I've had problems with the update method in subsonic, so instead of using:
a.Update()
I used
var qry = dbAnimals.Update<Notification>().
Set(n => n.NotName == notification.NotName).
Set(n => n.NotRecStatus == notification.NotRecStatus).
Set(n => n.NotModified == notification.NotModified).
Where(n => n.NotRecID == id).Execute();
The thing is that I now want to delete, and I get the same nullreference exception. so
a.Delete()
does not work. What would be the equivalent to delete that row depending on its ID? I have tried to find it but didn't get a clue.
Thank you:
To solve this problem I finally used this:
var query = new SubSonic.Query.Delete<Notification>(dbAnimals.DataProvider).
From<Notification>().
Where(NotificationTable.NotRecIDColumn).IsEqualTo(id).
Execute();
Related
Is there a simple way to modify this code to return only records where LocationID matches the id I'm trying to pass as a parameter? Needless to say, this doesn't compile. I thought Entity Framework was meant to make things easier, but I've searched online and can't find an understandable example of how to assign a simple query where a field in a single table/entity matches a number.
public async Task<List<PC>> GetPCsAsync(int id)
{
// Get our data. Don't yet know how to feed the variable to EF/Linq
PCList = await (from p in db.PC
select new PC {p.LocationID = id}).ToListAsync();
return PCList;
}
Thanks.
And also if you want to do it using Query Syntax it would be something like this:
PCList = await (from p in db.PC
where p.LocationID == id
select p).ToListAsync();
Here's a link to understand the differences between Query and Method syntax.
var list = db.PC.Where(x=>x.LocationID == id).ToList();
for async
var listAsync = await db.PC.Where(x=>x.LocationID == id).ToListAsync();
I hope it's help you!
I'm still learning ASP.NET and I've been trying to find an answer online for my question but I couldn't find any related to what I wanted. So, my question is:
Is there a way for a Session variable to hold a record from database and then be used to access only some of the columns from it ? Kinda like this:
Session["Student"] = db.Students.Where(x => x.StudentNumber == studentNumber);
Session["Student"].SingleOrDefault().StudentId;
You're getting that error because this:
Session["Student"] = db.Students.Where(x => x.StudentNumber == studentNumber);
does not store a Student in your Session variable. It saves a query in session. Change it to Session["Student"] = db.Students.Single(x => x.StudentNumber == studentNumber); or Session["Student"] = db.Students.SingleOrDefault(x => x.StudentNumber == studentNumber);
to achieve what you're looking for.
As a side note, you should probably do some null reference checks before you try to store the object in session and also when you retrieve it before using it. More like this:
var student = db.Students.SingleOrDefault(x => x.StudentNumber == studentNumber);
if(student != null)
{
Session["Student"] = student;
}
And getting it back out:
var student = (Student)Session["Student"];
if(student != null)
{
// do something with the student
}
And finally, there is almost certainly a better way to do what you want without using session variables.
I want to assign Linq Query result to dropdownlist which contain a
Distinct function
My Code:-
var area = de.City_Area_View
.Select(m => new { m.Area_Id, m.Area_Name})
.Distinct()
.ToList();
drpfilter.DataTextField = "Area_Name";
drpfilter.DataValueField = "Area_Id";
drpfilter.DataSource = area;
drpfilter.DataBind();
Problem :- When I write this code then I get Below Error
Error:- The method 'Distinct' is not supported.
I get System.NotSupportedException.
I want to assign a Distinct name of area to the DropDownList
So please help me for this problem.
If your set is small enough (so you don't mind fetching all the values from the database), the simplest thing would be to force the distinct part to be performed locally:
var area = de.City_Area_View
.Select(m => new { m.Area_Id, m.Area_Name})
.AsEnumerable()
.Distinct()
.ToList();
AsEnumerable simply "changes" the expression type to IEnumerable<T> instead of IQueryable<T>, so that the compiler calls Enumerable.Distinct instead of Queryable.Distinct - and Enumerable.Distict will definitely work.
After hours of trying and searching, I think its time to share my problem with you right now.
Problem Definition :
I have a Dictionary of KeyValuePairs(named filterPool) which includes an integer (PropertyID) and a string(McValue). What I am trying to do is filtering products depending on those KeyValuePairs and return them as a DataTable/List.
You may consider this as building dynamic "Where ... And .." clauses as SQL.
Here is the code that I am using :
foreach (KeyValuePair<int, string> filter in filterPool)
{
products = products.Where(i => i.PROPERTYID == filter.Key && i.MCVALUE.Equals(filter.Value));
}
return products.ToDataTable();
The problem is the foreach loop above seems to work only once, for the latest KeyValuePair available in the Dictionary.
As far as I could find on Stackoverflow, the closest solution to my problem was : this one, also using a Dictionary of values for filtering
There must be a way to achieve the goal of filtering using Dictionary and LINQ; or there's a huge thing that I am missing/ignoring to see somehow.
Hope the problem given is clear enough for all,
Thanks
^^
This is a closure issue. You can solve it by making a temporary:
foreach (KeyValuePair<int, string> filterTmp in filterPool)
{
var filter = filterTmp; // Make a temporary
products = products.Where(i => i.PROPERTYID == filter.Key && i.MCVALUE.Equals(filter.Value));
}
return products.ToDataTable();
For details on what's happening, see Eric Lippert's post Closing over the loop variable considered harmful.
Also note that this behavior has changed for C# 5. In C# 5/VS2012, this code would work as expected.
You're overwriting your products collection on every iteration of your foreach. I'm not sure what the data type on your collection is, but you'll want to do something like this in your foreach instead:
products.AddRange(products.Where(i => i.PROPERTYID == filter.Key && i.MCVALUE.Equals(filter.Value)));
I'm not sure if that makes sense, but it seems like you're trying to create a collection full of products that match your filterPool.
I think that it's better solved with aggregate:
return filter
.Aggregate(products, (acc, filter) => acc.Where(i => i.PROPERTYID == filter.Key && i.MCVALUE.Equals(filter.Value)));
.ToDataTable();
I have the following function that returns results from the database based on LINQ Expressions:
IQueryable<TEntity> FindAll<TEntity>(Expression<Func<TEntity, bool>> expression)
When I try pulling data from the function while using the .Any function from a list I get a null reference exception.
However when I pull the data without that specific condition and use the same .Any function in a for each loop everything works correctly.
Here is the call trying to use the .Any function which does not work:
var ppcReports = repository.FindAll<PPCReport>(
x => x.ClientId == clientId &&
(campaigns.Any(c=> c.Id == x.CampaignId))
).ToList();
And the way it does work properly:
var ppcReports = repository.FindAll<PPCReport>(
x => x.ClientId == clientId).ToList();
foreach (var item in ppcReports)
{
if (campaigns.Any(c => c.Id == item.CampaignId))
{
// do something
}
}
I was wondering why was this happening, am I doing something wrong is it just not possible to filter the results before the query finished?
By calling .ToList() before filtering the results it does work, so I suppose I cannot do such an operation on an IQueryable<T> implementation?
var ppcReports = repository.
FindAll<PPCReport>(x => x.ClientId == clientId).
ToList().
Where(w => campaigns.Any(c => c.Id == w.CampaignId)).
ToList();
Like those who commented, I'm surprised that you got a NullReferenceException rather than a complaint about not being able to compile that statement to SQL. However, the following code should let you do this in 1 query (and will do all filtering in SQL):
var campaignIds = (campaigns ?? Enumerable.Empty<Campaign>())
.Select(c => c.Id);
var ppcReports = repository
.FindAll<PPCReport>(pr => pr.ClientId == clientId
&& campaignIds.Contains(pr.CampaignId))
.ToList();
This should work in both EF and Linq-to-SQL.
Queryable.Any() returns an ArgumentNullException when the source is null, as documented here: https://msdn.microsoft.com/it-it/library/bb343630(v=vs.110).aspx