I want to select my price level in database to compare with the an integer number. But It is error : Operator '==' cannot be applied to operands of type 'System.Linq.IQueryable' and 'int'.
This is my code :
if (Request.IsAuthenticated){
CustomerModels cm = new CustomerModels();
string userName = Page.User.Identity.Name;
var list_pricelevel = from c in cm.DataContext.Customers
where c.WebAccount == userName
select c.PriceLevel;
if (list_pricelevel == 3) {
Response.Write("Welcome");
}
}
var list_pricelevel
This is per definition not an int because more than one row can be returned.
I don't use SQL syntax (only lambda) but at the end you want the equivalent of a .FirstOrDefault or Single or First. Basically taking the first row.
replace:
if (list_pricelevel == 3)
with:
if (list_pricelevel.First() == 3)
as you can see here: http://msdn.microsoft.com/en-us/library/bb291976.aspx, if you are sure there is a result or use FirstOrDefault...
when you have the result from LinQ expression you will always have the list of result set.
So in your code when you are querying as below :
var list_pricelevel = from c in cm.DataContext.Customers
where c.WebAccount == userName
select c.PriceLevel;
The list_pricelevel will be in the form of List ie IQueryable list,
so you have to get only one element to check with one element
so use the below code :
if (list_pricelevel.Single() == 3)
{
Response.Write("Welcome");
}
or
if (list_pricelevel.First() == 3)
{
Response.Write("Welcome");
}
both the above code gives you only one result set value so you can equate with 3 for validation.
Here is my suggestion:
if (list_pricelevel.First() == 3)
{
Response.Write("Welcome");
}
This may rise a NullReferenceException in case there is no item in Customers satisfying where c.WebAccount == userName.
Explanation:
list_pricelevel is a IEnumerable of items satisfying your where clause.
You need get the first item from your collection.
if (list_pricelevel.First() == 3) {
Response.Write("Welcome");
}
That's the beauty of Linq that every query returns an IQueryable so you can defer getting the final result until you really decided what you want. On the other word you can execute a query over another query :)
So in order to get the real data out of a query you should execute a command over it that actually returns what you want.
In your case since you are expecting your query to return only one value any methods like "FirstOrDefault","Single" or "First" will do the trick
Related
I'm testing a simple LINQ select query and want to get two DateTime values from the table, but I'm doing something wrong here and need to know what I'm doing/thinking wrong?
My query:
var test = from x in db.Projects where x.ID == 1 select x;
Then I try to get on of the values like this:
DateTime Date = test. ?????
Here I thought I should get a suggestion from the Intellisense after the dot to pick the value from the column StartDate the table, but this isn't working.
If you need multiple matches...
Are you sure that you have multiple Project objects that have the same ID of 1 which your query currently suggests? If that is the case, then your query should return all of the records that meet that constraint via the Where() method :
// Get your Projects that meet your criteria
var test = db.Projects.Where(p => p.ID == 1);
If you need to access properties from these elements, you could either loop through them explicitly :
// Iterate through each match that was found
foreach(var t in test)
{
// Access your properties here
DateTime dt = t.YourDateProperty;
}
Or you could accomplish this using a Select() statement to only pull the properties that you need :
// This will return a collection of Dates mapped from each element in your collection
var testDates = db.Projects.Where(p => p.ID == 1)
.Select(x => x.YourDateProperty);
If you only need a single match...
If you only need to match a single element within your collection, you might consider using the First(), Single() or their equivalent FirstOrDefault() and SingleOrDefault() methods, which will return a single entity that you can use as expected :
// This will return the first Project with an ID of 1
var test = db.Project.FirstOrDefault(p => p.ID == 1);
// If it was successful
if(test != null)
{
// Then you can safely access it here
DateTime dt = test.YourDateProperty;
}
The only difference between the methods mentioned (normal vs OrDefault()) is that the OrDefault() methods will return null if no matching elements are found, so they generally require a null check as seen above.
test is going to be an enumeration (IEnumerable<>, IQueryable<>, etc... many are applicable) of your Project type. So if, for example, you want the first record, you might do this:
DateTime Date = test.First().SomeDateProperty;
All of the data returned from your query is in test. It could be zero records, one record, many records, etc.
In test you will have a collection which matches the condition x.ID == 1. You should iterate through that collection and take your needed properties.
Edit
I suggest you to use the syntax:
var result = db.Projects.FirstOrDefault(x => x.ID ==1);
this is such as:
var date = test.FirstOrDefault();
DateTime? Date = date != null ? date.StartDate : null;
Just some details. Get Records is a variable where it contains the results of my stored procedure. Now, what I want to ask is what if I want to remove the group by function but I still want to get the key and items? Is there a way to do it?
var sortResCinema = GetRecords.Where(x => test2.branch == x.Bbranch && test.movieName == x.MovieName && x.MovieName != null)
.GroupBy(x => x.MovieName,
(key, elements) =>
new
{
Id = key,
Items = elements.ToList()
}).ToList();
There's no need for GroupBy here since you are looking for a specific movieName.
I guess you wanted something like this:
var sortResCinema = GetRecords.Where(x => test2.branch == x.Bbranch && test.movieName == x.MovieName).ToList();
You can replace the GroupBy with a Select. The Select statement can be used to alter the type of the results returned, which is what you appear to want to do. Should work with exactly the same syntax as the second parameter. So replace "GroupBy" with "Select" and remove the first argument. The key and elements properties that are being used in the GroupBy statement are internal to that function so you'd need to work out what function you want to replace these by, for instance the key might be x.MovieName.
I have a function like that :
public int? calculateContractPrice(int? comid)
{
int? sum = 0;
var q = from i in dbconnect.tblMaterialGroups
where i.tenderId == _tenderId
select i.id;
foreach (int i in q )
{
var q2 = from g in dbconnect.tblMaterialTenderAnnouncePrices
where g.MaterialGroupId == i && g.companyId == comid
select g;
sum = q2.First().amount*q2.First().price + q2.First().amount*q2.First().PriceForElse + sum;
}
return sum ;
}
When i try to execute this :
List<presentationcontract> q = (from i in dbconnect.tblContracts
where i.tender == _tenderId
select new presentationcontract()
{
tax =(calculateContractPrice(i.companyId)*(6/100)).ToString()
}).ToList();
Tax is string .after executing i got this error :
couldn't translate expression calculateContractPrice(i.companyId)*(6/100),invoke(value(system.Func1[system.nullable1[system.Int32]]))).ToString() into SQL and could not treat it as a local expression
Your edit makes clear the issue. You're trying to do
tax =(calculateContractPrice(i.companyId)*(6/100)).ToString()
in a sql statement but calculateContractPrice is in c#! To understand what's going on you really need to understand a bit how LINQ works.
First of all, stop using the silly sql-style syntax for LINQ. It is less powerful than the lambda syntax and hides what is going on under the hood in a way that makes it hard to understand.
Second consider a LINQ statement
users.Where(u => u.Name == "George").ToList();
where users is IEnumerable<User>. What happens here is that the lambda part is of type Func<User, bool> and gets compiled to a method that gets run against every single instance of User.
Now consider this LINQ statement
db.Users.Where(u => u.Name == "George").ToList();
where db.Users is IQueryable<T>. This looks the same but what happens is VERY different. What happens is that lambda is actually of type Expression<Func<User, bool>> this doesn't get compiled to a method, instead it gets compiled into something called an expression tree. This gets passed to the LINQ provider (in your case Entity Framework I'm guessing) which examines it and converts that into a SQL statement
SELECT Id, Name, Address FROM users WHERE Name = 'George'
What is happening in your case is that it sees the call to calculateContractPrice and simply has no way of converting that to SQL.
What you should therefore do is ensure the query runs first, then use the IEnumerable<T> form of LINQ that runs in c# to call your method.
var contracts = dbconnect.tblContracts.Where(i => i.tender == _tenderId)
.ToList() //Query executes here, now you have IEnumerable<T>
.Select(i => new PresentationContract {
Tax = ...
}).ToList(); //this ToList is only necessary if you want to prevent multiple iteration
You will want to solve all the other problems everyone else pointed out as well of course.
A few other notes - you will want to read up on .Net naming conventions. Usually, anything public,protected, or internal (classes, fields, properties, etc.) is recommended to be PascalCase. Also you probably want to move the division portion into the PresentationContract class. This class that has a Tax property should probably be the one that knows how to generate it.
Try this:
int? ret = calculateContractPrice(i.companyId);
if(ret.HasValue)
{
tax =(ret.Value*(6/100)).ToString();
}
You should make sure that the function indeed returned a value and then you use that integer value in calculation.
I want to query my item in table Items, where the last update of each item must be less than 91 days old (from last update till now) and the quantity > 0.
This is my code in the Model:
public IList<Item> GetAllProducts()
{
var ien_item = from i in this.DataContext.Items
orderby i.LastUpdated descending
select i;
return ien_item.ToList().Where(
s =>
HelperClasses.HelperClass.IsLastUpdate(s.LastUpdated.Value) == true
&&
(s => s.Quantity) > 0
)
.ToList();
}
Anyone can solve it? Thanks.
We don't really know what's not working here. EDIT: Merlyn spotted it; your lambda syntax is messed up. There's more to do here though.
However, I'd have thought you'd want this:
public IList<Item> GetAllProducts()
{
var lastUpdateLimit = DateTime.UtcNow.Date.AddDays(-91);
var query = from item in DataContext.Items
where item.Quantity > 0 && item.LastUpdated >= lastUpdateLimit
orderby item.LastUpdated descending
select item;
return query.ToList();
}
Note that this is able to do all the querying at the database side instead of fetching all the items and filtering at the client side. It does assume that HelperClasses.HelperClass.IsLastUpdate is simple though, and basically equivalent to the filter I've got above.
(One additional point to note is that by evaluating UtcNow.Date once, the result will be consistent for all items - whereas if your code evaluates "today" on every call to IsLastUpdate, some values in the query may end up being filtered against a different date to other values, due to time progressing while the query is evaluating.)
EDIT: If you really need to use HelperClasses.HelperClass.IsLastUpdate then I'd suggest:
public IList<Item> GetAllProducts()
{
var query = from item in DataContext.Items
where item.Quantity > 0
orderby item.LastUpdated descending
select item;
return query.AsEnumerable()
.Where(s => HelperClass.IsLastUpdate(s.LastUpdated.Value))
.ToList();
}
... then at least the quantity filter is performed at the database side, and you're not creating a complete buffered list before you need to (note the single call to ToList).
The problem is your lambda syntax. You're trying to define a second lambda while in the middle of defining a first lambda. While this is possible to do, and useful in some contexts, it is sort of an advanced scenario, and probably won't be useful to you until you know you need it.
Right now, you don't need it. Unless you know you need it, you don't need it :)
So -
Instead of what you've written:
.Where(
s =>
HelperClasses.HelperClass.IsLastUpdate(s.LastUpdated.Value) == true
&& (s => s.Quantity) > 0
)
Write this instead:
.Where(
s =>
HelperClasses.HelperClass.IsLastUpdate(s.LastUpdated.Value) == true
&& s.Quantity > 0 // Notice I got rid of the extra lambda here
)
If you're morbidly curious:
The compile error you got is because you didn't define your second lambda correctly. It redefined a variable you'd already used (s), and you were trying to check if a lambda was greater than zero. That makes no sense. You can only compare the result of a lambda to some value. It's like calling a function. You don't compare functions to numbers - you compare the result you get when calling a function to a number.
Easy ...
public IList<Item> GetAllProducts()
{
var ien_item =
from i in DataContext.Items
where
HelperClasses.HelperClass.IsLastUpdate(i.LastUpdated.Value)
&& s.Quantity > 0
orderby i.LastUpdated descending
select i;
return ien_item.ToList();
}
Linq to SQL: Methods are not allowed (linq is not magic and can not convert C# methods to TSQL)
http://msdn.microsoft.com/en-us/library/bb425822.aspx
Linq to Object: while looking the same, it is much more powerful than linq to SQL... but can not query SQL databases :)
http://msdn.microsoft.com/en-us/library/bb397919.aspx
Linq to XML: same as linq to Object, with xml object
http://msdn.microsoft.com/en-us/library/bb387098.aspx
Linq to Dataset: not the same as Linq to SQL !
http://msdn.microsoft.com/en-us/library/bb386977.aspx
Other linq providers:
http://en.wikipedia.org/wiki/Language_Integrated_Query
What is the best way to assemble a dynamic WHERE clause to a LINQ statement?
I have several dozen checkboxes on a form and am passing them back as: Dictionary<string, List<string>> (Dictionary<fieldName,List<values>>) to my LINQ query.
public IOrderedQueryable<ProductDetail> GetProductList(string productGroupName, string productTypeName, Dictionary<string,List<string>> filterDictionary)
{
var q = from c in db.ProductDetail
where c.ProductGroupName == productGroupName && c.ProductTypeName == productTypeName
// insert dynamic filter here
orderby c.ProductTypeName
select c;
return q;
}
(source: scottgu.com)
You need something like this? Use the Linq Dynamic Query Library (download includes examples).
Check out ScottGu's blog for more examples.
I have similar scenario where I need to add filters based on the user input and I chain the where clause.
Here is the sample code.
var votes = db.Votes.Where(r => r.SurveyID == surveyId);
if (fromDate != null)
{
votes = votes.Where(r => r.VoteDate.Value >= fromDate);
}
if (toDate != null)
{
votes = votes.Where(r => r.VoteDate.Value <= toDate);
}
votes = votes.Take(LimitRows).OrderByDescending(r => r.VoteDate);
You can also use the PredicateBuilder from LinqKit to chain multiple typesafe lambda expressions using Or or And.
http://www.albahari.com/nutshell/predicatebuilder.aspx
A simple Approach can be if your Columns are of Simple Type like String
public static IEnumerable<MyObject> WhereQuery(IEnumerable<MyObject> source, string columnName, string propertyValue)
{
return source.Where(m => { return m.GetType().GetProperty(columnName).GetValue(m, null).ToString().StartsWith(propertyValue); });
}
It seems much simpler and simpler to use the ternary operator to decide dynamically if a condition is included
List productList = new List();
productList =
db.ProductDetail.Where(p => p.ProductDetailID > 0 //Example prop
&& (String.IsNullOrEmpty(iproductGroupName) ? (true):(p.iproductGroupName.Equals(iproductGroupName)) ) //use ternary operator to make the condition dynamic
&& (ID == 0 ? (true) : (p.ID == IDParam))
).ToList();
I came up with a solution that even I can understand... by using the 'Contains' method you can chain as many WHERE's as you like. If the WHERE is an empty string, it's ignored (or evaluated as a select all). Here is my example of joining 2 tables in LINQ, applying multiple where clauses and populating a model class to be returned to the view. (this is a select all).
public ActionResult Index()
{
string AssetGroupCode = "";
string StatusCode = "";
string SearchString = "";
var mdl = from a in _db.Assets
join t in _db.Tags on a.ASSETID equals t.ASSETID
where a.ASSETGROUPCODE.Contains(AssetGroupCode)
&& a.STATUSCODE.Contains(StatusCode)
&& (
a.PO.Contains(SearchString)
|| a.MODEL.Contains(SearchString)
|| a.USERNAME.Contains(SearchString)
|| a.LOCATION.Contains(SearchString)
|| t.TAGNUMBER.Contains(SearchString)
|| t.SERIALNUMBER.Contains(SearchString)
)
select new AssetListView
{
AssetId = a.ASSETID,
TagId = t.TAGID,
PO = a.PO,
Model = a.MODEL,
UserName = a.USERNAME,
Location = a.LOCATION,
Tag = t.TAGNUMBER,
SerialNum = t.SERIALNUMBER
};
return View(mdl);
}
Just to share my idea for this case.
Another approach by solution is:
public IOrderedQueryable GetProductList(string productGroupName, string productTypeName, Dictionary> filterDictionary)
{
return db.ProductDetail
.where
(
p =>
(
(String.IsNullOrEmpty(productGroupName) || c.ProductGroupName.Contains(productGroupName))
&& (String.IsNullOrEmpty(productTypeName) || c.ProductTypeName.Contains(productTypeName))
// Apply similar logic to filterDictionary parameter here !!!
)
);
}
This approach is very flexible and allow with any parameter to be nullable.
You could use the Any() extension method. The following seems to work for me.
XStreamingElement root = new XStreamingElement("Results",
from el in StreamProductItem(file)
where fieldsToSearch.Any(s => el.Element(s) != null && el.Element(s).Value.Contains(searchTerm))
select fieldsToReturn.Select(r => (r == "product") ? el : el.Element(r))
);
Console.WriteLine(root.ToString());
Where 'fieldsToSearch' and 'fieldsToReturn' are both List objects.
This is the solution I came up with if anyone is interested.
https://kellyschronicles.wordpress.com/2017/12/16/dynamic-predicate-for-a-linq-query/
First we identify the single element type we need to use ( Of TRow As DataRow) and then identify the “source” we are using and tie the identifier to that source ((source As TypedTableBase(Of TRow)). Then we must specify the predicate, or the WHERE clause that is going to be passed (predicate As Func(Of TRow, Boolean)) which will either be returned as true or false. Then we identify how we want the returned information ordered (OrderByField As String). Our function will then return a EnumerableRowCollection(Of TRow), our collection of datarows that have met the conditions of our predicate(EnumerableRowCollection(Of TRow)). This is a basic example. Of course you must make sure your order field doesn’t contain nulls, or have handled that situation properly and make sure your column names (if you are using a strongly typed datasource never mind this, it will rename the columns for you) are standard.
System.Linq.Dynamic might help you build LINQ expressions at runtime.
The dynamic query library relies on a simple expression language for formulating expressions and queries in strings.
It provides you with string-based extension methods that you can pass any string expression into instead of using language operators or type-safe lambda extension methods.
It is simple and easy to use and is particularly useful in scenarios where queries are entirely dynamic, and you want to provide an end-user UI to help build them.
Source: Overview in Dynamic LINQ
The library lets you create LINQ expressions from plain strings, therefore, giving you the possibility to dynamically build a LINQ expression concatenating strings as you require.
Here's an example of what can be achieved:
var resultDynamic = context.Customers
.Where("City == #0 and Age > #1", "Paris", 50)
.ToList();