LINQ To Dataset - How To Handle DBNull
Jan 29, 2008
This is what I'm doing to select and display 3 columns of my datatable in GridView:
Dim myCustomers = From Cust In myDataset.Customers _
Select CustomerId = Cust.CustomerId, _
CustomerName = Cust.CustomerName, _
ExpiryDate = Cust.ExpiryDate
DataGridView1.DataSource = myCustomers.ToList
The expiry date has null values in many records whose expiry date is not set.On executing the query I get runtime error:System.Data.StrongTypingException "The value for column 'ExpiryDate' in table 'Customers' is DBNull."
Obviously, my LINQ query is wrong or incomplete, but I am not able to figure the way to do it correctly.
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May 20, 2010
I've got the following LINQ Statement:[code]invRecord.Pack_Num is a field of type Integer. This means that when I try to access it, if it is DBNull I get a StronglyTypedException. The above code throws this exception. If, however, I remove the "invRecord.Pack_Num = PSNum" and in its place put something like "True", the code works fine. So I guess my question is, why is that that invRecord.IsPack_NumNull() returns False when the value is in fact DBNull and what can I use as a conditional instead? I've been beating my head against the wall for a while now and I can't find a solution to this problem.
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Nov 12, 2011
I've got the following LINQ Statement:
[code]...
invRecord.Pack_Num is a field of type Integer. This means that when I try to access it, if it is DBNull I get a StronglyTypedException. The above code throws this exception. If, however, I remove the "invRecord.Pack_Num = PSNum" and in its place put something like "True", the code works fine. So I guess my question is, why is that that invRecord.IsPack_NumNull() returns False when the value is in fact DBNull and what can I use as a conditional instead? I've been beating my head against the wall for a while now and I can't find a solution to this problem.
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Mar 22, 2012
I'm trying to handle DBNull exception while reading data from database. It's my code:
...
Dim SQLRDAs SqlDataReader
...
val1= GetStringFromDB(Trim(SQLRD("Name")))
val2= GetStringFromDB(Trim(SQLRD("Level")))
[Code]...
But still I get Conversion from type 'DBNull' to type 'String' is not valid. error.
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Oct 12, 2011
how to set a field in a dataset table back to <DBNULL>?
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Mar 8, 2010
Why does the following query raise the error below for a row with a NULL value for barrel when I explicitly filter out those rows in the Where clause?
Dim query = From row As dbDataSet.conformalRow In dbDataSet.Tables("conformal") _
Where Not IsDBNull(row.Cal) AndAlso tiCal_drop.Text = row.Cal _
AndAlso Not IsDBNull(row.Tran) AndAlso tiTrans_drop.Text = row.Tran _
[code]....
Run-time exception thrown : System.Data.StrongTypingException - The value for column 'barrel' in table 'conformal' is DBNull.
How should my query / condition be rewritten to work as I intended?
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Oct 6, 2009
Im using VB in VS 2005.I have found several methods of testing if a value from a dataset is null. Are there differences between these two ways of doing it:
[code...]
Is one faster than the other? From some searching online, there seems to be debate about the best way to handle null values.
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Nov 3, 2010
I am trying to work programmatically with a dataset. The tableAdapter was created by dragging and dropping the dataset on my form, and then deleting the binding navigator and dataviewgrid. The problem I am having is when I am trying to delete a record that is DBNull. The dataset.delete( ) call creates fields for each of the fields, along with their data type (say Description as String). but I get an error whenever the field has a DBNull value.
CourseTableAdapter.Delete(DBDataSet.Course.Item(1).CourseID, DBDataSet.Course.Item(1).CourseDescription)
The error is Throw New Global.System.Data.StrongTypingException("The value for column 'Description' in table 'Course' is DBNull."I do have Option Strict On, but the problem exists even if I turn it off.
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Apr 29, 2011
I am querying a datagridview and it works great unless one of the cells has nothing (dbnull). How to over come this?Exceptions: Operator '=' is not defined for type 'DBNull' and type 'DBNull'.
Dim query = From row As DataGridViewRow In DataGridView1.Rows _
Where row.Cells(SelectedColumnIndex).Value = filter _
And row.Visible = False _
Select row Distinct
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Mar 10, 2010
In this example, an error is raised if either row.FirstName or row.LastName are NULL.
How do I rewrite the Select clause, to convert a DBNull value to a blank string ""?
Dim query = From row As myDataSet.myDataRow in myDataSet.Tables("MyData") _
Select row.FirstName, row.LastName
NOTE: Since the DataSet is strongly-typed. I can use row.isFirstNameNull(), but IIF(row.isFirstNameNull(), "", row.FirstName) will not work since all parameters are referenced.
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Oct 12, 2009
I've created a small WPF app in VB9 to catalog our sizable DVD collection. This exercise has been more about learn programming than the need to have a movie database app, but I digress.The SQL database I created has a field for "Genre" that does not have an entry for all the movies in our collection. As such, when I run the method that filters the movie listing by "Genre" the compiler throws the "StrongTypingException was unhandled" error stating "The value for column 'Genre' in table 'Movies' is DBNull.".
I now understand where the DBNull problem comes from and have been searching for several hours to find a VB solution that converts the DBNull fields to strings on the fly during the LINQ query. I've found many solutions people have tried but, since I'm quite new to all this, none of them have worked for me. Pasted below is the method that throws the exception.
[Code]...
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Apr 27, 2011
In the following linq query I have a division by zero exception wich I can't seem to code around. It occurs when the Sum of fldDevider returns 0.[code]...
How can I Make sure the ValuePerMonth field is 0 when the Group.Sum(Function(dr) CDec(dr.Field(Of String)(fldDevider))) function returns a 0. Or how do I return a 1 for Sum fldDevider if it is actually 0.
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May 24, 2009
I've got a subquery that returns the most recent value from a child table. In some cases the subquery returns nothing. The query below fails at runtime because the inferred type of MemberPrice is decimal and is not nullable. [code]...
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Jun 11, 2009
In a WinForms app (VS2008 / .Net Framework 3.5) I bind data from SQL Server to controls. If there's a null value e.g. an employee has no middle name (allowed by business rules), when I browse that record at runtime an exception is thrown. So I code: If Not (row("middle_name") Is DbNull.Value then txtMiddleName.DataBindings.Add(New Binding("Text", row, "middle_name)). Say someone adds a middle name to this record, because the control had no databindings the dataset.HasChanges property is false and update stored proc doesn't run. I guess I could handle this like this: If txtMiddleName.Databindings.Count = 0 AndAlso txtMiddleName.Text <> "" Then row("middle_name") = txtMiddleName.Text. I guess I would have to do something similar to handle a value changing from not null to null.
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May 18, 2010
I have a dataset that is populated via a SQL Stored Procedure, the format of which is below: Country | Brand | Variant | 2004 | 2005 | 2006 | 2007 | 2008 The number of rows varies between 50 and several thousand. What I'm trying to do is use Linq to interrogate the dataset (there will be several Linq queries based on user options), but a simple example would be to SUM the year columns based on Brand. I have the following that I believe creates a template for me to work with: But from here on I'm absolutely stuck!
[Code]...
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Jul 14, 2011
Background: I'm importing data from a MySQL database into a SQL-Server database(for reports and later a SSAS-Cube). I want to normalize the data at the same time. I want to group repeating Ticket_IDs to one record in a table Contact with other useful informations and leave the rawdata in the sub-table ContactDetail(with foreignkey to Contact).Hence every record in Contact has a unique Ticket_ID.
I've decided to use strong typed datasets for the import. Now i'm wondering what's the best way to detect if i've already added a Ticket_ID. I could check for it in every loop(~100000 records) but i'm assuming that there is a better/faster way.
Simplified sample-data:
Ticket_ID ID fiContact
89442226 1 1
89442226 2 1[code].....
I will have a look if this is faster than the iterating approach with a HashSet to detect if the contact was already created.
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Apr 4, 2009
linq querying various nested tables in a dataset? I can't find something like that on the net.
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Jun 4, 2012
I am using VS 2008, VB.NET and Windows XP r3.I am trying to use LINQ to DataSet in my app. In my test app, I can get it to work find. My test form has an OleDbDataAdapter, an OleDbConnection and a DataSet. It also has a DataGridView bound to the one table in the DataSet, a BindingSource and a Button.[code]
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Aug 20, 2010
I have a single columned datatable inside a single tabled dataset.I just want to convert this dataset to distinct rows. Here is my code, it gives compile error '.' expected.
Dim query = _
From email In ds.Tables(0) _
Select email.Field<string>("Email").Distinct()
EDIT: I changed to (Of String) and it works... BUT NOW 'query' is an ienumerable collection of characters... not a datatable... so how do I convert back easily without manually doing a loop?
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Jun 14, 2012
i think i have not represented my question correctly here My LEFT OUTER JOIN MSDN reference is here to visualize my problem in access databaseTABLE "T1"1234567TABLE "T2"1,sunday2,monday i need output as 1,sunday2,monday34567 i need it to done through LINQso far i did is here
Dim RIGHT_table As DataTable = ds1.Tables("t1")
Dim LEFT_table As DataTable = ds2.Tables("T2")
Dim OutPut = From LEFTsource In LEFT_table.AsEnumerable(), RIGHTsource In RIGHT_table.AsEnumerable() _
[code]...
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Feb 7, 2012
I have following code that creates Linq query.
I've never used Linq until today and having problem with "Order By Clause"[code..]
If I run the code, I get following error.Name 'p' is either not declared or not in the current scope. How come p!Weight in "Select Clause" works but not in "Order By Clause"?
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Apr 19, 2010
I have used a datarepeater with a dataset and it was simple to implement by dragging fields from the Data Sources window onto the DataRepeater control
I am trying to use Linq as a data source instead of dataset.
How do I bind data to textbox with Linq for Datarepeater?
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May 26, 2011
I have a collection (IList(Of Sample)) of the following class:
Public Class Sample
Public sampleNum As String
Public volume As Integer[code].....
This collection is filled from a regex that gets passed over a file.What I want to do is use Linq to generate a collection of these for each unique samplenum using the following conditions.For each samplenum: Have the highest volume where the final is greater then one
If the sample has multiple records for this volume then pick the one with the the highest final
If the previous step leaves us with no records pick the record with the highest final ignoring volume
I am extremely new to Linq and just can't get my head around this. For now I have solved this using for each's and temporary lists but I am interested in how this would be handled using pure Linq.Sample Data:
samplenum | volume | initial | final
1 | 50 | 8.47 | 6.87
1 | 300 | 8.93 | 3.15[code]......
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Mar 17, 2009
Right now our queries can be driven through the DataTable Select statement. However, utimately we want to end up with more complex queries. We think Linq is the answer, but can't find anyexamples of this.Here is the sysnopsis:I have a dataset containingdatatable(s) of informationI have a datatable containing business rules as strings like:
(ISNULL(MEMLNAME, '') = '')
(#1880/01/01# <= CONVERT(BIRTHDATE, 'System.DateTime') AND CONVERT(BIRTHDATE, 'System.DateTime') <= #2009/12/31#)
[code].....
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Aug 26, 2011
I have a DataSet where I need to filter the collection where one of the date columns is not null.
How do you term this in LINQ?
This is what I have so far but there is a compiler error on the w.CompletedDate field in the Where clause.
Dim completed = From ins In InspectionDataset.Inspection.AsEnumerable _
Where (Function(w) w.CompletedDate IsNot Nothing) _
Order By ins.Field(Of DateTime)("UpdatedDate")
I have also tried it the below way but the compiler will not allow the DBNull comparison.
Dim completed = From ins In Inspection.AsEnumerable _
Where ins.Field(Of DateTime)("CompletedDate") <> DBNull.Value _
Order By ins.Field(Of DateTime)("UpdatedDate")
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Feb 8, 2012
I have been searching for quite a while about joining multiple datatables via LINQ to dataset (When i say multiple, i don't mean just 2!!!), and it seems there aren't lot of examples in the net. I have seen examples of joining two datatables, done that, no problem there. Now i want to join three datatables. And there's nothing i can find out there.For example i have a 3 datatables:
PERSON (columns: person_id, name, gender_code, ethnic_code)
GENDER (columns: gender_code, gender_description)
ETHNICITY (cols: ethnic_code, ethnic_description)
[code].....
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Mar 2, 2012
I am trying to convert my EnumerableRowCollection into a DataTable, but I can't see the CopyToDataTable method. The enm.CopyToDataTable() has the blue saw tooth line under it with error saying " 'CopyToDataTable' is not a member of 'System.Data.EnumerableRowCollection' ". I have set a Reference to System.Data, System.Data.Linq, and System.Data.DataSetExtensions. I thought the CopyToDataTable method was part of the System.Data.DataSetExtensions, right? Anyone know why I can't access this method?
[Code]...
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Apr 30, 2009
from a performance point of view is it faster to work with linq to sql or dataset.datatable?
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Jan 13, 2011
How can I use dynamic fields in LINQ in the PART OF THE where query? I mean that I want to write where x = *dynamic fields* I want to do the select query in all of the tables that are in the dataset, not on one datatable.
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Jun 13, 2012
i am having 2 tables one table stores the WEEK-days another table stores the WEEK-days + other fields.i need the LINQ query to return all WEEK-days + other fields just like SQL LEFT JOIN query so far i did is [code]....
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