How do Hide/Show process by using buttons

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I created test application and opened one process, when i trying to hide it by clicking hide button the message i get : Object reference not set to an instance of an object.

The code i use:

Imports System.Runtime.InteropServices

Public Class Form1

    Const SW_HIDE As Integer = 0
    Const SW_SHOW As Integer = 1

    Dim process As Process


    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function ShowWindow(ByVal hwnd As IntPtr, nCmdShow As Integer) As Boolean
    End Function


    'Button for Hide process
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

        ShowWindow(process.MainWindowHandle, SW_HIDE)

    End Sub

    'Button for Show process
    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click

        ShowWindow(process.MainWindowHandle, SW_SHOW)

    End Sub

    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load

        Dim psi As ProcessStartInfo = New ProcessStartInfo()

        psi.FileName = "Calc.exe"
        TextBox1.Text = "Start calculator"

        process.Start(psi)

        'Wait until the process has a main window handle.
        While process.MainWindowHandle = IntPtr.Zero
            process.Refresh()
        End While

    End Sub
End Class

The Error Message appears when i push Hide button
Line : ShowWindow(process.MainWindowHandle, SW_HIDE)

Thank you.

Posted 3-Nov-14 0:18am

Leon Barkan

Updated 3-Nov-14 18:48pm

v3

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Comments

Ch Smrutiranjan 3-Nov-14 8:07am

Could you paste the exact error so that we can get a idea of what causes error and help you.It seems fishy about the ShowWindow() method. if you could paste the code for that method it will be good.

Sinisa Hajnal 4-Nov-14 2:01am

Please post the stack trace or at least full error message. Thank you.

[no name] 4-Nov-14 2:30am

Full Exception details:

System.NullReferenceException was unhandled
Message=Object reference not set to an instance of an object.
Source=WindowsApplication1
StackTrace:
at WindowsApplication1.Form1.Button1_Click(Object sender, EventArgs e) in C:\hideTestApp\WindowsApplication1\WindowsApplication1\Form1.vb:line 19
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(ApplicationContext context)
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
at WindowsApplication1.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81
at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:

Sinisa Hajnal 4-Nov-14 3:52am

Can you check that process is not nothing? Same for WinHandle...

1 solution

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Pikoh · Accepted Answer · 2014-11-03T21:48:00

Solution 2

Ok,try this:

Instead of this line:

VB

process.Start(psi)

Use this:

VB

process.StartInfo = psi
process.Start()

EDIT
After looking at MSDN, I see that Process.Start(ProcessStartInfo) returns a process,so this would also work:

VB

process = process.Start(psi)

Posted 3-Nov-14 21:48pm

Pikoh

Updated 3-Nov-14 22:07pm

v2

Comments

Sinisa Hajnal 4-Nov-14 3:54am

Why would that help?

Pikoh 4-Nov-14 4:02am

Well...i'm not an expert in VB,but it seems in VB Process works slightly different to C#. If you process.Start(ProcesssStartInfo) it seems that the process started does not get attached to the process variable. In the way i suggested,the process get correctly attached.

Pikoh 4-Nov-14 4:03am

And by the way,i tried it and it works,so i think yes,it would help :)

Sinisa Hajnal 4-Nov-14 5:29am

Well done! :)

Pikoh 4-Nov-14 4:08am

And see updated solution

[no name] 4-Nov-14 5:08am

process = process.Start(psi), Works! Thx.

Pikoh 4-Nov-14 5:16am

You're welcome :) If it works please mark the solution as accepted so others with your same problem would know how to solve it