Im using VB.net and I have a variable that contains a string which is the name of another variable. I want to access that variable based off of that string. The scenario goes like this
I am doing a log DataTable with variable name "LogChanges"
Now I have a DataTable variable name "dtbEmployee"
the value "dtbEmployee" will be stored in DataTable "LogChanges" as string
Now
while i loop through the "LogChanges" DataTable and get the "dtbEmployee" in string. Is there a way to use this string to point back to the "dtbEmployee" DataTable
Something like this
For Each dr As DataRow In LogChanges.Select("", "DtbId")
If dr.Item("RecordSeQ") > 0 Then
Dim tempDataTable As DataTable
tempDataTable = "Convert dr.Item("dtbNAme")" to DataTable code
End If
Next
Note: RecordSEQ is just a counter to see how many records have been modified for this table
You would need to store the DataTable against that name somewhere, e.g. the Tables collection of a DataSet or a Dictionary. If the variable is a field and you have the object that has the correct DataTable assigned to that field then you could use Reflection but that would be cumbersome at best. The variable is not the DataTable so you have to some actual relationship to use to get the DataTable via the use of the name.
Related
I have had an issue I have after much stress, narrowed down to assigning a string to a datatable (cell?)
Dim dc As New DataColumn("Col1")
dc.DataType = System.Type.GetType("System.String")
ParameterTable.Columns.Add(dc)
For i = 0 To whereInValues.Count - 1
r = ParameterTable.NewRow
r.Item("Col1") = whereInValues(i).ToString
ParameterTable.Rows.Add(r)
Next
This is a small snippet of the code in question. It's a loop iterating through the whereInValues List(Of String) containing strings for the IN() values of an SQL Statement.
ParameterTable is my DataTable
This is assigned to the .value property of the parameter, which is set to Structured.
Basically, when I set to the value of the datatable using the .ToString method of the List, it shows fine in the debugger, but doesn't return any results from the database. When I set the value directly like:
r.Item(column) = "Barratt Homes"
It returns the correct results. I have tried wrapped a CStr() around the expression, but to no effect.
What is the difference between assigning the datatable value this way? Is .ToString to same as a System.String?
Sorry, I have found the answer to my question.
My code was automatically wrapping the strings of the List(Of String) with a ' character. When using parameters you obviously doesn't need that as it forms part of the string you are searching for. So when I was assign the string manually, I didn't have these characters there.
Stupid mistake...
Dim dtModifiedData As DataTable = dtMain.GetChanges(DataRowState.Modified)
As the definition of the function says, it does copy the rows after filtering it from the main table.
Is it possible to get them by reference?
Yes:
Dim modifiedRows = From row In dtMain.AsEnumerable()
Where row.RowState = DataRowState.Modified
If you need it in a DataTable, that's not possible because a DataRow always belongs to one DataTable. If you try to move it from one to another you'll get an exception. You need a method like ImportRow which creates a new DataRow with the values from the source-row. For the same reason CopyToDataTable on above LINQ query will also create new DataRows.
I use below code in try catch block but it gives exception"'table' argument cannot be null. Parameter name: table" .My table name is caste and that table two columns are there srno and castename .But it say that my table has no data.Memory table is a datatable.
Dim Dset As New DataSet()
Dset = New DataSet()
Dset.Tables.Add(MemoryTable)
DataGridView1.DataSource = Dset.Tables("caste")
I tried data connect with database with using datasource but it gives service pack 1 error
'One is to use data binding on your TextBox controls and assigning the same DataSource.but its gives error
You don't show the definition of MemoryTable, but you do say it's a data table. If it's an object of type System.Data.DataTable, then it will have a property called TableName.
When you access a DataTable in a DataSet with a string index value, the value you are passing is the table's TableName property. So Dset.Tables("caste") is looking for a DataTable whose TableName property is set to "caste". If it can't find one, it will return Null. That looks like what's happening.
So set MemoryTable.TableName to "caste" and the error may go away.
I assume that MemoryTable actually has rows in it? If not, that may be a reason why you're getting the message about your table having no data.
So your code should look something like this:
Dim Dset As New DataSet() ' You don't have to do a separate assignment to Dset
' if you use New in the declaration, so we can omit that line.'
MemoryTable.TableName = "caste"
Dset.Tables.Add(MemoryTable)
DataGridView1.DataSource = Dset.Tables("caste")
And, actually, you can use MemoryTable as your data source without having to add it to a DataSet, unless you need to for some other reason.
DataGridView1.DataSource = MemoryTable
I hope this helps.
I have this part of code:
Dim SqlCommand_customer As New SqlCommand("selectcustomer", conn.con)
SqlCommand_customer.CommandType = CommandType.StoredProcedure
Dim da_customer As New SqlDataAdapter
Dim SqlDataAdapter_customer As New SqlDataAdapter(SqlCommand_customer)
Dim ds_customer = New DataSet()
SqlDataAdapter_customer.Fill(ds_customer, "customer")
DataGridView_customer.DataSource = ds_customer.Tables("customer")
So I want to know what does "customer" stands for in the last two lines
There is nothing named customer in the whole file
and MSDN states that the second parameter in Fill must be of type iDataReader:
dataReader
Type: System.Data.IDataReader
An instance of IDataReader.
The SqlDataAdapter class derives from DbDataAdapter that has a method overload for Fill that takes as second parameter a string.
This string is the name that will be assigned to the table created to store your records or, if you have already tables in the dataset, it is the name of a preexisting table wich will be loaded/refreshed with the records returned by the command.
Of course the syntax in
DataGridView_customer.DataSource = ds_customer.Tables("customer")
is the way in which you reference this table by its name (that would be the same as ds_customer.Tables(0))
The second argument to the fill command is the name of a table.
You are looking at the wrong reference. You want the Fill command off of the SQLDataAdapter.
I would like to fill a datatable with results from a SQL select statment but using a transaction. The reason that I am using a transaction is because I have a list of names (as a datatable), and I want to iterate through the list of names and select the database rows where the name = the name on the list. There are 500,000 names in the database and I only want to retreive the relevant rows. I have the code for the procedure as I think it should look like (untested) BUT I dont know HOW to place the data into a datatable .... so Im missing something where I declare the datatable and the 'fill' of that table , could someone help with this ? Or suggest how else I can get the information out of the batabase without looking up each name individually.
Using connection As New SQLite.SQLiteConnection(R2WconectionString)
connection.Open()
Dim sqliteTran As SQLite.SQLiteTransaction = connection.BeginTransaction()
Try
oMainQueryR = "SELECT NameID, Address, Ocupation FROM Employees Where Name= :Name"
Dim cmdSQLite As SQLite.SQLiteCommand = connection.CreateCommand()
With cmdSQLite
.CommandType = CommandType.Text
.CommandText = oMainQueryR
.Parameters.Add(":Name", SqlDbType.VarChar)
End With
'Prevent duplicate selects by using a dictionary
Dim NameInalready As New Dictionary(Of String, String)
For Each row As DataRow In TheLIST.Rows
If NameInalready.ContainsKey(row.Item("Name")) Then
Else
NameInalready.Add(row.Item("Name"), "")
cmdSQLite.Parameters(":Name").Value = row.Item("Name")
cmdSQLite.ExecuteNonQuery()
End If
Next
sqliteTran.Commit()
Catch ex As Exception
End Try
End Using
First, you don't need a transaction because you aren't updating the database.
Second, depending on the possible number of Names in TheLIST, it might be worthwhile for you to change the name selector to IN (i.e. SELECT * FROM Employees WHERE Name IN ('name1', 'name2'). However, if you expect more than about 10, this is probably not worth trouble.
Finally, you need to create a new DataTable to hold the results. Then you need to create a DataAdapter passing cmdSqlLite as the constructor parameter. And finally, replace your ExecuteNonQuery with DataAdapter.Fill(DataTable).
For example (after Dim cmdSQLite):
Dim oDataTable As New DataTable("Employees")
Dim oAdapter As New SqliteDataAdapter(cmdSQLite)
and replacing the ExecuteNonQuery line with:
oAdapter.Fill(oDataTable)
I will qualify this code by saying it may need some tweaks. I only work with class objects and collections, so my preference would have actually been to load a collection of Employee class instances.
I would have done that by replacing ExecuteNonQuery with ExecuteReader and then the loading the read data into a new class instance. This type of approach resolves various issues with serializing the data across service boundaries (i.e. Xml for web services) and also lets you embed business logic, if needed, into the classes.