System.I.O the process cannot access the file because it is being used by another process - vb.net

Question

I'm designing a program that calculates the average of scores. the program works great except when I try to add a name and a number from textbox into the textfile it returns an error. I believe that I have closed the file every time it has been used. my code is:
Public Class Form4
Public Function GetNumberOfLines(ByVal file_path As String) As Integer
Using sr As New StreamReader("C:\file.txt")
Dim NumberOfLines As Integer
Do While sr.Peek >= 0 'a pre test loop to read lines
sr.ReadLine()
NumberOfLines += 1
Loop
Return NumberOfLines
sr.Close() 'closes file after it is read
sr.Dispose()
End Using
End Function
Private Sub Form4_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Me.Hide()
End Sub
Private Sub Button9_Click(sender As Object, e As EventArgs) Handles Button9.Click
Dim FILE_NAME As String = "C:\file.txt"
If System.IO.File.Exists(FILE_NAME) = True Then
Dim objWriter As New System.IO.StreamWriter(FILE_NAME)
objWriter.Close()
objWriter.Write(TextBox5.Text)
MsgBox("Text written to file")
End Sub
Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
Using sr As New System.IO.StreamReader("C:\file.txt")
Dim LofInt As New List(Of Integer)
While sr.Peek <> -1
Dim line As String = sr.ReadLine
Dim intValue As String = String.Empty
For Each c As Char In line
If IsNumeric(c) Then
intValue += c
End If
Next
LofInt.Add(intValue)
End While
sr.Close()
sr.Dispose()
LofInt.Sort()
For Each i In LofInt
Scores.Items.Add(i)
Next
Dim sum As Decimal
For Each decAdded As Decimal In Me.Scores.Items
sum += Double.Parse(decAdded)
Next
TextBox4.Text = sum
sr.Close()
sr.Dispose()
End Using
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
TextBox1.Text = My.Computer.FileSystem.ReadAllText("C:\file.txt")
TextBox2.Text = GetNumberOfLines("C:\file.txt")
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim sum As Decimal
Dim avg As Decimal
For Each decAdded As Decimal In Me.Scores.Items
sum += Double.Parse(decAdded)
Next
avg = sum / (GetNumberOfLines("C:\file.txt"))
TextBox3.Text = avg
End Sub
Private Sub Button6_Click(sender As Object, e As EventArgs) Handles Button6.Click
End Sub
Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
Close()
End Sub
Private Sub TextBox5_TextChanged(sender As Object, e As EventArgs)
End Sub
Private Sub Button8_Click(sender As Object, e As EventArgs) Handles Button8.Click
TextBox1.Clear()
TextBox2.Clear()
Scores.Items.Clear()
TextBox4.Clear()
TextBox1.Text = My.Computer.FileSystem.ReadAllText("C:\file.txt")
TextBox2.Text = GetNumberOfLines("C:\file.txt")
Using sr As New System.IO.StreamReader("C:\file.txt")
Dim LofInt As New List(Of Integer)
While sr.Peek <> -1
Dim line As String = sr.ReadLine
Dim intValue As String = String.Empty
For Each c As Char In line
If IsNumeric(c) Then
intValue += c
End If
Next
LofInt.Add(intValue)
End While
LofInt.Sort()
For Each i In LofInt
Scores.Items.Add(i)
Next
Dim sum As Decimal
For Each decAdded As Decimal In Me.Scores.Items
sum += Double.Parse(decAdded)
Next
TextBox4.Text = sum
sr.Close()
sr.Dispose()
End Using
End Sub
Dim objWriter As New System.IO.StreamWriter(FILE_NAME) is where the error occurs