Обработка текстовых файлов с помощью базы данных SQL — Visual Basic

Попробуй это:

Dim newLines As List(Of String) = New List(Of String)
Dim sqlConn As New SqlConnection(connectionString)
Dim SQLCmd As New SqlCommand()
SQLCmd.Connection = sqlConn
Dim lines As String() = File.ReadAllLines(filename)
sqlConn.Open()
For Each line As String In lines
    Dim parts As String() = line.Split(";")
    SQLCmd.CommandText = "Select X from T where C=""" & parts(3) & """"
    Dim dr As SqlDataReader = SQLCmd.ExecuteReader
    While dr.Read()
        parts(3) = dr("X")
    End While
    newLines.Add(String.Join(";", parts))
Next
File.WriteAllLines(filename, newLines.ToArray())
sqlConn.Close()