I have an object where a property may exist or may not exist.
if(response.AddressInformation.AddressResponses.Any(inf => inf.AddressResponse.matchCodeStatus.ToLower().Equals("usps_match")))
{
}
I have two array items of AddressResponse. First item has null for matchCodeStatus and thats where I get object not set to an instance exception. How can I achieve my target and escape this exception?
I tried to put a null check before my IF, but it didnt work
if(response.AddressInformation.AddressResponses.Any(inf => inf.AddressResponse.matchCodeStatus != null)
As of C# 6, you can also use a null conditional operator ?. :
inf => inf.AddressResponse.matchCodeStatus?.ToLower().Equals("usps_match"))
Related
So I've got a collection of structs (it's actually a WCF datacontract but I'm presuming this has no bearing here).
List<OptionalExtra> OptionalExtras;
OptionalExtra is a struct.
public partial struct OptionalExtra
Now I'm running the below statement:
OptionalExtra multiOptExtra = OptionalExtras.Where(w => w.Code == optExtra.Code).FirstOrDefault();
if (multiOptExtra != null)
{
}
Now this won't compile:
the operator != cannot be applied to opperands of type OptionalExtra
and '<null>'
After a little googling I realised it's because OptionalExtra is a struct. Which I believe is not nullable unless defined as a nullable type?
So my question is, if my where statement returns no results what will be the outcome of the FirstOrDefault call? Will it thrown an exception?
Incidently this should never happen but better safe than sorry.
If your collection is empty, FirstOrDefault will return default(OptionalExtras). The default value of a struct is the struct with all its values in turn default initialized (i.e. zero, null, etc.).
If you assume that there will be an element and your code doesn't work with an empty collection, Use First() instead, since that will throw an exception when your collection is empty. It's generally better to fail fast than to return wrong data.
If you cannot assume that there will be an element, but also cannot deal with struct default initialization, you might make the structs in the collection a nullable value type, for example as follows:
OptionalExtras
.Where(w => w.Code == optExtra.Code)
.Cast<OptionalExtra?>()
.FirstOrDefault();
This way you can get a null return even for a struct. The key idea here is to extend the set of possible values to include something other than an OptionalExtra to allow detection of an empty list. If you don't like nullables, you could instead use a Maybe<> implementation (not a .NET builtin), or use an empty-or-singleton list (e.g. .Take(1).ToArray(). However, a nullable struct is likely your best bet.
TL;DR;
.FirstOrDefault<T>() returns default(T) if the sequence is empty
Use .First() instead if you assume the list is non-empty.
Cast to nullable and then use .FirstOrDefault<T>() when you cannot assume the list is non-empty.
As others have said, the result of your code when no elements match will be:
default( OptionalExtra )
If you want a null returned, you can cast your list to OptionalExtra?
OptionalExtra? multiOptExtra = OptionalExtras.Cast<OptionalExtra?>().Where( ...
You can then test for null
If default(OptionExtra) is still a valid value, it's better to change your code to this
var results = OptionalExtras.Where(w => w.Code == optExtra.Code).Take(1).ToList();
if (results.Any()) {
multiOptExtra = results[0]
}
The result will be the default value of your struct, e.g. default(OptionalExtras).
Whereas for a reference type the default value is null.
its provide you defualt value for your structure like as below
int[] numbers = { };
int first = numbers.FirstOrDefault();
Console.WriteLine(first);//this print 0 as output
other option to handle is make use of default value like as below
List<int> months = new List<int> { };
// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);
If you want to check for null, use System.Nullable collection:
var OptionalExtras = new List<OptionalExtra?>();
/* Add some values */
var extras = OptionalExtras.FirstOrDefault(oe => oe.Value.Code == "code");
if (extras != null)
{
Console.WriteLine(extras.Value.Code);
}
Note that you have to use Value to access the element.
Assuming Code is a string for the purposes of my answer, you should be able just to test that value for its default.
OptionalExtra multiOptExtra = OptionalExtras.Where(w => w.Code == optExtra.Code).FirstOrDefault();
if (multiOptExtra.Code != null)
{
}
Searching through a column in a spreadsheet I notice that the second line will exception ("Nullable object must have a value.") when it comes across a blank field, however the first line will succeed. I have to add the Bool cast to the second line because otherwise I get "Cannot convert Nullable<bool> to bool". I assume this is the issue, but is there a way around that to allow blank fields to be checked?
keyFoundCell = _ws.Cells["a:a"].FirstOrDefault(cell => cell.Value?.ToString() == field.Key);
keyFoundCell = _ws.Cells["a:a"].FirstOrDefault(cell => (bool)cell.Value?.ToString().Equals(field.Key, StringComparison.OrdinalIgnoreCase));
In both EPPlus and Excel Interop you can read a cell's contents using the Text property instead of the Value property if you want to operate on the cell's visible contents and avoid nulls. Value returns an object which might be null, but Text returns the visible text as a string which can be empty but won't be null.
If we're using Value.ToString() or Value?.ToString() then chances are we'd be better off with Text because that's a giveaway that we want the text we see, not the value.
What's happening here is that the ?. operator will return null right away if the left hand side of the operator is null.
So, when cell.Value is null, the first line works because you're doing a direct comparison using the == operator, which will return a bool. In other words, null == field.Key returns false (unless field.Key is null, of course).
The second line does not work without a cast because if the value is null, then the ?. operator returns null and the rest of the line is ignored (.ToString() is never called). So the exception you're getting is due to the fact that the if condition must return a bool, yet it's returning a Nullable<bool> instead.
One way to fix this is to simply check for null first. This will not compare any objects where cell.Value == null:
keyFoundCell = _ws.Cells["a:a"].FirstOrDefault(cell =>
cell.Value != null &&
cell.Value.ToString().Equals(field.Key, StringComparison.OrdinalIgnoreCase));
Another way you can do this is to use the static Equals method of the string class, which will allow one or more null arguments. This will include results where cell.Value == null (and will return true for those cases where field.Key is also null):
keyFoundCell = _ws.Cells["a:a"].FirstOrDefault(cell =>
string.Equals(cell.Value?.ToString(), field.Key, StringComparison.OrdinalIgnoreCase));
maybe try:
keyFoundCell = _ws.Cells["a:a"].FirstOrDefault(cell => (bool?)cell.Value?.ToString().Equals(field.Key, StringComparison.OrdinalIgnoreCase)) ?? false;
This question already has answers here:
What is a NullReferenceException, and how do I fix it?
(27 answers)
Closed 5 years ago.
I am trying to get a list item that contains a certain string and I am using this code:
string myListLine= myList.FirstOrDefault(stringToCheck => stringToCheck.Contains("mystring "));
All works well but if the list does not contain a particular string it throws an error:
object reference not set to an instance of an object
I think that I should somehow validate if the string first exists but not sure what would be the best approach.
if the list does not contain a particular string it throws an error:
That is not correct. The exception is thrown, because you have a null value inside your list! If all of your items in the list are valid strings then there will be no exception thrown. In this case FirstOrDefault will simply return null.
I think that I should somehow validate if the string first exists
You can check whether it is null the oldscool way first and combine it with an logical AND && with the Contains method
string myListLine= myList.FirstOrDefault(stringToCheck =>
stringToCheck != null && stringToCheck.Contains("mystring "));
This way the second Contains will not be evaluated if the first condition evaluates to FALSE
You can use the safe navigation operator and the null-coalescing operator to fix this:
System.Collections.Generic.List<string> myList = new System.Collections.Generic.List<string> { null, "Test123" };
string myListLine = myList.FirstOrDefault(stringToCheck => stringToCheck?.Contains("mystring ") ?? false);
The problem occurs if you're calling a method on a null list entry.
Note: If the list is empty FirstOrDefault returns null.
Hope that helps.
Your code simply goes through myList, for each item, it checks if it contains "mystring ".
Only reason you may get a null reference while running that line is your list myList is null. It wouldn't get any error if it is empty.
if you get a null reference after that line, that would mean that myListLine is null, which would mean myList did not contain any items that matched "mystring ".
To make sure you do not get a null reference error with myListLine you should check if it is null before accessing it by using something like this;
if( myListLine != null ){
<Do something to myListLine>
} else {
<list doesnt contain the correct string, alert user.>
}
I think this sample code of mine is one of the safest way by accessing the myList object. If it is null then return an indicator that value was not found or empty.
List<string> myList = new List<string>()
{
"name","adress","phoneNumber"
};
myList.RemoveAll(item => item == null);
string myListLine = myList.FirstOrDefault(stringToCheck => stringToCheck.Contains("myString")) ?? "Not found/Empty";
A variation:
var myList = new List<string>(){"foo","Bar", null,"the mystring"};
string myListLine = myList.FirstOrDefault(s => s == null? false : s.Contains("mystring"));
Same notes as already mentioned regarding FirstOrDefault being null if empty list or no match (does not Contains)
I've jumped into such a thing in my code and I don't really know how to read this. Would be nice if somebody could help me with this :)
return Company?.Call?.SingleOrDefault(cf => cf.Name == Client?.CallID)
?? Company?.Call?.SingleOrDefault(cf => cf.IsDefault)
?? new CallData();
The ?. operator is called a Null-conditional operator and is a new feature from C# 6, and was announced in October 2014. https://msdn.microsoft.com/en-us/library/dn986595.aspx
The first part: Company?.Call?.SingleOrDefault can be seen as similar to this:
if (Company == null)
{
return null;
}
else if (Company.Call == null)
{
return null;
}
else
{
return Company.Call.SingleOrDefault(....
}
But then the original coder uses the ?? Null-coalesce operator, which has been a part of C# since at least 2007. It means that if whatever is left of the ?? is null, evaluate what is right of the ?? and return that instead.
So the code basically means:
If Company is null, return new CallData()
If Company.Call is null, return new CallData()
If the first call to Company.Call.SingleOrDefault returns a CallData instance (having a certain value for the Name property) , return that instance
If the first call returns null, but the second call returns a CallData instance (being the default one) , return that instance
If both calls to SingleOrDefault return null, return new CallData()
This piece of code has some issues.
First of all, it is unreadable and should be refactored into something that is easier to understand. New language features are nice, but only when used responsibly.
Second: If there are more than one CallData instances with the same Name, the SingleOrDefault will throw an exception, but there might be a unique index in the database the prevents this. The same goes for IsDefault property - if there are more than one records with IsDefault = true, the SingleOrDefault call will throw an exception.
Split it to 3 expressions first, separated by the ??:
Company?.Call?.SingleOrDefault(cf => cf.Name == Client?.CallID)
??
Company?.Call?.SingleOrDefault(cf=>cf.IsDefault)
??
new CallData();
The returned value of the entire expression would be the first expression that is returning a non-null value.
Within each segment, once any of the properties accessed using ?. is null, the entire expression will be evaluated to null.
See Null-conditional Operators
I have the following line of code:
var selectedDomainID = lkuDomainType.EditValue.Equals(null)
? string.Empty
: lkuDomainType.EditValue;
Sometimes this generates a NullReferenceException. What I don't understand is why. Isn't the whole point of my code to check for null and if so assign string.empty? When I check in DEBUG it is stating that EditValue == null so what am I missing?
Use lkuDomainType.EditValue == null, otherwise you are trying to call an instance method on a null object. But the better option might be lkuDomainType.EditValue ?? String.Empty. Also watch out for lkuDomainType being null, unless it is a class not an object.
When you use Object.Property and Object is undefined, you are dereferencing a null pointer and that's why you get the exception. Instead, use:
var selectedDomainID = lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;
If EditValue is null then you can't call Equals. In this cas you would have to do:
var selectedDomainID = lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;
Or you can simplify it by doing:
var selectedDomainID = lkuDomainType.EditValue ?? string.Empty;
you are trying to call the Equals method on a null object - do it like this instead:
lkuDomainType.EditValue == null
The problem is that you are using the object before checking if it's null. You are calling the Equals method of the object, which fails if the reference is null.
You have to exchange your lkuDomainType.EditValue.Equals(null) for lkuDomainType.EditValue == null.
EditValue == null. That means that there is no object there. You cannot call functions on null objects, even if the function is .Equals().
You're better off just saying "(lkuDomainType.EditValue == null)" in this case.
You should use String.IsNullOrEmpty here. Like this:
var selectedDomainID = String.IsNullOrEmpty(lkuDomainType.EditValue) ? string.Empty : lkuDomainType.EditValue;
Equals is a method, you're trying to call a method on a null object which is throwing an exception.
when EditValue is null you cannot call the Equals method on it so the best way to check is to use
lkuDomainType.EditValue == null ? string.Empty : lkuDomainType.EditValue;
If lkuDomainType.EditValue is null, then "lkuDomainType.EditValue.Equals(someObject)" is the same as coding "null.Equals(someObject)". Well, obviously "null" doesn't have any members or methods (it wouldn't be null if it did). That's why you get a NullReferenceException.
Most of the examples from the other posts will work, including String.IsNullOrEmpty, which is a method that returns a boolean value.
All current answers fail to address a critical point: the difference between calling Equals on a Nullable type (which is a struct) vs. class type.
Calling Equals on a Nullable typed object whose value is null will not issue an exception, while doing the same on a class typed object will.
Consider the following code:
int? num = null; // int? is Nullable<int>
String s = null; // String is a class
var equalsNullable = num.Equals(null); // works
var equalsClass = s.Equals(null); // throws
The reason is that Nullable's HasValue method is called in such case, see Jon Skeet's answer for more info.
As other people said, the way to go in the OP's case is to use == or ??.