SQL - INSERT with Scope_Identity() - getting the record id - asp.net

Question

I have an ASP.NET page written in VB.NET that gets the items into a GridView by using a SELECT statement with INNER JOIN and also allows you to add an item to the invoice.
INNER JOIN that gets data from items and project_items.
SELECT Items.item_id, Items.item_name, Items.item_cost, project_items.item_quantity
FROM Items
INNER JOIN project_items
ON items.item_id = project_items.item_id
WHERE project_items.project_id = #parameter
#parameter is Session("ProjectID")
(There is a foreign key project_items.item_id -> items.item_id.)
I have an trying to use an SQL statement in VB.NET to try and INSERT into two tables simultaneously. What I tried is I tried to get the item_id of the last record created and insert into another table (project_items) by using that data. However, data is only being entered into the first table.
Any idea what I can do?
This is the code:
Protected Sub btnAddItem_Click(sender As Object, e As EventArgs) Handles btnAddItem.Click
Dim conn As New SqlConnection("Data Source=BRIAN-PC\SQLEXPRESS;Initial Catalog=master_db;Integrated Security=True")
Dim addItemComm As String = "SELECT item_id FROM project_items WHERE project_id=#ProjectID"
Dim user_id_select As New Integer
Dim addItemSQL As New SqlCommand
conn.Open()
addItemSQL = New SqlCommand(addItemComm, conn)
addItemSQL.Parameters.AddWithValue("#ProjectID", Convert.ToInt32(Session("ProjectID")))
Dim datareader As SqlDataReader = addItemSQL.ExecuteReader()
datareader.Close()
conn.Close()
Dim AddNewItemComm As String = "INSERT INTO Items (item_name, item_cost, item_code) VALUES (#ItemName, #ItemCost, #ItemCode); SELECT SCOPE_IDENTITY()"
Dim AddNewItem2Comm As String = "INSERT INTO project_items (item_id, project_id, item_quantity) VALUES (#ItemID, #ProjectID, #ItemQuantity) "
Dim AddNewItemSQL As New SqlCommand
conn.Open()
AddNewItemSQL = New SqlCommand(AddNewItemComm, conn)
AddNewItemSQL.Parameters.AddWithValue("#ItemName", txtItemName.Text.Trim)
AddNewItemSQL.Parameters.AddWithValue("#ItemCost", Convert.ToInt32(txtItemCost.Text))
AddNewItemSQL.Parameters.AddWithValue("#ItemCode", txtItemCost.Text.ToString.ToUpper)
Dim ItemId As Integer
ItemId = AddNewItemSQL.ExecuteScalar()
AddNewItemSQL.ExecuteNonQuery()
conn.Close()
conn.Open()
AddNewItemSQL = New SqlCommand(AddNewItem2Comm, conn)
AddNewItemSQL.Parameters.AddWithValue("#ItemID", ItemId)
AddNewItemSQL.Parameters.AddWithValue("#ProjectID", Convert.ToInt32(Session("ProjectID")))
AddNewItemSQL.Parameters.AddWithValue("#ItemQuantity", Convert.ToInt32(txtItemQuantity.Text))
AddNewItemSQL.ExecuteNonQuery()
conn.Close()
End Sub

Answer 1

Why are you doing this in multiple statements in the first place? Why not:
INSERT dbo.Items (item_name, item_cost, item_code)
OUTPUT inserted.ItemID, #ProjectID, #ItemQuantity
INTO dbo.project_items(item_id, project_id, item_quantity)
VALUES (#ItemName, #ItemCost, #ItemCode);
Now you only have to call one ExecuteNonQuery() and your app doesn't have to care about the actually SCOPE_IDENTITY() value generated. (You can still retrieve SCOPE_IDENTITY() if you want, of course, using ExecuteScalar - but as Nenad rightly points out, pick one instead of calling both.)
Since we now know that there is an explicit foreign key here, we can still reduce your C# code to one call even if we can't use the OUTPUT clause.
DECLARE #i INT;
INSERT dbo.Items (item_name, item_cost, item_code)
SELECT #ItemName, #ItemCost, #ItemCode;
SELECT #i = SCOPE_IDENTITY();
INSERT dbo.project_items(item_id, project_id, item_quantity)
SELECT #i, #ProjectID, #ItemQuantity
SELECT #i; -- if necessary
Would be even cleaner to put this into a stored procedure.

Answer 2

ItemId = AddNewItemSQL.ExecuteScalar()
AddNewItemSQL.ExecuteNonQuery()
These two rows next to each other will execute the command twice. You should remove the second one - ExecuteNonQuery. This will have your data inserted twice in the Items - two same rows but with different IDs.
Since you only retrieve ItemID from the first row, that one should be inserted in project_items, but the other one that was last inserted in items will have no matching row.
Also - complete section from beginning of button click method up before Dim AddNewItemComm As String - where you open and close DataReader and do nothing with it seems completely unnecessary.