Conversion from type dbnull to type double is not valid

<!-- Function 1-->   
Function GetStudentResultTotal(ByVal schid As String, ByVal level As String, ByVal session As String, ByVal term As String, ByVal klass As String, ByVal regno As String) As Double
If SCH_ID <> "" Then
schid = SCH_ID
End If
Dim total As Double = 0
Dim subjectCount As Integer = 0
Dim fields As New ArrayList
fields.Add("SUM(" & StudentData.Total & ") AS GrandTotal")
Dim filterValues As New Hashtable
filterValues.Add(StudentData.SchoolID, schid)
filterValues.Add(StudentData.Level, level)
filterValues.Add(StudentData.Session, session)
filterValues.Add(StudentData.Term, term)
filterValues.Add(StudentData.Klass, klass)
filterValues.Add(StudentData.RegNo, regno)
Dim filterArgs As String = "WHERE " & StudentData.SchoolID & "=@" & StudentData.SchoolID & " AND " & StudentData.Level & "=@" & StudentData.Level & " AND " & StudentData.Session & "=@" & StudentData.Session & " AND " & StudentData.Term & "=@" & StudentData.Term & " AND " & StudentData.Klass & "=@" & StudentData.Klass & " AND " & StudentData.RegNo & "=@" & StudentData.RegNo
Dim data As DataSet = _Data.GetData(StudentData.tblStudentResult, fields, filterValues, filterArgs)
'If data.Tables(0).Rows.Count > 0 Then
'    For Each dr As DataRow In data.Tables(0).Rows
'        total += CDbl(NormalizeRecord(dr(StudentData.Total)))
'        subjectCount += 1
'    Next
'End If
Dim dr As DataRow = data.Tables(0).Rows(0)
total = CDbl(dr("GrandTotal"))
Return total
End Function
<!-- Function 2-->  
Function GetData(ByVal tbl As String, ByVal values As ArrayList, ByVal filters As Hashtable, ByVal filterArgs As String) As DataSet
Dim _ds As New DataSet
Dim sql As String = "SELECT "
Dim fields As String = ""
Using conn As New MySqlConnection(connString)
conn.Open()
If values IsNot Nothing Then
For i = 0 To values.Count - 1
If fields = "" Then
fields = values.Item(i).ToString
Else
fields &= "," & values.Item(i).ToString
End If
Next
sql &= fields & " "
End If
sql &= "FROM " & tbl
If filterArgs <> "" Then
sql &= " " & filterArgs
End If
Dim cmd As New MySqlCommand(sql, conn)
If filters IsNot Nothing Then
For i = 0 To filters.Count - 1
cmd.Parameters.AddWithValue("@" & filters.Keys(i), filters.Values(i))
Next
End If
Dim da As New MySqlDataAdapter(cmd)
da.Fill(_ds)
conn.Close()
End Using
Return _ds
End Function
<!-- Function 3-->  
Function NormalizeRecord(ByVal value As String) As String
If value = "-" Then
value = "0"
End If
Return value
End Function

Function 1 as described in my code is supposed to sum the column total and return the result but it always throw error (Conversion from type dbnull to type double is not valid) if it returns null value especially when am inserting record for the first time. how can i control null value?

Well, there are two ways to deal with this.

First up, you might have no rows, or the ONLY row returned has a null value.

If you doing a "sum()", then if ANY of the rows are not null, then you WILL get a value back.

however, no rows, or rows with the column that are null, then you see/get/find a null.

So, one easy fix would be to use isnull.

So, your code could say use this:

.Add("IsNull(SUM(" & StudentData.Total & "),0) AS GrandTotal")

The above is probably your best bet, since EVEN if the query were to not even return any rows due to filter, you still get a 0 back.

Edit: I see you tagged this as MySQL, and not SQL server - my mistake, so I suggest using below solution.

however, often, in fact quite a bit, you will encounter null values in your data tables
(BTW, why are you using a dataset in place of a data table? You don't need a dataset here, since you don't have a colleciton of tables).

So, next up, since you often have do to this?

Place in your "system wide" bag of utility routines this:

Public Function Nz(ByVal Value As Object, 
Optional ByVal MyDefault As Object = "") As Object
If Value Is Nothing OrElse IsDBNull(Value) Then
Return MyDefault
Else
Return Value
End If
End Function

So, now you can say do this:

total = nz(dr("GrandTotal"),0)

You could/can modify the SQL query, and have it return a 0 value for those null rows.