Develop Reference

Filter a list of class with element which starts with specific letters

I have a list of a class object, I need to filter that list with element which starts with these letters "GHB" and then set a listview control dataconext to it to display the elements
if(myList.ToList().FindIndex(x=> x.Name !=null)!=-1 )
{
listview1.DataContext = myList.ToList().where(x=> x.Name.StarstWith("GHB"))
}
But it gives me an error when an element is null

It gives you the error because your if condition is actually useless. You check whether Name in at least one element is not null, if so you try to access the variable. This of course will fail, because you need only 1 element with a valid name and the rest still can have null values which will lead to the NullReferenceException
What you can do is: check in the where clause additionaly whether Name is not null and if so only then check whether it StartsWith("GHB"):
listview1.DataContext = myList.Where(x => x?.Name != null && x.Name.StartsWith("GHB")).ToList();
this way you can save yourself the if condition.
I guess what you where trying to check is if Name in all elements is not null. In this case you can use the All method:
if (myList.All(x=>x.Name != null)
EDIT: using the ? will avoid that Name is checked if an element in the List is entirely null:
myList.Where(x => x?.Name != null && x.Name.StartsWith("GHB")).ToList();

Try this:
listview1.DataContext = myList
.Where(x => x != null
&& !string.IsNullOrEmpty(x.Name)
&& x.Name.StarstWith("GHB"))
.ToList();
...and remove the if statement.

Null value in linq where clause

I'm having an issue where I want to return results where something matches and I get an error if one of the properties I'm trying to match is null.
if (!string.IsNullOrEmpty(searchString))
{
Infos = Infos.Where(
x =>
x.FirstName.ToLower().Contains(searchString) ||
x.LastName.ToLower().Contains(searchString) ||
x.ContractNum.ToLower().Contains(searchString) ||
x.VIN.ToLower().Contains(searchString) ||
x.Claim.InitiatedBy.ToLower().Contains(searchString)
).ToList();
}
If ContractNum or VIN, for example, are null then it throws an error. I'm not sure how to check if one of these are null inside of a linq query.

You can add explicit null checks:
Infos = Infos.Where(
x =>
(x.FirstName != null && x.FirstName.ToLower().Contains(searchString)) ||
(x.LastName != null && x.LastName.ToLower().Contains(searchString)) ||
(x.ContractNum != null && x.ContractNum.ToLower().Contains(searchString)) ||
(x.VIN != null && x.VIN.ToLower().Contains(searchString)) ||
(x.Claim != null && x.Claim.InitiatedBy != null && x.Claim.InitiatedBy.ToLower().Contains(searchString))
).ToList();

You have multiple options, first is to do an explicit check against null and the other option is to use Null propagation operator.
x.FirstName != null && x.FirstName.ToLower().Contains(searchString)
or
x.FirstName?.ToLower()?.Contains(searchString) == true
But I would suggest you to use IndexOf instead of Contains for case
insensitive comparison.
something like:
x.FirstName?.IndexOf(searchString, StringComparison.CurrentCultureIgnoreCase) >= 0)

Checking the property is null or empty before comparing it it's the only way I know
if (!string.IsNullOrEmpty(searchString))
{
Infos = Infos.Where(
x =>
(!String.IsNullOrEmpty(x.FirstName) && x.FirstName.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.LastName) && x.LastName.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.ContractNum) && x.ContractNum.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.VIN) && x.VIN.ToLowerInvariant().Contains(searchString)) ||
(x.Claim != null && !String.IsNullOrEmpty(x.Claim.InitiatedBy) && x.Claim.InitiatedBy.ToLowerInvariant().Contains(searchString))
).ToList();
}
EXTRA: I added a check on the Claim property to make sure it's not null when looking at InitiatedBy
EXTRA 2: Using the build in function IsNullOrEmpty to compare string to "" and nullso the code is clearer.
Extra 3: Used of ToLowerInvariant (https://msdn.microsoft.com/en-us/library/system.string.tolowerinvariant(v=vs.110).aspx) so the lowering action will act the same no matter of the culture.

You could use ?? to replace it with a acceptable value.
(x.ContractNum??"").ToLower()

I would use the null conditional operator ?, this will however, return a nullable bool? so you will need to handle that appropriately.
Some examples on how to do this:
x?.FirstName?.ToLower().Contains(searchString) == true;
x?.FirstName?.ToLower().Contains(searchString) ?? false;

An alternative method to keep the comparison logic in one place to use a sub collection of the properties and check on those:
Infos = Infos.Where(i=>
new[] {i.FirstName,i.LastName,i.ContractNum /*etc*/}
.Any(w=> w?.ToLower().Contains(searchString) ?? false))
.ToList();
(It does read out all properties, but that shouldn't cost much performance and gains much maintainability )

Where function on list with multiple conditions returns errors

I'm trying to filter a list based on a few criteria and the .Where() function gives me an error in 2 parts of the same method.
if (string.IsNullOrWhiteSpace(champs))
{
data = dal.GetVueTache().Where(t =>
t.ProjetDescription64.ToLower().Contains(filtre.ToLower())
// *This Line || t.ProjetDescription256.ToLower().Contains(filtre.ToLower())
|| t.Description256.ToLower().Contains(filtre.ToLower())
||t.ResponsableNomCourt.ToLower().Contains(filtre.ToLower())
|| t.PrioriteDesc.ToLower().Contains(filtre.ToLower())
).ToList();
}
If I use any of the previous conditions except the one on the nullable field alone I get a perfectly filtered list on that criteria, if I add an OR "||" then I get a System.NullReferenceException on the first criteria.
I also have a similar issue in another part of the same method
else
{
data = dal.GetVueTache().Where(t =>
t.GetType().GetProperty(champs).GetValue(t).ToString().ToLower().Contains(filtre.ToLower())
).ToList();
}
This one filters my list based on the criteria "filtre" on the property "champs". It works on every property but the second one, which is a nullable one. I understand that this is where the issue comes from, but I can't find a way to test if the property is null before evaluating it and work around this from inside the .Where() Method.
Any advice will be greatly appreciated!!
Edit :
Thanks to Ivan Stoev for his solution!
The correct syntax for testing the null value in the first case is:
|| (t.ProjetDescription256 != null && t.ProjetDescription256.ToLower().Contains(filtre.ToLower()))
In the second case:
(t.GetType().GetProperty(champs).GetValue(t) != null && t.GetType().GetProperty(champs).GetValue(t).ToString().ToLower().Contains(filtre.ToLower()))

Just do a null check either the old way:
|| (t.ProjetDescription256 != null && t.ProjetDescription256.ToLower().Contains(filtre.ToLower()))
or the C# 6 way (utilizing the null conditional operator):
|| t.ProjetDescription256?.ToLower().Contains(filtre.ToLower()) == true
Btw, you can greatly simplify similar checks and avoid such errors by writing a simple custom extension methods like this:
public static class StringExtensions
{
public static bool ContainsIgnoreCase(this string source, string target)
{
return source == null ? target == null : target != null && source.IndexOf(target, StringComparison.CurrentCultureIgnoreCase) >= 0;
}
}
so your snippet becomes simply:
data = dal.GetVueTache().Where(
t => t.ProjetDescription64.ContainsIgnoreCase(filtre)
|| t.ProjetDescription256.ContainsIgnoreCase(filtre)
|| t.Description256.ContainsIgnoreCase(filtre)
|| t.ResponsableNomCourt.ContainsIgnoreCase(filtre)
|| t.PrioriteDesc.ContainsIgnoreCase(filtre)
).ToList();

Linq and anonymous type

select new
{
Selected = (cvf != null && cvf.Deleted==false)
}
The above statement proceeds to check cvf.Deleted even if cvf is null. Then it throws an invalid object reference error.
How do I fix this?

Probably something else going on since && will short-circuit evaluate. That said, try this instead:
select new
{
Selected = cvf != null
? !cvf.Deleted
: false
};

more short code about if statement

i wanted to try the following code:
//all arrays are List<T> type.
if (m.terms[0] != null && m.terms[0].labels != null && m.terms[0].labels[0].title == "Part-of-speech")
{
result = true;
}
but it occured runtime error occasionly in following situation
i. m.terms == null
ii. m.terms != null, but m.terms[0] does not intialized.
iii. m.terms != null, and m.terms[0] has been exist but
m.terms[0].label does not initialized.
...
so i did modify it to like this:
if (m.terms[0] != null)
{
if (m.terms[0].labels != null)
{
if (m.terms[0].labels[0].title == "Part-of-speech") { result = true; }
}
}
is it the best way?

&& is a short circuiting operator, so the first way you wrote it and the second way will be functionally equivalent.
if (a && b && c)
{
// work
}
b will only be evaluated if a returns true. (Same goes for c).
In your code, checking m.terms[0].labels will not be a problem because you would have short-circuited out of the expression if m.terms[0] had been null.
To completely cover yourself, you'd want to possibly add checks for m and m.terms, however.
m != null && m.terms != null && m.terms.Count > 0 && m.terms[0] != null ...
As it evaluates from left to right, it will break on the first condition that doesn't pass and the rest will go unchecked.

int index = 0;
int labelIndex = 0;
string titleToCheck = "Part-of-speech";
if (m != null && m.terms != null && m.terms.Count > index)// or m.Length...
{
if (m.terms[index] != null && m.terms[index].labels != null &&
m.terms[index].labels.Count > labelIndex)
{
if (m.terms[index].labels[labelIndex].title == titleToCheck)
{
result = true;
}
}
}

This is all about readability. C# uses Short-circuit evaluation so in functionality there is no difference.

try this
if (m!=null && m.terms!= null && m.terms[0].labels!=null && m.terms[0].labels[0].title!=null && m.terms[0].labels[0].title == "Part-of-speech")

Yes, it would be better to split off each null check into a separate if statement.
The reason is that the second and third conditions require the first to not be null. If the first is null, then the second and third conditions will in turn throw errors because their parent is null yet is trying to be accessed.