SQL Server connection related error - vb.net

Question

Below error is thrown in a already opened program which connects to different database
A transport-level error has occurred when sending the request to the server. (provider: TCP Provider, error: 0 - An existing connection
was forcibly closed by the remote host.)
Program details: code done using vb.net with SQL Server 2008 as back-end
Two instance of same exe where running simultaneously in same PC but error was thrown only by one instance
Program uses both SqlConnection (ADO.NET) and ADODB connection (upgraded from VB6) and the error is thrown by both types of connection
If the error is due to network problem with the server then why few programs work fine? I am not able to trace the reason for this behavior of the program
Can I know why this error occurs and why only in few programs of same instance

Answer 1

When each instance of your exe connects, it gets its own SQL Server process ID (SPID). You are getting the error because your instance's connection was either killed at the server level or disrupted at the network level, at some point before it tried to execute the SQL command where the error occurred.
In SSMS, check your SQL Server Log file. In this example I killed a SPID, and the killing of the SPID was logged:
If you don't see a logged event, you are probably dealing with a network issue. Further troubleshooting might involve setting one or more break points in your code somewhere after it opens the ADODB connection, but before the error, and then verifying through SSMS that a SPID for that instance is running at the break point. exec sp_who2 is a good command to list all currently active SPIDs on the server.

Answer 2

Imports System.Data
Imports System.Data.SqlClient
Public Class Form2
Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Function call_add()
Dim tam, eng, mat, sci, soc, tot As Decimal
tam = 0
eng = 0
mat = 0
sci = 0
soc = 0
tot = 0
If tamil.Text <> "" Then
tam = Convert.ToDecimal(tamil.Text)
End If
If english.Text <> "" Then
eng = Convert.ToDecimal(english.Text)
End If
If maths.Text <> "" Then
mat = Convert.ToDecimal(maths.Text)
End If
If science.Text <> "" Then
sci = Convert.ToDecimal(science.Text)
End If
If social.Text <> "" Then
soc = Convert.ToDecimal(social.Text)
End If
tot = tam + eng + mat + sci + soc
total.Text = tot.ToString
percentage.Text = total.Text / 500 * 100
Return False
End Function
Private Sub total_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles total.TextChanged
call_add()
End Sub
Private Sub percentage_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles percentage.TextChanged
call_add()
End Sub
Private Sub tamil_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles tamil.TextChanged
call_add()
End Sub
Private Sub english_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles english.TextChanged
call_add()
End Sub
Private Sub maths_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles maths.TextChanged
call_add()
End Sub
Private Sub science_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles science.TextChanged
call_add()
End Sub
Private Sub social_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles social.TextChanged
call_add()
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim con As SqlConnection
Dim cmd As SqlCommand
'Dim str As String
con = New SqlConnection("server=WHITE-PC\WHITEPC;initial catalog=markreg;Integrated Security=True")
cmd = New SqlCommand
con.Open()
Dim command As New SqlCommand
cmd.Connection = con
command = New SqlCommand("stdregno,stdname,tamil,english,maths,science,social,total,percentage", con)
command.ExecuteNonQuery()
con.Close()
MsgBox("Added Sucessfully", MsgBoxStyle.Information, "Succesfully")
call_clear()
End Sub
Private Function call_clear()
stdregno.Text = ""
stdname.Text = ""
tamil.Text = ""
english.Text = ""
maths.Text = ""
science.Text = ""
social.Text = ""
total.Text = ""
percentage.Text = ""
End Function
Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
Form1.Show()
Me.Hide()
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
InputBox("Enter the Regno You want search")
End Sub
Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
InputBox("Enter the Regno You want Modify")
End Sub
End Class