ToString() ?? : "";
I want to make sure that if null is returned, that reason = "" and not null.
i am getting an error on this line:
Error 3 Invalid expression term ':'
How can this be fixed?
EDIT:
thank you for the changes on the colon, now i am getting this exception on the same line:
string reason = cmd.ExecuteScalar().ToString() ?? "";
System.NullReferenceException occurred
Message="Object reference not set to an instance of an object."
Try this:
string reason = cmd.ExecuteScalar().ToString() ?? "";
BUT: this will still fail, since if .ExecuteScalar() returns a NULL, you're already causing a Null Reference Exception by calling .ToString() on that NULL value......
So I guess the ?? operator really doesn't help you here... do the "usual" dance:
object result = cmd.ExecuteScalar();
if(result != null)
{
reason = result.ToString();
}
else
{
reason = "(NULL value returned)";
}
First, you shouldn't have the : when using the ?? operator.
Second, to do what you are trying to do here without getting an error, you need to do it differently:
object objReason = cmd.ExecuteScalar();
string reason = objReason == null ? "" : objReason.ToString();
This will check whether or not your returned value is null and if it is, the second line will set reason to a blank string, otherwise it will use your returned value.
Since ExecuteScalar() returns object that might be null you should not call .ToString() since that may throw and exception.
string reason = Convert.ToString(cmd.ExecuteScalar());
This works because Convert.ToString() will convert null to string.Empty
or if you must use ?? because you really like it:
(cmd.ExecuteScalar() ?? (object)"").ToString();
Just get rid of the colon.
string reason = cmd.ExecuteScalar().ToString() ?? "";
For reference, check the MSDN page.
When using the null-coalescing operator, you don't need the colon:
string reason = cmd.ExecuteScalar().ToString() ?? "";
As others have pointed out though, ToString() would cause a NullReferenceExcpetion to be thrown anyway...so you don't gain anything here. You'd be much better off splitting this into multiple lines:
var result = cmd.ExecuteScalar();
string reason = result == null ? "" : result.ToString();
You're confusing the ? conditional operator, the syntax for which looks like this:
String x = condition ? valueIfConditionIsTrue : valueIfConditionIsFalse;
with the ?? null-coalesce operator whose syntax is as follows:
String x = possiblyNull ?? valueIfPossiblyNullIsNull;
So, apart from all that... this is the part you really want:
String reason = (cmd.ExecuteScalar() ?? "").ToString();
This takes care of your exception where ToString() was causing a null-reference exception.
Just use
string reason = cmd.ExecuteScalar() ?? "";
Does anyone know how can I check whether a session is empty or null in .net c# web-applications?
Example:
I have the following code:
ixCardType.SelectedValue = Session["ixCardType"].ToString();
It's always display me error for Session["ixCardType"] (error message: Object reference not set to an instance of an object). Anyway I can check the session before go to the .ToString() ??
Something as simple as an 'if' should work.
if(Session["ixCardType"] != null)
ixCardType.SelectedValue = Session["ixCardType"].ToString();
Or something like this if you want the empty string when the session value is null:
ixCardType.SelectedValue = Session["ixCardType"] == null? "" : Session["ixCardType"].ToString();
Cast the object using the as operator, which returns null if the value fails to cast to the desired class type, or if it's null itself.
string value = Session["ixCardType"] as string;
if (String.IsNullOrEmpty(value))
{
// null or empty
}
You can assign the result to a variable, and test it for null/empty prior to calling ToString():
var cardType = Session["ixCardType"];
if (cardType != null)
{
ixCardType.SelectedValue = cardType.ToString();
}
When looping through a DataRow and encountering types such as
DataRow dr;
dr["someString"]
dr["someInteger"]
dr["somedata"]
What's the best way to get them into their corresponding data types? dr["foo"] is just a generic object.
Also, are these able to be easily converted to nullable types? dr["someInteger"] could be null.
When reading from a DataRow, your biggest enemy is a null value. In a DataRow, when a value is null, it is not equals to null: It is equals to DBNull.Value.
if(DBNull.Value == null)
{
// Will never happen
}
Unless you know that your field cannot be null, it is not safe to cast. For example, the following example will fail if the data is DBNull:
string name = (string)dr["Name"];
If you can use the LINQ extensions, you can include the reference System.Data.DataSetExtensions and the namespace System.Data and call
string name = dr.Field<string>("Name");
If you cannot use LINQ, then you have to fall back to checking for null value with
string name = null;
if(!dr.IsNull("Name"))
name = (string)dr["Name"];
Or you could code your own Field function like this:
public static T GetValue<T>(object value)
{
if (value == null || value == DBNull.Value)
return default(T);
else
return (T)value;
}
and get your value this way:
string name = GetValue<string>(dr["Name"]);
If you can use .net 3.5, then you can use the Field extension method to more easily access the data if you know the type. An example would be:
string somestring= row.Field<string>("SomeString");
Otherwise you're stuck with casting the field to the type of the object the old fashioned way.
Simply casting the values to the right type should work:
(string) dr["someString"];
(int?) dr["someInteger"];
(byte[]) dr["somedata"];
string GetString(DataRow dr, string ColumnName)
{
if (dr.IsNull(ColumnName))
{
return null;
}
return (string)dr[ColumnName];
}
Another option is to use "as"
string str = dr["someString"] as string;
if it's DBNull.Value (or any other object not of type string), then str will get a real "null". Otherwise it will get the proper string value.
For value types, you can use nullable, i.e.
int? i = dr["someint"] as int?;
Again, it will get a real "null" instead of DBNull.Value. However, with nullable types you have to remember to use .Value, i.e.
int x = i.Value + 5;