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I have used LastOrDefault in other cases , and there were no problems translating in SQL Query.
so I think there is a SQL equivalent for LastOrDefault.
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Allow me to rephrase. That error message says the method isn't supported by the whatever the underlying provider is.
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ok , but what's wrong with my query , and what should I do ?
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Try this:
Dim query1 = (From t In context.students order by t.Name select t).ToList
Dim query2 = From t1 In query1
Let p = t1.status.Where(Function(t2) t2.year<=year1).OrderBy(Function(t3)
t3.year).LastOrDefault
Where Not IsNothing(p) AndAlso p.active = True
Select t1
Dim results As IEnumerable(Of students) = query2.ToList()
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Again , on Local Cache I have all the students.The same problem as my query on my first post.
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I don't think, I know it is not part of SQL92. It is something from a local dialect.
Bastard Programmer from Hell 
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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That's annoying. Try switching to OrderByDescending, and use FirstOrDefault.
Dim queryv As IQueryable(Of students)
queryv = From t In context.students order by t.Name select t
queryv = From t1 In queryv
Let p = t1.status.Where(Function(t2) t2.year <= year1).OrderByDescending(Function(t3) t3.year).FirstOrDefault()
Where Not IsNothing(p) AndAlso p.active = True
Select t1
Dim results As IEnumerable(Of students) = queryv.ToList()
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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As usual, as soon as I post, if figure it out; sorry. 
This
Me._Column = Activator.CreateInstance(GetType(T))()
Should be this
Me._Column = Activator.CreateInstance(GetType(T)).
However, now I have another problem; I'll work on this awhile.
I hope I am in the right group.
I have a Class that creates an xml file that currently works.
However, I would like to change the definition to define 1 of 2
generic types (T) when creating the file; Column1 or Column2. The code below is what
I have come up with through searches.
Everything compiles but when I create an instance of MyXmlDef I get this error.
'No Default Member Found for type 'Column1'
A search on the error says:
A late-bound index Get or Set call has been attempted.
Ensure that the object has a default property with a Get or Set, depending on which you are trying to make.
I have tried to define a Property for Column below but it doesn't fix the error.
I don't know what 'object' the error refers to.
Can someone help?
Thank you
' I am just showing the relevant code
' I have hardcoded this to Column1 to test
public Class MyXmlDef
<XmlElement("ColumnList")> _
Public _ColumnList As ColumnList(Of Column1)
Public Sub New()
Me._ColumnList = New ColumnList(Of Column1)
end sub
end Class
Class ColumnList(Of T)
<XmlElement("Column")> _
Public _Column As T
' This is test code to try and fix the error; it doesn't fix.
Public Property Column() As T
Get
Return Me._Column
End Get
Set(ByVal value As T)
Me._Column = value
End Set
End Property
Public Sub New()
' THis is the code I found on the internet to create an object from generic.
' However, it does not compile.
' Me._Column = Activator.CreateInstance(T)()
' This compiles but get the error described above for Column1.
' I do not know if this is valid or not.
Me._Column = Activator.CreateInstance(GetType(T))()
End Sub
End Class
Class Column1
<XmlElement("ProductName")> _
Public _ProductName As XMLProductName
' ... other XmlElments
Public Sub New()
'... initialize elements
end sub
End Class
Class Column2
<XmlElement("ProductName")> _
Public _ProductName As XMLProductName
' ... other different XmlElments
Public Sub New()
'... initialize elements
end sub
End Class
-- modified 3-Apr-19 13:40pm.
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You just need a constraint on your generic type parameter:
Class ColumnList(Of T As New)
...
Public Sub New()
Me._Column = New T()
End Sub
...
Generic Types in Visual Basic (Visual Basic) | Microsoft Docs[^]
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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Hello,
I have a program in Visual Basic 2010 32 bit now i want to convert it to visual Studio 2019 since the old 2010 is almost end of life.
I have opened the program in Visual studio 2019 there was a message that there are conversions needed and when this was finished i clicked run There is a error
--> An error occured while signing. Failed to sign bin\debug\app.publish\projectname.exe . Signtool Error: No Certificates where found that met all the given criteria.
The program was written in Visual basic 2005, i imported it in VB 2008 and at last in VB 2010 . The application is running without any problem on a Old Windows XP wich is not connected to Internet so there is no issue of security since it is stand alone but the pc is getting old and need to be replaced so when i want to use the application on new Windows 10 64 bit wich will be on the internet …
Is there a solution to import and use the program in 2019 without any special issues ? if not is there a way to do this on a visual studio 2012 ? 2013 2015 ??? otherwise i try with the older version of visual studio
If it will be the same what can i do to get it up and running on Visual Studio 2019 what i prefer.
Many thanks
best regards
D.
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You probably need to resign the application as the certificate used is too old, wrong format or expired.
Go to right click on the project and open properties, there will be a tab called signing.
Never underestimate the power of human stupidity -
RAH
I'm old. I know stuff - JSOP
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Thanks. Unchecked Signing and problem solved.
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Hello !
I'm using VB.net 2017 , Entity framework 6.
On my form I have loaded several entities and its childs.
Mylist=context.myobjs.Include("mychilds").Tolist.
Now , another user that use the same program from another computer , make some changes in some of the records and/or some childs. He save the data to database.
On my form I have a refresh button that execute the same query as above.
The problem is that when I press this button , on my list I have no updated values from database but the old values. It seems that that query does not read the database but use the data on local cache ( Why ???)
I know that disposing the context and creating a new one will resolve this problem , but I don't want to do this because on my forms I have other records from other entities that I have loaded . And if I recreate the context I need to load again all these but I don't want to do this because only records form 1 entity and its childs can be updated from other users as I described above.
Searching in internet , I've found 2 general methods doing this :
METHOD 1
For Each en As myobj In mylist
context.Entry(en).Reload()
For Each chld In en.mychilds
context.Entry(chld).Reload()
Next
Next
METHOD 2
For Each en As myobj In mylist
For i= en.mychilds.count-1 to 0 step -1
context.Entry(en.mychilds(i)).State = EntityState.Detached
Next
context.Entry(en).State = EntityState.Detached
Next
Mylist=context.myobjs.Include("mychilds").Tolist
The problem is that both methods are very slow when mylist is very large.
Is there any other way ?
Thank you !
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Use different context instances to maintain different sets of data that you can reload separately?
There's nothing that says you have to use a single context instance for everything. That way you can just destroy the context instance and create a new one, telling it to load the data you want all over again.
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But the entities have relation between each other , how can I keep different contexts !!!!!?????
one entity is a child for other and is parent for others. How can I use these in different contexts ????
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It wasn't clear in your original post if these records you're trying to reload were related to the other entities or not.
There is no fast way to do this. You could have the context execute a SQL query to reload just the records you want, but the context is still going to go through each and every result record to update the cached objected.
I think your design needs to be reconsidered if you have a user making changes that forces other users to reload a large amount of data. Minimize the data each user is caching as much as possible.
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This is what i'm asking : a query to reload all the records from an entity.
If some of objects inside myobj are updated from other users , I need a query to reload all the objects in myobj from database. But I can't find a such query to do this so I have to reload one by one as on my 2 methods described in my first question.
It's strange why there's no a method on entity framework to force reloading all object from an entity from database and not read on local cache. !!!!!
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Then you're going to have to tell the clients which records updated from the client that wrote to the database.
Now, every client is going to have to retrieve the object again using that records ID value. Retrieving a record by ID is easy. Reloading EVERY record you have cached is going to take forever, no matter what you do.
There is no solution around this.
Even without an ORM like EF, there's no real good way to do this! It's your application design that has the problem.
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Sorry , I think this is a problem of Entity Framework Design.
Because , if other users add or delete records , when I run my query these changes are "known" by EF and are reflected in results.
But the updated records are not read from database and old values are kept from local cache.
For me , It's ridiculous that EF does not have a command to "READ ALL THE DATA AGAIN FROM DATABASE"
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Those changes are NOT known by EF. At least not until you run a query again to retrieve the entire set of records again. But now you have a problem. How does EF know that the records are deleted? It doesn't. The records just don't show up in the result set from the database. That in no way means that they were deleted. The only way to know that would be to get the record ID of what EF has cached and try to retrieve that record ID from the database. Now do that for every record you have cached and it'll take a long time.
Move on to the problem of just telling EF to reload a specific table. Great, you now have all of the data from that table, but how to do handle the case where the cached data is "dirty"? That means you have data in the cache that's been modified but not written back tot he database. Do you through those changes out in favor of the current data? Do you skip that changes? Do you modify only certain fields? How is the ORM supposed to know which fields to keep and which to throw out and refresh?
Another problem. What query returned the data the generated the object in the cache? There is ORM that I'm aware of that caches the query that generated a set of object graphs. What if there's multiple queries that can be run that generate a particular object in a graph? How do you know which query to run to PROPERLY regenerate that object with the correct data? It's impossible for the ORM to know that.
Google "database concurrency" for the difficulties in doing what you want to do. There is no easy solution to this problem, and not every solution is going to work for every application.
You're looking at this through the lens of what you want EF to do and not from the standpoint of how any ORM would even accomplish this in a generic way usable by all applications. I know you just want to call .Reload() and have it work, but you're not seeing the complexity behind actually making that happen.
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Quote: How does EF know that the records are deleted? It doesn't. The records just don't show up in the result set from the database.
Ok , and why EF does not keep the records that are in local cache , but instead "use the information" that those records are not show up in the result set of database ? And why in the case of modified records , "the information from database" is not used , and it's just keep the records from local cache.
Quote: Move on to the problem of just telling EF to reload a specific table. Great, you now have all of the data from that table, but how to do handle the case where the cached data is "dirty"?
If the cached data is modified , i think EF should let the programmers to choose . If i try to reload data from database when cached data is modified , then i'm responsible to get the new data from database and overwrite those changes. But In my application for example , if a modification was made , i don't let the users to reload data from database. It's simple. And how you will resolve the problem when cached data is modified and i create a new context ( as many have suggested this idea for my problem). This is the same problem. I think these problems EF should leave in the hand of programmers.
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Quote: Ok , and why EF does not keep the records that are in local cache , but instead "use the information" that those records are not show up in the result set of database ? And why in the case of modified records , "the information from database" is not used , and it's just keep the records from local cache.
That made no sense at all.
EF maintains a cache of objects reconstituted from result sets returned by the database. Once the result set is returned, there is no connection between the objects and the database. If the database changes, EF gets no notification at all that anything happened to the database copy.
If you have modified records, EF maintains the original "as loaded" copy of the object and the modifications to it. When EF goes to update the records in the tables, all of the original field values go into the WHERE clause of the SQL DELETE OR UPDATE statements. If any of those fields do not match the current data in the fields in the database table, the query fails to update the record because of a concurrency problem. You're not updating the latest version of the record. You're trying to update a newer version of the record than what you retrieved. That is a problem not solved by EF or the database.
Seriously, go Google "database concurrency" and start reading.
desanti wrote: If the cached data is modified , i think EF should let the programmers to choose
And how do you propose you do that in the EF code? EF avoids the problem by not handling it at all, letting YOU decide how to handle it, which is why you even asked this question in the first place because of performance. Having EF do it internally will not make it any faster. It still has to same limitations you do handling it yourself, row-by-row from the cached data.
There is no solution to the performance problem! Again, your only chance at making either of the two solutions "faster" is to cache less data at the client.
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Quote: And how do you propose you do that in the EF code? EF avoids the problem by not handling it at all, letting YOU decide how to handle it, which is why you even asked this question in the first place because of performance. Having EF do it internally will not make it any faster. It still has to same limitations you do handling it yourself, row-by-row from the cached data.
With "Letting in the hand of programmers" , i mean informing them some rules , that if the local data is modified and they try to load from database , the local cache will be overwritten. So before loading from database , save or Undo those changes , but if you want to do without save or undo , you will loose those changes. I think is simple . The police does not close the road for people because there are cars moving on it. But there are semaphores and people are informed that when the semaphore is green the can walk safely, if is red they should not walk but they are free to do , keeping the risk by itself.
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desanti wrote: hat if the local data is modified and they try to load from database , the local cache will be overwritten
So is that behavior desirable in all applications and situations? Nope.
So how do you configure that behavior in code, and all possible variants? Like either keeping the modified data, dumping all the modified data, or just keeping some of it and replacing the rest, or replacing some of it based on rules, or dumping some of it based on rules, ... there's too many possibilities to list... And that's the exact reason why it doesn't show up in ORM's.
You're trying to get EF, and ORM's in general, to do something that is best left up to the coder. Hell, that's even what you said in your previous post!
By the way, your analogy doesn't even apply to this.
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Quote: So how do you configure that behavior in code, and all possible variants? Like either keeping the modified data, dumping all the modified data, or just keeping some of it and replacing the rest, or replacing some of it based on rules, or dumping some of it based on rules, ... there's too many possibilities to list
no no , i'm telling that EF could have some rules , for example if you load data from database ( with a special command like reload ) the cached data will be overwritten. So a programmer can use this and i think it's easy to resolve the situations.EF has a Reload command but for a single object at time . If a have a list of entries that i want to reload , i should do one by one ??? why not all with a single command !!!!
By your opinion , better to have a full working Reload command and let programmers to resolve the possibilities . or to have something like my second solution where i'm fooling EF telling that those objects are detached in order to resolve my problem ????!!!
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