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How can i compare the variable var with the enum in C#

I am using a variable var for getting the result from my database and I want to compare it with my enum value to do some processes. But I am facing the problem var cant compare with the enum value.I have tried to convert both of the variable to String but still cant make it. Any idea of doing so?
//Fetch the value from database
var resultActivateQR = from value in db.QrCodes
where value.Code == qrCode
select value.Status;
//Want to compare the value
if (resultActivateQR.Equals((int)Status.New))
{
return true;
}
else
{
return false;
}
//My Enum In other class
public enum Status
{
New = 0,
Activated = 1,
Void = 2,
}

Your variable resultActivateQR would be an IEnumerable collection object returned from your LINQ statement. You would need to use a LINQ method like .Single() or .First() to get the Status of the one record that matched your qrCode (assuming there should always just be one record return from the database from the LINQ statement).
So your code could look something like this (this is more concise but essentially does what it seems like you needed):
var resultActivateQR = db.QrCodes.Where(x => x.Code == qrCode).Single();
//Want to compare the value
return resultActivateQR.Status == (int)Status.New;
This will bring back the single object the represents the database record that matches your QR Code and then compares that objects status to your enum Status.New and will return true or false from the comparison.

Writing a LINQ query, as you did, creates just queryable object, which does not contain any data, just query itself. It is called deferred exectuion, meaning that data is fetched when it is needed.
So one way is to call method such as ToList() or ToArray().
From your question I guess you expect one record to be fetched or you want to get one record, which requires calling First() method or FirstOrDefault(), which would also materialise data.
Second matter is the type of Value column, but I guess it's an int, so you can easily compare with your enum. Otherwise, you should cast your enum to appropriate datatype in order to successfully compare it with value in column.

Error: “Cannot implicitly convert type”

I am working on one project and I have a question regarding the converting type. I want to create simple search for my project, but it can't return result with this message :
Error 1 Cannot implicitly convert type 'System.Collections.Generic.List' to 'EmployeeDataAccess.TimeWorkMonthly'
public TimeWorkMonthly Get(int id)
{
using (EmployeeDbEntities Entities = new EmployeeDbEntities())
{
List<TimeWorkMonthly> persons = new List<TimeWorkMonthly>();
var result = Entities.TimeWorkMonthlies
.Where(e => e.KartNo == id)
.Select(e => e)
.ToList();
return result.ToList();
}
}

The return type of your method is TimeWorkMonthlies but inside the method body return List<TimeWorkMonthlies>
You should either
change your method return type to IEnumerable<TimeWorkMonthlies>
(You could use List<TimeWorkMonthlies> but using an interface to abstract a collection type is better for many reasons)
Use FirstOrDefault, First, SingleOrDefault or Single extension methods of IEnumerable if you aim to return only one element and you do not care about anything except for the first element
Which of those methods is better depends on your data and search criteria - i.e. whether you expect this ID to be unique or not.
From your semantics it looks like you're doing a sort of repository like ID lookup, so my guess would be solution 2) and using Single or SingleOrDefault
The last choice is how you want your program to behave if nothing is found by ID
If you want an exception, use Single
If you want a null use SingleOrDefault
In Summary, all you have to do is change your last line of code to
return result.Single();
(And ofcourse, you don't need a call to ToList() just before that)

Your method signature indicates you just want to return a single object. But you're returning a List of objects. Using .ToList() is not appropriate when you just want to return one object. There are four appropriate extension methods:
First - will return the first item from the collection and throw an exception if the collection is empty.
FirstOrDefault - will return the first item in the collection, or the default of the type if the collection is empty.
Single - if there is one item in the collection, it will return it. If there are no items in the collection an exception is thrown. If there are multiple items in the collection, an exception is thrown.
SingleOrDefault - if there is one item in the collection it will return it. If there are no items in the collection it will return the default value for the type. If there are multiple items in the collection it will thrown an exception.
Since you're searching by ID, you probably don't ever to expect to match two or more elements. So that rules out First and FirstOrDefault. You should use Single or SingleOrDefault depending on what you want the behavior to be if there is no item found that has the matching ID.
public TimeWorkMonthly Get(int id)
{
using (EmployeeDbEntities Entities = new EmployeeDbEntities())
{
var result = Entities.TimeWorkMonthlies.Where(e => e.KartNo == id).Single();
return result;
}
}
Note I eliminated the persons variable because you never did anything with it. And your usage of the .Select extension method was superflous since you just selected the same object already being iterated over. Select is for when you want to transform the object.

The problem is your qry only . If you want to convert it with Tolist() function you have to change your qry
like this
public TimeWorkMonthly Get(int id)
{
using (EmployeeDbEntities Entities = new EmployeeDbEntities())
{
var result = from x in Entities.TimeWorkMonthlies
Where x.KartNo == id
Select x;
return result.ToList();
}
}
You can now convert it to list by tolist() and use it according to your need.

Can't find property or field on a dynamic object

I using Dynamic Linq to do some database queries and it's working really well up to now. I can pass a string to a Select to select fields like so:
var output = myDatabaseTable.Select("Foo, Bar");
For example. The power obviously being when you pass a string variable rather than hardcoded strings. The problem I'm running into now is that the library using IEnumerable instead of IEnumerable<T> because, obviously, it can't know T until runtime. I'm using this to select data and eventually return it to a client and it works fine for spitting out raw data, but now I want to be able to do some more processing before returning the data and that requires getting the query to run on the database first. I can do something like this:
var materializedResults = output.Cast<dynamic>().ToList();
And that will make the query run. But the problem is, once I've done that, it seems I can't use dynamic linq anymore. For example, if I did something like this:
var foos = materializedResults.Select("Foo");
I now get a System.Linq.Dynamic.ParseException with the message No property of field 'Foo' exists in type 'Object' (Note: I can see in the debugger that the materializedResults does actually have all the expected properties).
So after casting to a List so I can potentially iterate through it and modify some of the values, I can no longer query it.
So my question is, how can I take a dynamic query (with select, group by, order by etc provided as strings), materialize the results and then actually process those result dynamically?
I thought maybe if I could cast to the actual type rather than dynamic it might work, so I tried this:
var d = output.Cast<dynamic>().ToList();
MethodInfo method = typeof(Enumerable).GetMethod("Cast", new[] {typeof(IEnumerable)});
method = method.MakeGenericMethod(d.First().GetType());
output = method.Invoke(d, new[] {d}) as IEnumerable;
Which is ugly and requires me to cast twice. The first time to dynamic so I can get the type from the first item then again to that type.

If you do YourStuff.Cast<dynamic>.ToList(), you will receive IEnumerable<object>, and there is no property Foo on the type object.
The question you might be asking, how can you get IList<TheActualType>?! You can do it this way:
// for IEnumerable
public static IList ToAnonymousList(this IEnumerable enumerable)
{
var enumerator = enumerable.GetEnumerator();
if (!enumerator.MoveNext())
throw new Exception("?? No elements??");
var value = enumerator.Current;
var returnList = (IList) typeof (List<>)
.MakeGenericType(value.GetType())
.GetConstructor(Type.EmptyTypes)
.Invoke(null);
returnList.Add(value);
while (enumerator.MoveNext())
returnList.Add(enumerator.Current);
return returnList;
}
// for IQueryable
public static IList ToAnonymousList(this IQueryable source)
{
if (source == null) throw new ArgumentNullException("source");
var returnList = (IList) typeof (List<>)
.MakeGenericType(source.ElementType)
.GetConstructor(Type.EmptyTypes)
.Invoke(null);
foreach (var elem in source)
returnList.Add(elem);
return returnList;
}
It's a simple extension method that can later be used, as such:
var test = (new[]
{
new
{
Property1 = "10",
Property2 = "10",
Property3 = 1
}
}
.Select("New(Property1, Property2)"))
.ToAnonymousList();

Your Cast is Defaulting to which will cause an error
(output as System.Collections.Generics.IEnumerable)
This Cast Specified the Correct Interface Try again
(output as System.Collections.IEnumerable).Cast<dynamic>().ToList()

Cannot convert type 'System.Linq.IQueryable<double?>' to 'float'

I have a table named InventoryItem which consists of columns ItemDescription and Item BalanceQty.I want to fetch the BalanceQty of the ItemName selected in a comboBox.For this,I created a method in my Data Access Layer And passed the string parameter representing the string value of ItemDescription to this method.This has been implemented using Entity Framework.This is how my code looks:
public float GetAvailableQty(string itemName)
{
float availableQty =(from table in context.InventoryItem
where table.ItemDescription == itemName
select table.BalanceQuantity);
return availableQty;
}
But it is giving me the following error-
Cannot convert type 'System.Linq.IQueryable' to 'float'
Where am I going wrong?

Probably you need this:
double availableQty =(from table in context.InventoryItem
where table.ItemDescription == itemName
select table.BalanceQuantity).Sum();
IQueriable returns an expression tree. The result of query like this is a rows set, and it can be materialized to IEnumerate by using of ToList() or implicitly by assigning to IEnumerable. But anyway it will be rows set, not a single value. If you sure the query returns the single one then use .Single() or SingleOrDefault. See other extension methods for IQueriable.
Otherwise, if you need an array then assign result to some IEnumerable variable.

Because your Linq returns an Iqueryable ...
Lets assume you have 3 rows with with an item with 3 different quatities (silly, i know, but think about other things that can have multiple values per item, like colors for a paint). Your linq will return the three quantities, and you're assuming it's a number
You could use First or FirstOrDefault to fetch the first item, or the default value for that object.
In your case, it shouldn't matter, but you should realize how Linq works and what it returns ...
Another example:
let's assume : List numbers = {1,2,3,4,5,6} (let's assume they are ints).
and you do : var small_numbers = numbers.Where(x => x<4)
What you get is something you can then query like: foreach (var small in small_numbers) {...}). The result is not an int.
You could take the first, last, and indeed, that would be an int. But what you get is a collection. so, even if you do: var one_result = numbers.Where(x => x<2), one_result is not an int.

IEnumerable<T>.Union(IEnumerable<T>) overwrites contents instead of unioning

I've got a collection of items (ADO.NET Entity Framework), and need to return a subset as search results based on a couple different criteria. Unfortunately, the criteria overlap in such a way that I can't just take the collection Where the criteria are met (or drop Where the criteria are not met), since this would leave out or duplicate valid items that should be returned.
I decided I would do each check individually, and combine the results. I considered using AddRange, but that would result in duplicates in the results list (and my understanding is it would enumerate the collection every time - am I correct/mistaken here?). I realized Union does not insert duplicates, and defers enumeration until necessary (again, is this understanding correct?).
The search is written as follows:
IEnumerable<MyClass> Results = Enumerable.Empty<MyClass>();
IEnumerable<MyClass> Potential = db.MyClasses.Where(x => x.Y); //Precondition
int parsed_key;
//For each searchable value
foreach(var selected in SelectedValues1)
{
IEnumerable<MyClass> matched = Potential.Where(x => x.Value1 == selected);
Results = Results.Union(matched); //This is where the problem is
}
//Ellipsed....
foreach(var selected in SelectedValuesN) //Happens to be integer
{
if(!int.TryParse(selected, out parsed_id))
continue;
IEnumerable<MyClass> matched = Potential.Where(x => x.ValueN == parsed_id);
Results = Results.Union(matched); //This is where the problem is
}
It seems, however, that Results = Results.Union(matched) is working more like Results = matched. I've stepped through with some test data and a test search. The search asks for results where the first field is -1, 0, 1, or 3. This should return 4 results (two 0s, a 1 and a 3). The first iteration of the loops works as expected, with Results still being empty. The second iteration also works as expected, with Results containing two items. After the third iteration, however, Results contains only one item.
Have I just misunderstood how .Union works, or is there something else going on here?

Because of deferred execution, by the time you eventually consume Results, it is the union of many Where queries all of which are based on the last value of selected.
So you have
Results = Potential.Where(selected)
.Union(Potential.Where(selected))
.Union(potential.Where(selected))...
and all the selected values are the same.
You need to create a var currentSelected = selected inside your loop and pass that to the query. That way each value of selected will be captured individually and you won't have this problem.

You can do this much more simply:
Reuslts = SelectedValues.SelectMany(s => Potential.Where(x => x.Value == s));
(this may return duplicates)
Or
Results = Potential.Where(x => SelectedValues.Contains(x.Value));

As pointed out by others, your LINQ expression is a closure. This means your variable selected is captured by the LINQ expression in each iteration of your foreach-loop. The same variable is used in each iteration of the foreach, so it will end up having whatever the last value was. To get around this, you will need to declare a local variable within the foreach-loop, like so:
//For each searchable value
foreach(var selected in SelectedValues1)
{
var localSelected = selected;
Results = Results.Union(Potential.Where(x => x.Value1 == localSelected));
}
It is much shorter to just use .Contains():
Results = Results.Union(Potential.Where(x => SelectedValues1.Contains(x.Value1)));
Since you need to query multiple SelectedValues collections, you could put them all inside their own collection and iterate over that as well, although you'd need some way of matching the correct field/property on your objects.
You could possibly do this by storing your lists of selected values in a Dictionary with the name of the field/property as the key. You would use Reflection to look up the correct field and perform your check. You could then shorten the code to the following:
// Store each of your searchable lists here
Dictionary<string, IEnumerable<MyClass>> DictionaryOfSelectedValues = ...;
Type t = typeof(MyType);
// For each list of searchable values
foreach(var selectedValues in DictionaryOfSelectedValues) // Returns KeyValuePair<TKey, TValue>
{
// Try to get a property for this key
PropertyInfo prop = t.GetProperty(selectedValues.Key);
IEnumerable<MyClass> localSelected = selectedValues.Value;
if( prop != null )
{
Results = Results.Union(Potential.Where(x =>
localSelected.Contains(prop.GetValue(x, null))));
}
else // If it's not a property, check if the entry is for a field
{
FieldInfo field = t.GetField(selectedValues.Key);
if( field != null )
{
Results = Results.Union(Potential.Where(x =>
localSelected.Contains(field.GetValue(x, null))));
}
}
}

No, your use of union is absoloutely correct.
The only thing to keep in mind is it excludes duplicates as based on the equality operator. Do you have sample data?

Okay, I think you are are haveing a problem because Union uses deferred execution.
What happens if you do,
var unionResults = Results.Union(matched).ToList();
Results = unionResults;