I am trying to initialize an object with data from database return in a dataset as below:
Class Pdf
Public FileId As Integer
Public AccountNumber As Integer
Public DateSaved As DateTime
Public FileName As String
Public DateImported As DateTime
Scenerio 1
I can intialize the object like this:
Dim pdf = New Pdf With {.FileId = ds.Tables(0).Rows(i)("fileid"),
.AccountNumber = ds.Tables(0).Rows(i)("accountnumber"),
.DateSaved = ds.Tables(0).Rows(i)("datesaved"),
.FileName = ds.Tables(0).Rows(i)("filename"),
.DateImported = ds.Tables(0).Rows(i)("dateimported")
}
But this is not working, because column data can be null and I am not if how to do a db null check in this approach.
Then I have scenerio 2:
Dim pdf As New pdf
If Not IsDBNull(ds.Tables(0).Rows(i)("fileid")) Then
PdfFileId = ds.Tables(0).Rows(i)("fileid")
Else
PdfFileId = 0
End If
If Not IsDBNull(ds.Tables(0).Rows(i)("accountnumber")) Then
pdf.AccountNumber = ds.Tables(0).Rows(i)("accountnumber")
Else
pdf.AccountNumber = 0
End If
If Not IsDBNull(ds.Tables(0).Rows(i)("datesaved")) Then
pdf.DateSaved = Format(ds.Tables(0).Rows(i)("datesaved"), "yyyy-MM-dd")
Else
pdf.DateSaved = Nothing
End If
If Not IsDBNull(ds.Tables(0).Rows(i)("dateimported")) Then
pdf.DateImported= Format(ds.Tables(0).Rows(i)("dateimported"), "yyyy-MM-dd")
Else
pdf.DateImported= Nothing
End If
How can I do this to avoid doing so many If statements below. This way seems inefficient to me, can anyone suggest an better approach to initializing the object in scenario one or two? If the question is unclear, please do let me know, I will try and explain.
Please note this is sample data.
From reading T Field<T>(this DataRow row, string columnName), I believe that there is a check for the DBNull.Value for both reference and value types, returning a default value if DBNull.Value is passed.
So you can use it instead of checking for DBNull.Value each time:
.FileName = ds.Tables(0).Rows(i).Field(Of String)("FileName")
If the value of the specified DataColumn is null and T is a reference type or nullable type, the return type will be null. The Field method will not return Value.
DataRowExtensions.Field
Since you cant use this then #TimSchmelter provided an answer which you could build upon:
.FileId = If(row.IsNull("fileid"), 0, Convert.ToInt32(row("fileid"))
Its not inefficient but a lot of code, so maybe there is a more readable/maintainable approach.
The If operator is one, you can also use DataRow.IsNull:
For i As Int32 = 0 To ds.Tables(0).Rows.Count - 1
Dim row = ds.Tables(0).Rows(i)
Dim pdf = New Pdf With {
.FileId = If(row.IsNull("fileid"), 0, row.Field(Of Int32)("fileid")),
.AccountNumber = If(row.IsNull("accountnumber"), 0, row.Field(Of Int32)("accountnumber")),
.DateSaved = If(row.IsNull("datesaved"), Date.MinValue, row.Field(Of Date)("datesaved")),
.FileName = If(row.IsNull("filename"), "", row.Field(Of String)("filename")),
.DateImported = If(row.IsNull("dateimported"), Date.MinValue, row.Field(Of Date)("dateimported"))
}
Next
(changed the name of your class from Object to Pdf)
You should also set Option Strict to On, then you need to code type safe but you gain compile time safety and avoid unwanted conversions.
Related
I have a list of records and for Employee R1005, I need to check if that Employee has been Enabled for login alert (i.e EnableLoginAlert = Yes), then a button will be displayed.
CompanyID EmployeeNo EnableLoginAlert
10046 R1005 Yes
20041 Ajax12 No
47021 Drek Yes
I have tried the below codes:
If dCompanyDetails.Tables(0).Rows.Count > 0 Then
Dim dataView As DataView = dCompanyDetails.Tables(0).DefaultView
dataView.RowFilter = "EmployeeNo = '" & strEmployeeNumber & "'"
Dim svalue As String = dataView.Table.Rows(0).ItemArray(0).ToString()
If svalue = "No" Then
AlertButton.Visible = False
ElseIf svalue = "Yes" Then
{
//Do something else
}
End If
End If
If you are going to use a DataView then use it. This:
Dim svalue As String = dataView.Table.Rows(0).ItemArray(0).ToString()
is simply going back to the DataTable and using it, ignoring the DataView. The DataView contains DataRowView objects so get the one you need and use it. It is similar to a DataRow and you can use it the same way in this case:
Dim enableLoginAlert = CStr(dataView(0)("EnableLoginAlert")) = "Yes"
Now you have an actual Boolean that represents the state you want.
That's not how you should do it though. Generally speaking, you would use a DataView when you want to bind data. In fact, if you bind a DataTable then the data you see in the UI actually comes from the DefaultView. That's why you can filter and sort it. In this case, there are better options.
If you want to find a row by its primary key then the Rows collection of a DataTable has a Find method, e.g.
Dim row = dCompanyDetails.Tables(0).Rows.Find(strEmployeeNumber)
Dim enableLoginAlert = CStr(row("EnableLoginAlert")) = "Yes"
If you're searching by other than the primary key, the DataTable itself has a Select method. Because multiple rows may match, it returns an array, so you need to get the row out of that, e.g.
Dim row = dCompanyDetails.Tables(0).Select($"EmployeeNo = '{strEmployeeNumber}'").First()
Dim enableLoginAlert = CStr(row("EnableLoginAlert")) = "Yes"
If you want to look up a single row it's perhaps easiest to use LINQ:
Dim row = dCompanyDetails.Tables(0).Rows.Cast(Of DataRow).AsQueryable().FirstOrDefault(Function(r) r("EmployeeNo").ToString() = strEmployeeNumber)
If row IsNot Nothing AndAlso row("EnableLoginAlert").ToString() = "Yes" Then
...
..though I'd be the first to claim that using LINQ on base DataTables is very verbose, because of the Cast/AsQueryable. I'd use strongly typed DataTables (in a dataset); if you were to convert your code to using strongly typed tables it would look like:
Dim r = someDataSet.AProperTableName.FirstOrDefault(Function(r) r.EmployeeNo = strEmployeeNumber)
If r?.EnableLoginALert = "Yes" Then
...
...using strongly typed datatables is much less messy..
nb: You need to Imports System.Linq for these to work
That LINQ is the same thing as:
For Each r as DataRow in dCompanyDetails.Tables(0)
If r("EmployeeNo").ToString() = "R1005" AndAlso r("EnableLoginAlert").ToString() = "Yes" Then
...
You also have the option of using DataTable.Select (not a LINQ thing, though LINQ has a Select too)
Dim matchingRows = dCompanyDetails.Tables(0).Select($"[EmployeeNo] = '{strEmployeeNumber}'")
If matchingrows.Count > 0 AndAlso matchingRows(0)("EnableLoginAlert").ToString() = "Yes"
I'm working on a Sindows Forms application to help keep inventory of some scanners. I'm using Linq2Sql, each table has an id column. On my repair history form. I'm trying to use the serial number from the inventory table so it goes to the database and looks up the sID from the table and it returns the correct value, but when I go to send all the entered data to the history table it gets a null reference exception.
Dim db As New DataClasses1DataContext
Dim rep As Scanner_Repair_History
Dim scan = (From Scanner_Inventory In db.Scanner_Inventories Where scannerid.Text = Scanner_Inventory.SN Select Scanner_Inventory.SID).FirstOrDefault
rep.SID = scan
rep.Date_Broken = datebroke.Value
rep.Description = description.Text
rep.Send_Date = senddate.Text
rep.Recieve_Date = recievedate.Text
rep.Cost = cost.Text
rep.PlantID = plantid.Text
rep.BID = brokenid.Text
rep.RMAnumber = rmanum.Text
db.Scanner_Repair_Histories.InsertOnSubmit(rep)
db.SubmitChanges()
is that me but you didn't instanciate your "rep" variable
You don't have a defined object for placement with a 'new' keyword but I am also curious if it is a system.type.
Update based on Jinx88909
You may be returning an entire POCO Object that may be null and have a null property. You can adjust this most times by doing a null condition if you are using .NET 4.5 and up. '?.' operator.
Dim db As New DataClasses1DataContext
'I need to be a new object and not instantiated as Nothing
Dim rep As New Scanner_Repair_History
'You have the potential for a nothing value here as 'FirstOrDefault' includes a potential Nothing'.
'I would get the entire object and then just a property of it after the fact
Dim scan = (From Scanner_Inventory In db.Scanner_Inventories Where scannerid?.Text = Scanner_Inventory?.SN Select Scanner_Inventory).FirstOrDefault?.Sid
If scan IsNot Nothing Then
rep.SID = scan 'Could you maybe want scan.Id or something similar?
rep.Date_Broken = datebroke.Value
rep.Description = description.Text
rep.Send_Date = senddate.Text
rep.Recieve_Date = recievedate.Text
rep.Cost = cost.Text
rep.PlantID = plantid.Text
rep.BID = brokenid.Text
rep.RMAnumber = rmanum.Text
db.Scanner_Repair_Histories.InsertOnSubmit(rep)
db.SubmitChanges()
Else
Console.WriteLine("I got nothing for you with the inputs you put in!")
End If
While CommitReader.Read()
Yield New Commit() With {
.FirstValue = CommitReader.GetInt32(CommitReader.GetOrdinal("FirstValue")),
.SecondValue = CommitReader.GetString(CommitReader.GetOrdinal("SecondValue")).Trim(),
'Lots of values
End While
I know I can do something like this; however there are 24 properties and I would like to make this part as clean as possible
While CommitReader.Read()
new Commit (){
Dim index As Integer = reader.GetOrdinal("FirstValue")
If reader.IsDBNull(index) Then
FirstValue = String.Empty
Else
FirstValue = reader(index)
End If
index = reader.GetOrdinal("SecondValue")
If reader.IsDBNull(index) Then
SecondValue = String.Empty
Else
SecondValue = reader(index)
End If
}
End While
Is there a better way to handle this type of thing? I am mainly a C# developer so if the syntax is off a little sorry, I am winging it in VB.
It's a shame that SqlDataReader doesn't have the generic Field extension method like DataRow does, but you could define your own extension method (has to be in a module in VB.NET) to help with the null checks, perhaps something like this:
<Extension>
Function GetValue(Of T)(rdr As SqlDataReader, i As Integer) As T
If rdr.IsDBNull(i) Then
Return Nothing
End If
Return DirectCast(rdr.GetValue(i), T)
End Function
And use it something like this:
While CommitReader.Read()
Yield New Commit() With {
.FirstValue = CommitReader.GetValue(Of Integer?)(CommitReader.GetOrdinal("FirstValue")),
.SecondValue = CommitReader.GetValue(Of String)(CommitReader.GetOrdinal("SecondValue")),
'Lots of values
End While
I haven't tested this fully to make sure it handles all data types appropriately (may be worth looking at DataRowExtensions.Field to see how it does it).
Note that you are using String.Empty as the "null" value for strings, while this will use Nothing/null (I also had to remove the .Trim call to avoid NREs). If you want empty string instead, you could use (adding the Trim back in):
.SecondValue = If(CommitReader.GetValue(Of String)(CommitReader.GetOrdinal("SecondValue")), String.Empty).Trim()
You may also want to move the GetOrdinal calls out of the loop to improve performance.
Obviously you have repetition in your code if ... else ... condition.
So you can extract it in another method.
For your case generic extension method seems good candidate.
Public Module Extensions
<Extension>
Public Function GetValueOrDefault(Of T)(originalValue As object,
defaultValue As T) As T
If originalValue = DbNull.Value Then
Return defaultValue
End If
return DirectCast(originalValue, T)
End Function
End Module
Then use it:
While CommitReader.Read() = True
Dim temp = new Commit With
{
Dim index As Integer = reader.GetOrdinal("FirstValue")
FirstValue = reader(index).GetValueOrDefault(String.Empty)
Dim index As Integer = reader.GetOrdinal("SecondValue")
FirstValue = reader(index).GetValueOrDefault(String.Empty)
}
End While
You can create another overload which return "default" value for given type if it is DbNull
<Extension>
Public Function GetValueOrDefault(Of T)(originalValue As object) As T
Return originalValue.GetValueOrDefault(Nothing)
End Function
Nothing in vb.net is default value, for reference types it is null for Integer it is 0 for example.
For using this overload you need provide type parameter explicitly
While CommitReader.Read() = True
Dim temp = new Commit With
{
Dim index As Integer = reader.GetOrdinal("FirstValue")
FirstValue = reader(index).GetValueOrDefault(Of String)()
Dim index As Integer = reader.GetOrdinal("SecondValue")
FirstValue = reader(index).GetValueOrDefault(Of String)()
}
End While
Notice that your solution executing reader twice, for checking is it null and for reading value. This can cause "tiny" performance issue.
So in extension method above we read value only once and then check value for DbNull.
If you concatenate a string with a Null you get the string:
FirstValue = reader(index) & ""
Kind of "unprofessional" but saves a lot of coding time if all you are doing is converting a possible Null to an empty string. Easy to forget however, so later data dependent errors may pop up.
I need to do some refactoring (actually it's A LOT, but this small step will be very helpful for the whole process). So, let's say I've got this code snippet:
If xmlDoc.SelectSingleNode("/dang") IsNot Nothing Then
universalNode = xmlDoc.SelectSingleNode("/dang")
Type = "dang"
ElseIf xmlDoc.SelectSingleNode("/nang") IsNot Nothing Then
universalNode = xmlDoc.SelectSingleNode("/nang")
Type = "nang"
ElseIf xmlDoc.SelectSingleNode("/lang") IsNot Nothing Then
universalNode = xmlDoc.SelectSingleNode("/lang")
Type = "lang"
ElseIf xmlDoc.SelectSingleNode("/tang") IsNot Nothing Then
universalNode = xmlDoc.SelectSingleNode("/tang")
Type = "tang"
ElseIf xmlDoc.SelectSingleNode("/xtang") IsNot Nothing Then
universalNode = xmlDoc.SelectSingleNode("/xtang")
Type = "xtang"
End If
It's in the body of a big function and I want to take it out to a separate function. So, what I was wondering is whether it will be better to pass both universalNode and Type by value and just assign to them the values, without the need of returning anything? Is that going to work well or it is risky?
If it was just the Type that I'm working on, i.e., then I would just return it, but it's more than 1 variable that is being changed, and both are local variables for the big function, from which I am taking out this code snippet.
Any other suggestions maybe?
You can pass the variables by reference (not by value) to the function, it's not a bad practice and should work:
Public Sub MyFunc(ByRef node As MyNode, ByRef typ As String)
node = ...
typ = ...
End Sub
Or you can return some complex data holder:
Public Class MyParams
Public node As MyNode
Public typ As String
End Class
Public MyParams MyFunc()
Dim result As New MyParams()
result.node = ...
result.typ = ...
Return result
End Sub
Hey,
I want to write a query that the "where" in the query is a string something like"
Dim query as string= "Name =xxxx and Date > 10 "
Dim t = from book in doc.Descendants("books") Select _
[Name] = book..value, [Date] = book..value....
Where (query)
I build the query string on run time
Thanks...
I'm not saying this is your case but I see this a lot from people that came from ASP classic where we used to build dynamic SQL strings all of the time. We tend to hope that LINQ will give us some more power in part of the code but let us use strings elsewhere. Unfortunately this isn't true. Where takes a boolean argument and there's no way around that. You can write your own parser that uses reflection and eventually returns a boolean but you'd be writing a lot of code that could be error prone. Here's how you really should do it:
Assuming this is our data class:
Public Class TestObject
Public Property Name As String
Public Property Job As String
End Class
And here's our test data:
Dim Objects As New List(Of TestObject)
Objects.Add(New TestObject() With {.Name = "A", .Job = "Baker"})
Objects.Add(New TestObject() With {.Name = "B", .Job = "President"})
Objects.Add(New TestObject() With {.Name = "C", .Job = "Bus Driver"})
Objects.Add(New TestObject() With {.Name = "D", .Job = "Trainer"})
What you want to do is create a variable that represents the data to search for:
''//This variable simulates our choice. Normally we would be parsing the querystring, form data, XML values, etc
Dim RandNum = New Random().Next(0, 3)
Dim LookForName As String = Nothing
Select Case RandNum
Case 0 : LookForName = "A"
Case 1 : LookForName = "B"
Case 2 : LookForName = "C"
End Select
''//Query based on our name
Dim Subset = (From O In Objects Select O Where (O.Name = LookForName)).ToList()
If sometimes you need to search on Job and sometimes and sometimes you don't you just might have to write a couple of queries:
Dim Subset As List(Of TestObject)
Select Case RandNum
Case 0
Subset = (From O In Objects Select O Where (O.Name = "A" And O.Job = "Baker")).ToList()
Case Else
Select Case RandNum
Case 1 : LookForName = "B"
Case 2 : LookForName = "C"
End Select
Subset = (From O In Objects Select O Where (O.Name = LookForName)).ToList()
End Select
And just to explain writing your own query parser (which is a path that I recommend you DO NOT go down), here is a very, very, very rough start. It only supports = and only strings and can break at multiple points.
Public Shared Function QueryParser(ByVal obj As Object, ByVal ParamArray queries() As String) As Boolean
''//Sanity check
If obj Is Nothing Then Throw New ArgumentNullException("obj")
If (queries Is Nothing) OrElse (queries.Count = 0) Then Throw New ArgumentNullException("queries")
''//Array of property/value
Dim NameValue() As String
''//Loop through each query
For Each Q In queries
''//Remove whitespace around equals sign
Q = System.Text.RegularExpressions.Regex.Replace(Q, "\s+=\s+", "=")
''//Break the query into two parts.
''//NOTE: this only supports the equal sign right now
NameValue = Q.Split("="c)
''//NOTE: if either part of the query also contains an equal sign then this exception will be thrown
If NameValue.Length <> 2 Then Throw New ArgumentException("Queries must be in the format X=Y")
''//Grab the property by name
Dim P = obj.GetType().GetProperty(NameValue(0))
''//Make sure it exists
If P Is Nothing Then Throw New ApplicationException(String.Format("Cannot find property {0}", NameValue(0)))
''//We only support strings right now
If Not P.PropertyType Is GetType(String) Then Throw New ApplicationException("Only string property types are support")
''//Get the value of the property for the supplied object
Dim V = P.GetValue(obj, Nothing)
''//Assumming null never equals null return false for a null value
If V Is Nothing Then Return False
''//Compare the two strings, return false if something doesn't match.
''//You could use String.Compare here, too, but this will use the current Option Compare rules
If V.ToString() <> NameValue(1) Then Return False
Next
''//The above didn't fail so return true
Return True
End Function
This code would allow you to write:
Dim Subset = (From O In Objects Select O Where (QueryParser(O, "Name = A", "Job = Baker"))).ToList()
No, there is nothing directly like what you're looking for where you can pass in a string. As they say, when all you have is a hammer, everything looks like a nail...The real problem is that you need to learn what LINQ is good at and apply it to your code (if it is a good fit), rather than try and make it do what you could with a dynamically built SQL query string.
What you should probably be doing is making those "Where" clauses strongly typed anyway. Your current code has a lot of potential to blow up and be hard to debug.
What you could do instead is something like this (sorry, using C#, been a while since I've touched VB.NET):
var query = from book in doc.Descendants("books")
select book;
if(needsNameComparison)
{
query = query.where(book.Name == nameToCompare);
}
if(needsDateComparison)
{
query = query.Where(book.Date > 10);
}
List<book> bookList = query.ToList();
With LINQ, "query" isn't actually run until the "ToList()" call. Since it uses late execution, the query is dynamic in that it's being built on until it actually needs to run. This is similar to the code you were looking to use before since you were building a query string ahead of time, then executing it at a specific point.