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I try to make a lamda expression in C# for filtering a list and can't make it work when I compare a variable to an item in an array

So I have the gui set up so that I have two main listboxes. I'm still figuring out what kind of gui I want for this application so there is another one but that's not relevant. One listbox is a list of options for what you want to check for a department or an employee. The other is a list of departments. Right now I have the functionality for the name option to view the names of employees for a department. I just need to know how I can filter a list so that the only employees that show up are the ones who are in the chosen department after I click on the submit button. I figured I would use a lambda expression for that and it hasn't been working for me. I really want to know how to use lambda expressions better so please only give me a solution that involves using them. If it's impossible or if it would be more efficient to do something else then let me know.
File where I put reads and set dept array to file contents
//list of employees
public static List<Employee> EmployeeList = new List<Employee>();
//array to hold the options users have for interacting with info
public static string[] OptionsArr;
//array to hold the departments
public static string[] DeptsArr;
//skipping around same file to relevant code
//set the departments array to the contents of the depts file
DeptsArr = File.ReadAllLines("..\\..\\departments.txt");
Not sure if needed
Method for populating DeptListBox
private void UpdateDeptListBox()
{
//set up for new info
DeptListBox.Items.Clear();
//prevent interfence with update
DeptListBox.BeginUpdate();
//set department listbox to depts array
DeptListBox.DataSource = Program.DeptsArr;
DeptListBox.EndUpdate();
}
Problem Method - the submit button method
List<Employee> ResultList = new List<Employee>();
//name
if (OptionsListBox.SelectedIndex == 1)
{
//user selects Marketing department
if (DeptListBox.SelectedIndex == 0)
{
//problem is either with lambda exp or Program.DeptsArr comparison
foreach (Employee empl in Program.EmployeeList.Where(empl => empl.Dept.CompareTo(Program.DeptsArr[0]) == 0).ToList())
{
//this doesn't happen
ResultList.Add(empl);
}
for (int i = 0; i<ResultList.Count; i++)
{
ResultListBox.Items.Add(ResultList[i].Lname + " " + ResultList[i].Fname + " " + ResultList[i].Dept);
}
}
}
}

For me it can be helpful when I am having issues to break stuff down and look at smaller pieces. Are you sure the issue is your lambda function? It may be your options listbox != 1 or that the data is not being read in correctly.
As far as I can tell, this part should work. Although there are some issues with it:
foreach (Employee empl in Program.EmployeeList.Where(empl =>empl.Dept.CompareTo(Program.DeptsArr[0]) == 0).ToList())
{
//this doesn't happen
ResultList.Add(empl);
}
You could start with just the Employee lambda function and hard code the values. Maybe something like this which does indeed produce the correct results (Bob and Brandon)
List<Employee> ResultList = new List<Employee>();
List<Employee> EmployeeList = new List<Employee> {
new Employee{ Name = "Bob", Dept = "Accounting" },
new Employee{ Name = "Larry", Dept = "A" },
new Employee{ Name = "Margret", Dept = "B" },
new Employee{ Name = "Brandon", Dept = "Accounting" }
};
string[] DeptsArr = new string[2];
DeptsArr[0] = "Accounting";
DeptsArr[1] = "A";
//user selects Marketing department
if (departmentIndex == 0)
{
foreach (Employee empl in EmployeeList.Where(empl => empl.Dept.CompareTo(DeptsArr[0]) == 0).ToList())
{
ResultList.Add(empl);
}
}
However your lamda function inside a foreach loop is redundant. You can think of a lambda function as an instruction for running a foreach loop. A foreach loop by itself could look like this:
List<Employee> ResultList = new List<Employee>();
foreach (Employee empl in EmployeeList)
{
if(empl.Dept == DeptsArr[0])
{
ResultList.Add(empl);
}
}
You could get the same result as the foreach loop above, by using the following lamda function:
List<Employee> ResultList = EmployeeList.Where(empl => empl.Dept == DeptsArr[0]).ToList();
A final note is that the "ToList()" on the end of that lambda function is what executes the loop and returns the result as a List. Many times this is not required. Without the "ToList()" part an IEnumerable will be returned which you may be able to use instead. Using an IEnumerable instead of calling ToList() can have better performance in many scenarios.

If you want to test whether a specific value is in an array then you call Contains on that array, e.g.
var allEmployees = new List<Employee>();
// Populate allEmployees here.
var selectedEmployees = allEmployees.Where(e => selectedDepartments.Contains(e.Department)).ToArray();
The selectedEmployees array will contain only the Employee objects from the allEmployees list with a Department property value that is contained in the selectedDepartments array/collection.

MVC List Error List<Model>

I'm using foreach to transfer data from list to another but when adding value updated automatically to last value added. For example:
list1 = [1,2,3]
list2 = new List<Model>()
foreach(var item in list1) {
list2.Add(item)
}
the result in list2 is [ 3, 3, 3]
Actually example is below :
var _sizes = new List<ProductsSize>();
var _size = new ProductsSize();
if (model.Dynamic_ProductsSize.Count > 0)
{
foreach (var item in model.Dynamic_ProductsSize)
{
_size.SizeId = item;
_sizes.Add(_size);
}
}
model.ProductsSize = _sizes.ToList();
I need to know why it only takes the last item and what is the solution for this case

You only have one ProductsSize object:
var _size = new ProductsSize();
And you keep modifying that same object. All references to that object, including any list elements it's been added to, get updated when you modify that one object.
Instead, create your new object in the loop:
foreach (var item in model.Dynamic_ProductsSize)
{
var _size = new ProductsSize();
_size.SizeId = item;
_sizes.Add(_size);
}
That way each element in the list is a new object instead of the same object added multiple times.
Side note, you have a few things in the code which aren't necessary. Checking the length before the loop, for example, as well as converting a list to a list at the end.
In fact, I imagine all of the code shown can be shortened to simply this:
model.ProductsSize = model.Dynamic_ProductsSize.Select(p => new ProductsSize { SizeId = p }).ToList();
In which case you're also just converting one model property to another model property. Why not put this logic in the model itself and skip the whole thing?
public IEnumerable<ProductsSize> ProductsSize
{
get { return this.Dynamic_ProductsSize.Select(p => new ProductsSize { SizeId = p });
}
Unless there's a particular reason you want the same data twice in two different properties that isn't clear from this code, having one set of data and just different views/calculations/etc. of that data is often preferred.

Create a new object before adding it to the list. You can use the object initializer syntax to keep it concise:
if (model.Dynamic_ProductsSize.Count > 0)
{
foreach (var item in model.Dynamic_ProductsSize)
{
_sizes.Add(new ProductsSize(){SizeId = item});
}
}

How to build List by using data from another List using LINQ

i have some List that hold 1 or more Guid.
var getMyfirstListGuids = (from dlist in db.MyTable
where dlist.id == theid
select dlist).ToList();
List<MyfirstList> myfirstList = new List<MyfirstList>();
foreach (var item in myLandingPageList)
{
Guid theguid = new Guid(item.guid);
MyfirstList addnewRow = new MyfirstList();
addnewRow.LpGuid = new Guid(theguid);
myfirstList.Add(addnewRow);
}
Now i have a list with 1 or more guids.
my next step is to make list with data from SQL by the first list guids.
In the SQL could be 1 row or more for each guid. with one row result i can guess what to do. but if there is many results i dont have an idea.

Okay so you wanna do it like this:
List<SecondListGUID> secondListGUID = new List<SecondListGUID>();
foreach (var item in myfirstList)
{
for(int i = 0; i<_yourDBEntity.GUIDs.Count(); i++)
{
if(item.LpGuid == _yourDBEntity.GUIDs[i].GUID)
secondListGUID.add(
new SecondListGUID() {
// add the corresponding GUID's here
});
}
}
Basically you have to do a foreach through your first List and then do a for loop (or foreach - whichever you prefer) through your DB table (entity in this case if you're using Entity framework) and simply compare the GUID's from your DB table and if they match you'll wanna add the item to your third list.
P.S. I've worked with the informations that you've provided, you can change the 2nd list type into the one you need, and change your entity framwork data model name into the one you actually use :)

You can try this
List<Guid>getMyfirstListGuids = new List<Guid>();
getMyfirstListGuids.addRange(from dlist in db.MyTable
where dlist.id == theid
select dlist).ToList());
List<MySecList> mySecList = new List<MySecList>();
mySecList.AddRange(_db.myLandingPageList.Where(p => p.Guid.Any(x => getMyfirstListGuids.Contains(x));

Adding element to a list in class, which is a part of List<class>, based on condition

Let's have a simple class with 2 fields
public class Sample
{
public int IdOfSample;
public string SampleName;
}
And another using this one
public class ListOfSamples
{
public int IdOfList;
public List<Sample> SampleList;
}
And finally, since we will use a couple of different ListOfSamples, make a list of them:
public static List<ListOfSamples> FinalList = new List<ListOfSamples>();
Now the problem:
I create a new Sample (let's call it NewItem), with some name and Id. I want to check if there's a ListOfSamples in my FinalList that as the same Id as the NewItem I have. Otherwise create new ListOfSamples in the FinalList with the IdOfList = NewItem.IdOfSample.
I think I got the first part which checks if you should add a new list (ie. a ListOfSamples with specified IdOfList does not exist:
Sample NewItem = new Sample()
{
IdOfSample = 12345,
SampleName = "Some name"
};
int index = FinalList.FindIndex(f => f.IdOfList == NewItem.IdOfSample);
if (!FinalList.Any() || index == -1)
{
ListOfSamples NewList = new ListOfSamples()
{
IdOfList = NewItem.IdOfSample,
SampleList = new List<Sample>()
};
NewList.SampleList.Add(NewItem);
FinalList.Add(NewList);
}
Now, I'm trying to construct a statement, that, if the list with specified Id already exists in the FinalList, just add the new item to it, but so far I think my limited experience with LINQ is showing, nothing I try seems to work.
So:
If there exists a ListOfSamples with IdOfList == NewItem.IdOfSample in FinalList, then add NewItem to that ListOfSamples.

How about
if (!FinalList.Any() || index == -1)
...
else
{
FinalList[index].SampleList.Add(NewItem);
}

If you just wanted to check whether the list item existed, a suitable LINQ statement could be:
if (FinalList.Any(l => l.IdOfList == NewItem.IdOfSample))
{
// ...
}
Given you want to work on the item then you could attempt to retrieve it as follows:
var existingList = FinalList.SingleOrDefault(l => l.IdOfList == NewItem.IdOfSample);
if (existingList != null)
{
existingList.Add( ... );
}
Though perhaps it's worth thinking about using a HashSet of lists if you want to guarantee uniqueness...

if i understand it right ...
// search for the list with the given Id
var listOfSamples = finalList.Where(fl => fl.IdOfList == newItem.IdOfSample).FirstOrDefault();
if (listOfSamples == null)
{
// not found
// add new List with the new item in final list
finalList.Add(new ListOfSamples {IdOfList = newItem.IdOfSample, SampleList = new List<Sample>{newItem}} );
}
else
{
// found
// add the new item into the found list
listOfSamples.SampleList.Add(newItem);
}

If you replace ListOfSamples with a Dictionary<int, List<Sample>> then you will gain the ability to do a lookup in O(1) time and guarantee that the ids at the top level are unique. and then you can just add stuff like this.
Dictionary<int, List<Sample>> FinalList = new Dictionary<int, List<Sample>>();
Sample NewItem = new Sample()
{
IdOfSample = 12345,
SampleName = "Some name"
};
List<Sample> list;
if (!FinalList.TryGetValue(NewItem.IdOfSample, out list))
{
list = new List<Sample>();
FinalList.Add(NewItem.IdOfSample, list);
}
list.Add(NewItem);
TryGetValue will see if the dictionary has an entry for the key you pass it and returns true if it does and false if it does not. If it does have an entry for the key it also assigns the value of the entry (in this case your list of samples) to the out parameter. So, we check if it returns false and in that case we create a new list and add it to the dictionary. Then we add the sample to the list that we either got from the dictionary, or just created and put in the dictionary.

Update One List of Objects Attribute with Other List using LINQ

I have a scenario as think
class a
{
String Username;
String val;
}
List<a> lst = new List<a>();
List<a> lstnew = new List<a>();
What i required is to that in lstnew i have some updated values in val Attribute (Only in Several Objects) , what i required is to update the lst with updated values in lstnew as the Username Attribute using LINQ

You can join the two lists on UserName, and then update the Values in the first list with those in the second.
For example, given this class and lists:
public class a
{
public string UserName { get; set; }
public string Value { get; set; }
}
List<a> list = new List<a>
{
new a { UserName = "Perry", Value = "A" },
new a { UserName = "Ferb", Value = "B" },
new a { UserName = "Phineas", Value = "C" }
};
List<a> newList = new List<a>
{
new a { UserName = "Phineas", Value = "X" },
new a { UserName = "Ferb", Value = "Y" },
new a { UserName = "Candace", Value = "Z" }
};
You can join to get the elements with common UserNames:
var common = from a1 in list
join a2 in newList on a1.UserName equals a2.UserName
select new { A1 = a1, A2 = a2 };
At this point, if I understand you correctly, you want to update the elements from the original list:
foreach(var c in common)
{
c.A1.Value = c.A2.Value;
}
at which point the elements in list look like:
UserName Value
-----------------
Perry A
Ferb Y
Phineas X

It sounds like you have two lists. One of which is named lst and contains a full list of usernames and a second one named lstnew that contains a list of usernames who have had their val property updated. I suggest unioning the untouched usernames with the ones that have been updated. This represents the most LINQ-friendly solution I can think of.
var updatedList = Enumerable.Union(
lst.Where(x => !lstnew.Any(y => y.Username == x.Username)),
lstnew).ToList();

you should be able to use the .Zip() method to execute this.
lst.Zip(lstNew, (orig, new) => {
orig.Username = new.Username;
return orig;
});
the idea that you are getting each pair together, then instead of returning a new one, changing the orig.Username value and return the orig.

This should also do the trick. Zip method, propsed by Alastair Pitts assumes that both collections have the same order of elements and each element from first list has correspondent element in second list. My approach is more generic, it simply looks for corresponding element by comparing Username property. Still it assumes that for each element in lstNew there is corresponding element in lst.
lstNew.ForEach(new => lst.First(orig => orig.Username == new.Username).val = new.val);

I know this is an old question but a more elegant solution that I have developed, which is a slight improvement over the one given by #JeffOgata would be:
var newList= lst.GroupJoin(lstnew ,
i => i.UserName ,
j => j.UserName ,
(i, j) => j.FirstOrDefault()?? i );
Where lst is the original list and lstnew is the new list.
This will just replace the entire object in the first list with the corresponding object in the second list (the join) if one exists.
It is a slight improvement over the answer given by #JeffOgata
The result is the same.
If you have complex objects then iterating through each object then going through all the properties was a problem, simply replacing the old object with the new one was quicker.
This hopefully will help someone.