trying to input data into my .NET program through a mySQL database - mysql

Question

Hi I am trying to import data from a database into variable in my program but I keep on getting the error:
System.InvalidCastException: 'Conversion from type 'DBNull' to type 'String' is not valid.'
Dim value As Integer
Dim MySqlConn As MySqlConnection
Dim sql As New MySqlCommand
Dim dataAdapter As New MySqlDataAdapter
Dim dataFromDB As New DataSet
Dim numrows As Integer
MySqlConn = New MySqlConnection
MySqlConn.ConnectionString = "server=localhost;user id=root;database=gamedata;"
Try
MySqlConn.Open()
sql = New MySqlCommand("SELECT Ccost FROM cards WHERE `UserName` = '" & UserName & "' AND `Game` = '" & game & "'", MySqlConn)
dataAdapter = New MySqlDataAdapter(sql)
dataAdapter.Fill(dataFromDB)
numrows = dataFromDB.Tables(0).Rows.Count
For counter = 1 To numrows - 1
value = dataFromDB.Tables(0).Rows(counter).Item(0)
Next
sql = New MySqlCommand("Select Level,Health,Score,PlayerTime FROM savedata WHERE `UserName` = '" & UserName & "' AND `Game` = '" & game & "'", MySqlConn)
dataAdapter = New MySqlDataAdapter(sql)
dataAdapter.Fill(dataFromDB)
numrows = dataFromDB.Tables(0).Rows.Count
'For counter = 0 To 1
level = dataFromDB.Tables(0).Rows(0).Item(0)
Phealth = dataFromDB.Tables(0).Rows(0).Item(1)
score = dataFromDB.Tables(0).Rows(0).Item(2)
time = dataFromDB.Tables(0).Rows(0).Item(3)
Catch ex As MySqlException
MsgBox("Error " & ex.Message)
End Try
database code:
use `Gamedata`;
create table `SaveData`
(`GameCode` int AUTO_INCREMENT not null,`Game` enum('1','2','3','4') not null,`UserName` varchar(20) not null,`level` int not null, `Health` int not null,`Score` int not null,`PlayerTime` time not null,
foreign key(`UserName`) REFERENCES `player` (`UserName`),
primary key(`GameCode`));

Answer 1

Here is a slightly different approach. This code requires the following Imports at the top of the file.
Imports System.Data.SqlTypes
You need to declare your database objects with Using blocks so they will be closed and disposed even if there is an error. Queries should always be written using parameters. Not it only does it avoid sql injection but it is easier to read and write.
A database integer can be null but not so in .net. The SqlInt32 has an .IsNull property that we can use for testing. If you try to convert a null string to a string by, for example calling .ToString, you will get an error.
Private ConStr As String = "server=localhost;user id=root;database=gamedata;"
Private Sub OpCode(UserName As String, Game As String)
Dim value As Integer
Dim dt As New DataTable
Dim strSql = "SELECT Ccost FROM cards WHERE `UserName` = #UserName AND `Game` = #Game"
Try
Using Sql As New MySqlCommand(strSql, New MySqlConnection(ConStr))
Sql.Connection.Open()
Sql.Parameters.Add("#UserName", MySqlDbType.VarChar).Value = UserName
Sql.Parameters.Add("#Game", MySqlDbType.VarChar).Value = Game
Dim obj = Sql.ExecuteScalar
value = If(obj Is Nothing, 0, CInt(obj))
Sql.CommandText = "Select Level,Health,Score,PlayerTime FROM savedata WHERE `UserName` = #UserName AND `Game` = #Game;"
Using reader = Sql.ExecuteReader
dt.Load(reader)
End Using
End Using
Dim level = If(CType(dt.Rows(0)(0), SqlString).IsNull, "", dt.Rows(0)(0).ToString)
Dim Phealth = If(CType(dt.Rows(0)(1), SqlString).IsNull, "", dt.Rows(0)(1).ToString)
Dim score = If(CType(dt.Rows(0)(2), SqlInt32).IsNull, 0, CInt(dt.Rows(0)(2)))
Dim time = If(CType(dt.Rows(0)(3), SqlInt32).IsNull, 0, CInt(dt.Rows(0)(3)))
Catch ex As MySqlException
MsgBox("Error " & ex.Message)
End Try
End Sub