LINQ - WPF Discussion Boards

4-Aug-09 0:44

Hi,
Am very new to silverlight, and am working on this module where i want to retrieve some data from database, the problem is when i mention column names it dosnt work properly and it gives an error at the return statement....the code is below...

Public Function GetLoginName(ByVal strLogin As String) As List(Of tbl_User) Implements IService1.GetLoginName
Dim db As New DataClasses1DataContext
Dim matchingLogin = From Login In db.tbl_Users Where Login.UserName.StartsWith(strLogin) _
Select Login.UserName, Login.Password
Return matchingLogin
End Function

Can any1 tell me how to achieve it ..

Thanks in advance

Michael Sync4-Aug-09 1:27

Michael Sync

4-Aug-09 1:27

Are you writing this code in Webservice (not Silverlight)? Then, it is not probably Silverlight problem. Never mind, we can still help.

What error are you getting now? What is "strLogin" or What do you have in "db.tbl_Users"?

Thanks and Regards,
Michael Sync ( Blog: http://michaelsync.net)

Microsoft MVP (Silverlight), WPF/Silverlight Insiders

sunil.n.cs4-Aug-09 2:32

4-Aug-09 2:32

Ya you are right am writing this code in web.service, "strlogin" contains the keyword depending on which i have to retrieve data from database. What am trying to do here is retrieve username and password from table tbl_user.

Am getting the following error
Unable to cast object of type 'System.Data.Linq.DataQuery`1[VB$AnonymousType_0`2[System.String,System.String]]' to type 'System.Collections.Generic.List`1[E_Learning.Web.tbl_User]'.

Is there any other way to retrieve data from database without using webservice, am very new 2 silverlight and dont know much about it, any support will be of great help.

Michael Sync4-Aug-09 4:24

Michael Sync

4-Aug-09 4:24

I don't have VB in my VS. But I guess you will need to cast it before returning the object.

Return matchingLogin.ToList()

sunil.n.cs wrote:
Is there any other way to retrieve data from database without using webservic

If you are using silverlight-ported object database like db4o or silverlightdb then you can retrieve the data without using any web service. Otherwise, No.

Thanks and Regards,
Michael Sync ( Blog: http://michaelsync.net)

Microsoft MVP (Silverlight), WPF/Silverlight Insiders

sunil.n.cs4-Aug-09 18:09

4-Aug-09 18:09

I already tried casting before returning the object, it works fine until i dont mention any column names in select, if i do mention any column names as in this case den i get the following error when i try to cast.

Value of type 'System.Collections.Generic.List(Of <anonymous type="">)' cannot be converted to 'System.Collections.Generic.List(Of E_Learning.Web.tbl_User)'.

Pete O'Hanlon4-Aug-09 22:05

4-Aug-09 22:05

As Michael said, you need to do:Return matchingLogin.ToList(). The reason you have to do this is because Linq uses a technique called deferred execution (please Google and have a read about this feature), which means that matchingLogin is actually IQueryable(Of tbl_User).

If you think about this, it makes a lot of sense because there are so many different operations that you could perform here on your select that you need to tell it which one you want. For instance, you could return a single row using SingleOrDefault. The onus is on you to tell the query which one you want, hence you use Return matchingLogin.ToList().

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

sunil.n.cs4-Aug-09 22:52

4-Aug-09 22:52

First of all i would like to thank u guys for the support u have been providing. Now as i mentioned in my last reply i am using deferred execution i.e :Return matchingLogin.ToList()...this will work fine for the query

Dim matchingLogin = From Login In db.tbl_Users Where Login.UserName.StartsWith(strLogin) _
Select Login
Return matchingLogin.ToList()

But when i use column names in select like this one

Dim matchingLogin = From Login In db.tbl_Users Where Login.UserName.StartsWith(strLogin) _
Select Login.UserName, Login.Password
Return matchingLogin.ToList()

it gives me a error in blue underline for matchingLogin.ToList() and when i move the mouse on top it, it says

Value of type 'System.Collections.Generic.List(Of )' cannot be converted to 'System.Collections.Generic.List(Of E_Learning.Web.tbl_User)'.

So to remove this error i have removed .Tolist() from Return

Pete O'Hanlon4-Aug-09 23:50

4-Aug-09 23:50

Ah - sorry, I missed the bit where you picked out the smaller set of data. The problem you've got here, is that you've created an anonymous class in the Select. If you want to return the whole class, use Select Login instead, which selects the lot for you.

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

sunil.n.cs5-Aug-09 1:00

5-Aug-09 1:00

I dont want to take the whole class, i want to take smaller set i.e only the required amount data from the database, is it that der is no option to take only required amount of data(Column selection option) here.

Pete O'Hanlon5-Aug-09 1:28

5-Aug-09 1:28

Then you'll have to create a class that matches these columns and return a list of that class instead. You'll use the select projection to fill that class.

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

sunil.n.cs5-Aug-09 1:50

5-Aug-09 1:50

Thanks a lot, if possible can u plz give me an example on how to achieve it.

Pete O'Hanlon5-Aug-09 2:13

5-Aug-09 2:13

Bear in mind that I'm a C# developer (not a VB.NET one), so I may not get the syntax 100% correct, but I should imagine that this will work:

Public Function GetLoginName(ByVal strLogin As String) As List(Of UserSubset) 
  Implements IService1.GetLoginName
  Dim db As New DataClasses1DataContext
  Dim matchingLogin = From Login In db.tbl_Users Where Login.UserName.StartsWith(strLogin) _
    Select New UserSubset With {.Name = Login.UserName, .Password = Login.Password }
  Return matchingLogin.ToList
End Function

You will need to create a class called UserSubset that contains the properties Name and Password. That should be good enough to get you started.

"WPF has many lovers. It's a veritable porn star!" - Josh Smith

As Braveheart once said, "You can take our freedom but you'll never take our Hobnobs!" - Martin Hughes.

sunil.n.cs5-Aug-09 3:11