This is my code:
DateTime? test;
test = ((objectParsed.birthday != null) ? DateTime.Parse((string)objectParsed.birthday : null));
Why can I not set that variable to null?
Aside from anything to do with Nullable<T> (in this case, DateTime?), the error is happening specifically here:
((objectParsed.birthday != null) ? DateTime.Parse((string)objectParsed.birthday : null))
Note that there's no mention of a nullable DateTime in this code. And before the result of this code is assigned to a nullable DateTime, this code by itself needs to be evaluated. It can't be, because of the error you're seeing.
The operator being used (: ?) needs to be able to infer types from all arguments to the operation, and those types need to be able to match. Here you're passing it a DateTime and null which can't be matched. Try casting one of the arguments:
((objectParsed.birthday != null) ? (DateTime?)DateTime.Parse((string)objectParsed.birthday : null))
You can't set null in this case because ternary operator must return values same types
try this:
test = (objectParsed.birthday != null) ? (DateTime?)DateTime.Parse((string)objectParsed.birthday): null;
Try with
test = ((objectParsed.birthday != null) ? DateTime.Parse((string)objectParsed.birthday): null;
Explanation: the structure of a ternary operator is:
variable = (condition)?(value if yes):(value if no);
Related
How can I check if any key from json object have null value
JsonObject itemObject = itemValue.GetObject();
string id = itemObject["id"].GetString() == null ? "" : itemObject["id"].GetString();
this is my code but app crashes on it if null value for key "id"
IJsonValue idValue = itemObject.GetNamedValue("id");
if ( idValue.ValueType == JsonValueType.Null)
{
// is Null
}
else if (idValue.ValueType == JsonValueType.String)
{
string id = idValue.GetString();
}
If you do this too much, consider adding extension methods.
To do the opposite use:
IJsonValue value = JsonValue.CreateNullValue();
Read here more about null values.
http://msdn.microsoft.com/en-us/library/ms173224.aspx
The ?? operator is called the null-coalescing operator. It returns the left-hand operand if the operand is not null; otherwise it returns the right hand operand.
if itemObject["id"] is null then the method null.GetString() doesn't exist and you'll get the error specified (null object never has any methods/fields/properties).
string id = itemObject["id"] == null ? (string)null : itemObject["id"].GetString(); // (string)null is an alternative to "", both are valid null representations for a string, but you should use whichever is your preference consistently to avoid errors further down the line
the above avoids calling .GetString() until you've asserted that the ID isn't null (check here for more in-depth), if you're using C#6 you should be able to use the new shorthand:
string id = itemObject["id"]?.GetString();
Here is solution for the issue
string id = itemObject["id"].ValueType == JsonValueType.Null ? "" : itemObject["id"].GetString();
I am trying to populate a combobox with a list my query returns. When I execute my program it gives me a specified cast is not valid error ( I have it execute on page load event). Every field in the database I have to work with can be null except the primary key. So I tried using DBNull.Value but it can't get my (int)reader fields to work. I have supplied my code below for a better understanding. How can I get my (int)reader's to work with my statements, so they can read when there is a null value?
CustData cd = new CustData();
cd.CustomerID = (int)reader["CustomerID"];
cd.Name = reader["Name"] != DBNull.Value ? reader["Name"].ToString() : string.Empty;
cd.ShippingAddress = reader["ShippingAddress"] != DBNull.Value ? reader["ShippingAddress"].ToString() : string.Empty;
cd.ShippingCity = reader["ShippingCity"] != DBNull.Value ? reader["ShippingCity"].ToString() : string.Empty;
cd.ShippingState = reader["ShippingState"] != DBNull.Value ? reader["ShippingState"].ToString() : string.Empty;
cd.ShippingZip = (int)reader["ShippingZip"];
cd.BillingAddress = reader["BillingAddress"] != DBNull.Value ? reader["BillingAddress"].ToString() : string.Empty;
cd.BillingCity = reader["BillingCity"] != DBNull.Value ? reader["BillingCity"].ToString() : string.Empty;
cd.BillingState = reader["BillingState"] != DBNull.Value ? reader["BillingState"].ToString() : string.Empty;
cd.BillingZip = (int)reader["BillingZip"];
cd.Territory = reader["Territory"] != DBNull.Value ? reader["Territory"].ToString() : string.Empty;
cd.Category = reader["Category"] != DBNull.Value ? reader["Category"].ToString() : string.Emptyy
That is because int is not nullable. You need to use int? or nullable<int> (long hand) to allow it to be an int OR a null value.
You can then use the usual .HasValue and .Value etc to get the value from the item.
EDIT: To enhance the visibility of my comment to this answer. I would advise against checking for NULL and storing Zero into your property because then when you save back you are changing a Null to a Zero even though nothing has been changed by the system. Now, reports etc may distinguish between NULL and Zero (very often) and could start doing strange things!
Null does NOT equal zero!! If you assume it does as a work around... What happens if I truly do want to record zero? How do you differentiate between a real zero and a "was null now zero" trick? Do it right, save yourself the pain!
Use nullable int, or just make your control for your int's too
reader["ShippingZip"] != DBNull.Value ? (int)reader["ShippingZip"] : default(int);
You should use a nullable int for your variable and cast it, like (int?). Int can only have a value; nullable types can also be null. When you use a nullable type, you can look at the property .HasValue. Here is the MSDN page: http://msdn.microsoft.com/en-us/library/2cf62fcy.aspx
EnumType = reader["EnumTypeId"] == DBNull.Value ? EnumType.None : (EnumType)(int)reader["EnumTypeId"];
I thought if reader["EnumTypeId"] is null, it should assign the EnumType.None value, but it is still trying to cast the null value to an int which is obviously causing an exception.
I tried the following and it did not work either:
EnumType = reader["EnumTypeId"] == null ? EnumType.None : (EnumType)(int)reader["EnumTypeId"];
Instead of using Enums, I went ahead and decided to use a nullable int, so now my code is slightly different, but it still does not work with DBNull.Value, null, or GetOrdinal...
intType= reader["intType"] == DBNull.Value ? null : (int?)reader["intType"];
Also, why do I have to do a (int?) cast instead of just a (int) cast?
Don't use DBNull, just use plain old null.
EnumType = reader["EnumTypeId"] == null ? EnumType.None : (EnumType)(int)reader["EnumTypeId"];
Edit
The issue could be that the database type of EnumTypeId isn't an int/Int32. If so, then reading as a string and then parsing should fix the problem.
EnumType? enumVal = null;
if (reader["EnumTypeId"] != null)
{
int intVal;
enumVal = (int.TryParse(reader["EnumTypeId"].ToString(), out intVal)) ? (EnumType)intVal : null;
}
EnumType = ? EnumType.None : (EnumType)(int)reader["EnumTypeId"];
Another way you can use is IsDBNull method:
int index = reader.GetOrdinal("EnumTypeId");
EnumType = reader.IsDBNull(index) ? EnumType.None :
(EnumType)reader.GetInt32(index);
I'm creating an object for my database and I found a weird thing, which I don't understand:
I've an object which should reference a "language" by an ID, but this can be null, so my property is a int?(Nullable<int>)
so firstly I tried to use the object initializer:
myObject = new MyObject()
{
myNullableProperty = language == null ? null : language.id;
}
but it doesn't work! It tell me that null cannot be converted to int
But if I it in a if/else structure, I can put null in a var and then assign it to my properties.
Why is this acting like this?
You may try casting the null to int? as the ?: operator requires both operands to return the same type:
myNullableProperty = language == null ? (int?)null : language.id
This is because of a type mismatch. You must cast your null value to the int type.
The reason is, when using the ? operator the left and the right side of the : are required to be from the same type and typeof(null)!=typeof(int) so:
myNullableProperty = language == null ? (int?)null : language.id;
Most likely null is interpreted as object which obviously can't be assigned to int. You might want to use myNullableProperty = language == null ? (int?)null : language.id;
I have defined Class Person property Birthday as nullable DateTime? , so why shouldn’t the null coalescing operator work in the following example?
cmd.Parameters.Add(new SqlParameter("#Birthday",
SqlDbType.SmallDateTime)).Value =
person.Birthday ?? DBNull.Value;
The compiler err I got was "Operator '??' cannot be applied to operands of type 'System.DateTime?' and 'System.DBNull'"
The following also got a compile error :
cmd.Parameters.Add(new SqlParameter("#Birthday",
SqlDbType.SmallDateTime)).Value =
(person.Birthday == null) ? person.Birthday:DBNull.Value;
I added a cast to (object) as recommended by Refactor, and it compiled, but didn’t work properly and the value was stored in the sqlserver db as null in both cases.
SqlDbType.SmallDateTime)).Value =
person.Birthday ?? (object)DBNull.Value;
Can someone explain what is going on here?
I needed to use the following clumsy code:
if (person.Birthday == null)
cmd.Parameters.Add("#Birthday", SqlDbType.SmallDateTime).Value
= DBNull.Value;
else cmd.Parameters.Add("#Birthday", SqlDbType.SmallDateTime).Value =
person.Birthday;
The problem is that DateTime? and DBNull.Value are not the same type so you can't use the null coalescing operator on them.
In your case you can do person.Birthday ?? (object)DBNull.Value to pass a value of type object through to Add()
I prefer to iterate over my parameters just before executing the query, changing all instances of null to DBNull as appropriate, for example:
foreach (IDataParameter param in cmd.Parameters)
if (param.Value == null)
param.Value = DBNull.Value;
This lets me leave null values as-is and simply swap them out en masse later.
Your first problem is that for the ?? or ?: operator, the objects for either choice must be the same type. Here they are different type.