I am using WinForms and trying to retrieve all the data from the database once on form Load so I won't have to go back and forth to the database a lot.
I created a class and wrote a function that returns a dataset but I'm not being able to refer to it from my forms.
This is the code:
Private Function FillKeywords() As DataSet
Dim ds1 As DataSet : Dim cmd as SqlCommand
Dim da As SqlDataAdapter
Try
cmd = New SqlCommand("Dbo.selectkeywords", cn)
cmd.CommandType = CommandType.StoredProcedure
da = New SqlDataAdapter(cmd)
ds1 = New DataSet
da.Fill(ds1, "Keywords")
Return ds1
Catch ex As SqlClient.SqlException
WriteExToFile(ex.ToString)
Catch ex As Exception
WriteExToFile(ex.ToString)
End Try
End Function
I'll take a wild guess and say you will need to lose the Private keyword if you want your function to be accessible from another class or module.
Related
I have looked at many different code snippets on this site looking that would show me how to do something that should be fairly simple once I have the knowledge.
I want to query a database table for an array of values and then populate a combobox with those results.
Here is what I have so far:
Public Sub getMachines()
Try
Dim SQL As String = "SELECT MachineName from machine"
Form1.machineName.DisplayMember = "Text"
Dim tb As New DataTable
tb.Columns.Add("Text", GetType(String))
Using cn As New MySqlConnection(ConnectionString)
Using cmd As New MySqlCommand(SQL, cn)
For Each cmd As String In cmd
'I want to add each value found in the database to "tb.Rows.Add"
'tb.Rows.Add(???)
Next
Form1.machineName.DataSource = tb
cn.Open()
cmd.ExecuteNonQuery()
End Using
cn.Close()
End Using
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
I proceeded much like you did. I used the Load method of the DataTable. It is not necessary to set the column name and type. The name of the column is taken from the Select statement and the datatype is inferred by ADO.net from the first few records.
Luckily a DataTable can be an Enumerable using the .AsEnumnerable method. Then we can use Linq to get all the values from the MachineName column. Calling .ToArray causes the Linq to execute. If you hold your cursor over names on this line you will see that the datatype is String(). Just what we need to fill a combo box.
Code for a class called DataAccess
Private ConnectionString As String = "Your Connection String"
Public Function GetMachineNames() As String()
Dim tb As New DataTable
Dim SQL As String = "SELECT MachineName from machine;"
Using cn As New MySqlConnection(ConnectionString)
Using cmd As New MySqlCommand(SQL, cn)
cn.Open()
dt.Load(cmd.ExecuteReader)
End Using
End Using
Dim names = dt.AsEnumerable().Select(Function(x) x.Field(Of String)("MachineName")).ToArray()
Return names
End Function
In the form load you combo box like this.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim DatAcc As New DataAccess()
Dim arr = DatAcc.GetMachineNames()
machineName.DataSource = arr
End Sub
If you just want the MachineName to be displayed in the ComboBox, then just use that as the DisplayMember; don't bother creating another column called Text.
Public Sub getMachines()
Try
Dim cmd As String = "SELECT MachineName from machine"
Dim ds As New DataSet()
Using con As New MySqlConnection(ConnectionString)
Using da As New MySqlDataAdapter(cmd, con)
da.Fill(ds)
With Form1.machineName
.DisplayMember = "MachineName"
.ValueMember = "MachineName"
.DataSource = ds
End With
End Using
End Using
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
I'll show a few examples, including using parameters, since that is important.
First up, a quick translation to run the existing query and loop through the results:
Public Sub getMachines()
Try
Dim SQL As String = "SELECT MachineName from machine"
Using cn As New MySqlConnection(ConnectionString), _
cmd As New MySqlCommand(SQL, cn)
cn.Open()
Using rdr As MySqlDatareader = cmd.ExecuteReader
While rdr.Read()
Form1.machineName.Items.Add(rdr("MachineName"))
End While
End Using
End Using
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
But better practice for a method like this is to isolate data access for the UI. This method should return results to the caller, which can decide what do with them. So I'll show two methods: one to get the data, and the other to loop through it and set up the combobox:
Private Function GetMachines() As DataTable
'No try/catch needed here. Handle it in the UI level, instead
Dim SQL As String = "SELECT MachineName from machine"
Dim result As New DataTable
Using cn As New MySqlConnection(ConnectionString), _
cmd As New MySqlCommand(SQL, cn),
da As New MySqlDataAdapter(cmd)
da.Fill(result)
End Using
Return result
End Function
Public Sub LoadMachines()
Try
For Each item As DataRow in getMachines().Rows
Form1.machineName.Items.Add(item("MachineName"))
Next
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
Or, we can use DataBinding:
Private Function GetMachines() As DataTable
Dim SQL As String = "SELECT MachineName from machine"
Dim result As New DataTable
Using cn As New MySqlConnection(ConnectionString), _
cmd As New MySqlCommand(SQL, cn),
da As New MySqlDataAdapter(cmd)
da.Fill(result)
End Using
Return result
End Function
Public Sub LoadMachines()
Try
Form1.machineName.DisplayMember = "FirstName";
Form1.machineName.ValueMember = "City"
Form1.machineName.DataSource = GetMachines()
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
If you ever want to use a filter, you might do this (notice the overloading):
Private Function GetMachines(ByVal machineFilter As String) As DataTable
Dim SQL As String = "SELECT MachineName from machine WHERE MachineName LIKE #Filter"
Dim result As New DataTable
Using cn As New MySqlConnection(ConnectionString), _
cmd As New MySqlCommand(SQL, cn),
da As New MySqlDataAdapter(cmd)
'Match the MySqlDbType to your actual database column type and length
cmd.Parameters.Add("#Filter", MySqlDbType.VarString, 30).Value = machineFilter
da.Fill(result)
End Using
Return result
End Function
Private Function GetMachines(ByVal machineFilter As String) As DataTable
Return GetMachines("%")
End Function
Query parameters like that are very important, and if you were doing string concatenation to accomplish this kind of thing on your old platform, you were doing very bad things there, too.
Finally, let's get fancy. A lot of the time, you really don't want to load an entire result set into RAM, as is done with a DataTable. That can be bad. Instead, you'd like be able to stream results into memory and only work with one at a time, minimizing RAM use. In these cases, you get to play with a DataReader... but returning a DataReader object from within a Using block (which is important) doesn't work that well. To get around this, we can use functional programming concepts and advanced language features:
Private Iterator Function GetMachines(ByVal machineFilter As String) As IEnumerable(Of String)
Dim SQL As String = "SELECT MachineName from machine WHERE MachineName LIKE #Filter"
Using cn As New MySqlConnection(ConnectionString), _
cmd As New MySqlCommand(SQL, cn)
'Match the MySqlDbType to your actual database column type and length
cmd.Parameters.Add("#Filter", MySqlDbType.VarString, 30).Value = machineFilter
cn.Open()
Using rdr As MySqlDatareader = cmd.ExecuteReader
While rdr.Read()
Dim result As String = rdr("MachineName")
Yield Return result
End While
End Using
End Using
Return result
End Function
Private Function GetMachines() As IEnumerable(Of String)
Return GetMachines("%")
End Function
I currently have the following code in my project which populates a DataGridView object with the results of an SQL query.
Sub PerformQuery(ByVal SQLText As String)
Dim DbConnection As New OleDb.OleDbConnection(createConnectionString)
Dim SQLQuery As String = SQLText
Dim Adapter As New OleDb.OleDbDataAdapter(SQLQuery, DbConnection)
Try
Using Table As New DataTable
Adapter.Fill(Table)
Table.Locale = Globalization.CultureInfo.InvariantCulture
DbConnection.Close()
DataGridView1.DataSource = Table
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Elsewhere in my project I can create a DataSet object using the code
Dim ds As New DataSet
And then extract data from it using code like:
MaxRows = ds.Tables("Dataset_Users").Rows.Count
Rather than populating a DataGridView, how can I use the PerformQuery code to create a dataset?
Thank you in advance for your help.
I think you are after the following:
Try
Dim ds As New DataSet
Using Table As New DataTable
Adapter.Fill(Table)
Table.Locale = Globalization.CultureInfo.InvariantCulture
DbConnection.Close()
DataGridView1.DataSource = Table
ds.Table.Add(Table)
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
Or as in your example you was after the Number of rows in the dataset you can do the same with the DataTable, For Example:
Try
Dim MaxRows As Integer
Using Table As New DataTable
Adapter.Fill(Table)
Table.Locale = Globalization.CultureInfo.InvariantCulture
DbConnection.Close()
DataGridView1.DataSource = Table
'' Getting the number of rows in the DataTable
MaxRows = Table.Rows.Count
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
Think in a more functional style. Return the table instead of setting to grid. While we're here, let's update the method so you don't have to write queries anymore that leave you wide open to sql injection attacks:
Function PerformQuery(ByVal SQLText As String, ByVal ParamArray Parameters() As OleDbParameter) As DataTable
Dim result As New DataTable()
Using cn As New OleDb.OleDbConnection(createConnectionString), _
cmd As New OleDb.OleDbCommand(SQLText, cn), _
Adapter As New OleDb.OleDbDataAdapter(cmd, cn)
If Parameters IsNot Nothing AndAlso Parameters.Length > 0 Then
cmd.Parameters.AddRange(Parameters)
End If
Adapter.Fill(result)
End Using
Return Result
End Function
Note that I also removed the error handling and locale code. You still need to do that stuff, but when you want to just return a datatable instead of interact directly with the user interface in a method you have effectively moved your code to a lower level of abstraction. When you do that, you probably don't want to deal with the error handling at this lower level any more; instead let the exceptions bubble up where you can handle them closer to the user interface.
Now you call the updated method like this:
Dim sql As String = "SELECT * FROM Customers WHERE CustomerID = ?"
Dim CustID As New OleDb.OleDbParameter("CustomerId", OleDbType.Integer)
CustID.Value = 123456
Try
DataGridView1.DataSource = PerformQuery(sql, CustID)
Catch Ex As Excpetion
MsgBox(Ex.Message)
End Try
I have a project without any parameters used in SQL queries.
Is there any solution so that i don't have to change the function and validate parameters from the Query string itself?
Query = "select * from tbl_Users where userName='"& textbox1.text &"' and password='"& textbox2.text &"' "
ds = obj.ExecuteQueryReturnDS(Query)
Function where query is passed:
Public Function ExecuteQueryReturnDS(ByVal stQuery As String) As DataSet
Try
Dim ds As New DataSet
Using sqlCon As New SqlConnection(connStr)
Dim sqlCmd As New SqlCommand(stQuery, sqlCon)
Dim sqlAda As New SqlDataAdapter(sqlCmd)
sqlCmd.CommandType = CommandType.Text
sqlAda.Fill(ds)
End Using
Return ds
Catch ex As Exception
End Try
End Function
I tried passing parameters into the function but the function is used in for other queries as well hence i cannot define the parameters inside the function .
Is there any work around
I think the only solution is create a new function and gradually migrate to it.
Public Function ExecuteQueryReturnDS(ByVal cmdQuery As SqlCommand) As DataSet
Try
Dim ds As New DataSet
Using sqlCon As New SqlConnection(connStr)
cmdQuery.Connection = sqlCon
Dim sqlAda As New SqlDataAdapter(cmdQuery)
sqlAda.Fill(ds)
End Using
Return ds
Catch ex As Exception
End Try
End Function
cmdQuery is intended to be an SqlCommand to which you already added all the parameters you need.
As an intermediate step before switching to a full parameterized application you could change your actual method to be able to receive an optional argument.
This optional argument will be your SqlParameter array defined in the point where you call this query
Public Function ExecuteQueryReturnDS(ByVal stQuery As String, Optional ByVal prms As SqlParameter() = Nothing) As DataSet
Try
Dim ds As New DataSet
Using sqlCon As New SqlConnection(connStr)
Dim sqlCmd As New SqlCommand(stQuery, sqlCon)
if Not prms Is Nothing Then
sqlCmd.Parameters.AddRange(prms)
End if
Dim sqlAda As New SqlDataAdapter(sqlCmd)
sqlCmd.CommandType = CommandType.Text
sqlAda.Fill(ds)
End Using
Return ds
Catch ex As Exception
End Try
End Function
I'm trying to learn VB.NET and I'm looking to retrieve data from an Access 2010 view, using Vb.NET 2008, problem is I'm getting the following error. System.ArgumentNullException was unhandled Message=Value cannot be null
I cannot work out how to solve the error, so hopefully someone with more experience can help.
Public Function GetMyoleDataAdapterStudentQuestionRepeat(ByRef mydataSet As DataSet, ByVal topicId As String, ByVal groupId As String) As OleDbDataAdapter
Try
Dim strAccessConn As String = _appConfigDbConn
Dim cn As OleDbConnection = New OleDbConnection(strAccessConn)
Dim ds As New DataSet
Dim da As New OleDbDataAdapter("qryStudentNameRandomBasedOnScore", cn)
da.SelectCommand.CommandType = CommandType.StoredProcedure
da.SelectCommand.Parameters.AddWithValue("#GroupID", groupId)
da.Fill(ds, "Student")
Return da
Catch ex As Exception
Throw New ApplicationException(ex.InnerException.Message.ToString())
End Try
End Function
Within button click
Dim myoleDataAdapter As OleDbDataAdapter = GroupData.GetMyoleDataAdapterStudentQuestionRepeat(mydataSet, topicId, groupId)
myoleDataAdapter.Fill(mydataSet)
txtStudentName.DataBindings.Add("Text", mydataSet.Tables(0), "studentname")
This line throws error: myoleDataAdapter.Fill(mydataSet)
And in case it helps, my view is
SELECT TOP 1 tblStudentNameAndScore.studentname
FROM tblStudentNameAndScore
WHERE (((tblStudentNameAndScore.[QuizCount]) Between 2 And 10)) AND tblStudentNameAndScore.GroupID = ?
ORDER BY Rnd(QuizCount);
Thanks for any hep
In this line
myoleDataAdapter.Fill(mydataSet)
you try to fill the dataset but this dataset is never initialized.
Following your actual pattern, you should change your code that initialize the dataadapter in this way
Public Function GetMyoleDataAdapterStudentQuestionRepeat(ByVal topicId As String, ByVal groupId As String) As OleDbDataAdapter
Try
Dim strAccessConn As String = _appConfigDbConn
Dim cn As OleDbConnection = New OleDbConnection(strAccessConn)
Dim ds As New DataSet
Dim da As New OleDbDataAdapter("qryStudentNameRandomBasedOnScore", cn)
da.SelectCommand.CommandType = CommandType.StoredProcedure
da.SelectCommand.Parameters.AddWithValue("#GroupID", groupId)
Return da
Catch ex As Exception
Throw New ApplicationException(ex.InnerException.Message.ToString())
End Try
End Function
and then in the caller code write
Dim myoleDataAdapter As OleDbDataAdapter = GroupData.GetMyoleDataAdapterStudentQuestionRepeat( topicId, groupId)
mydataSet = new DataSet()
myoleDataAdapter.Fill(mydataSet)
....
In the method that returns the adapter you don't need to pass the dataset because you don't use it in any way and there is no need to create and fill another dataset. Just return the adapter and work on the caller code
Of course you could also change the method and return a dataset filled with your data instead of the DataAdapter. Probably this solution could be considered more encapsulated
Public Function GetStudentQuestionRepeat(ByVal topicId As String, ByVal groupId As String) _
As Dataset
....
Dim da As New OleDbDataAdapter("qryStudentNameRandomBasedOnScore", cn)
da.SelectCommand.CommandType = CommandType.StoredProcedure
da.SelectCommand.Parameters.AddWithValue("#GroupID", groupId)
Dim ds = new Dataset()
da.Fill(ds, "Student")
Return ds
....
End Function
By the way, what is the purpose of topidID variable passed and never used?
Why not return a Dataset instead of DataAdapter, like:
Public Function GetMyoleDataAdapterStudentQuestionRepeat(ByVal topicId As String, ByVal groupId As String) As DataSet
Try
Dim strAccessConn As String = _appConfigDbConn
Dim cn As OleDbConnection = New OleDbConnection(strAccessConn)
Dim ds As New DataSet
Dim da As New OleDbDataAdapter("qryStudentNameRandomBasedOnScore", cn)
da.SelectCommand.CommandType = CommandType.StoredProcedure
da.SelectCommand.Parameters.AddWithValue("#GroupID", groupId)
da.Fill(ds, "Student")
Return ds
Catch ex As Exception
Throw New ApplicationException(ex.InnerException.Message.ToString())
End Try
End Function
And then when you call it would be shorter:
Dim myDataSet As DataSet = GroupData.GetMyoleDataAdapterStudentQuestionRepeat(topicId, groupId)
txtStudentName.DataBindings.Add("Text", myDataSet.Tables(0), "studentname")
Good day,
I have been trying to figure out the source of error in my application. What this function (GetConcentrations()) is to display data from my database onto a DataGridView. The function is called in the PageLoad but works fine but when I call it in another Sub, I get this error.
Could anyone help me? I already tried different ways.
Thanks!
Here is the code:
Private Sub GetConcentrations()
Dim conString As String = ConfigurationManager.ConnectionStrings("dbAsthmaConnectionString").ConnectionString
Me.dataAdapter = New SqlDataAdapter("Select * from tblConcentrations", conString)
'Dim adapter As New SqlDataAdapter("Select * from tblConcentrations", conString)
Dim dataset As New Data.DataSet
Try
Dim commandBuilder As New SqlCommandBuilder(Me.dataAdapter)
Dim table As New DataTable()
table.Locale = System.Globalization.CultureInfo.InvariantCulture
Me.dataAdapter.Fill(table)
'Dim datasetgetconcentrations = New DataSet
Me.dataAdapter.Fill(dataset)
Me.bindingSource1.DataSource = table
DataGridView3.AutoResizeColumns(DataGridViewAutoSizeColumnMode.AllCellsExceptHeader)
DataGridView3.DataSource = dataset.Tables(0)
dataset.Dispose()
Catch ex As Exception
MsgBox("Failed to display the concentration table (GetConcentrations)!", MsgBoxStyle.Information)
End Try
'Dim ConcentrationValue As Double = DataGridView2.Rows.Contains
'TextBoxCurrentConcentration.Text = txtMessage.Text
End Sub
I don't understand why you call the Fill method two times.
Once to fill a DataTable and one again to fill a DataSet. Only the one for the dataset is really needed.
Could you try if this changes something in your problem?
Dim ds as New DataSet
ds.Locale = System.Globalization.CultureInfo.InvariantCulture
Me.dataAdapter.Fill(ds)
DataGridView3.AutoResizeColumns(DataGridViewAutoSizeColumnMode.AllCellsExceptHeader)
DataGridView3.DataSource = ds.Tables(0)
Also, don't dispose your dataset on exiting, you are still using it.
In this case it's better to let the Garbage Collection handle this task.