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How can i SendMessage to other processes windows then notepad?

In the top of form1
const int WM_SETTEXT = 0X000C;
//include FindWindowEx
[DllImport("user32.dll")]
public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);
//include SendMessage
[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hWnd, int uMsg, int wParam, string lParam);
Then in the constructor
//getting notepad's process | at least one instance of notepad must be running
Process notepadProccess = Process.GetProcessesByName("devenv")[0];
//getting notepad's textbox handle from the main window's handle
//the textbox is called 'Edit'
IntPtr notepadTextbox = FindWindowEx(notepadProccess.MainWindowHandle, IntPtr.Zero, "Edit", null);
//sending the message to the textbox
SendMessage(notepadTextbox, WM_SETTEXT, 0, "Testing");
This is working for the notepad.
But if i want to send a message to another visual studio opened window of a project ?
For example i created a new project in a new visual studio and created in the project a new class called it test
And i want to send to the class some text for example:
using system;
For example this is the new class window:
After sending the message it will be like this:

Hide/Show desktop Items using c#

I use the following program for Hide/Show desktop Items using c#. It was working Fine.
public partial class Form1 : Form
{
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr GetWindow(IntPtr hWnd, GetWindow_Cmd uCmd);
enum GetWindow_Cmd : uint
{
GW_HWNDFIRST = 0,
GW_HWNDLAST = 1,
GW_HWNDNEXT = 2,
GW_HWNDPREV = 3,
GW_OWNER = 4,
GW_CHILD = 5,
GW_ENABLEDPOPUP = 6
}
[DllImport("user32.dll", CharSet = CharSet.Auto)]
static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);
private const int WM_COMMAND = 0x111;
static void ToggleDesktopIcons()
{
var toggleDesktopCommand = new IntPtr(0x7402);
IntPtr hWnd = GetWindow(FindWindow("Progman", "Program Manager"), GetWindow_Cmd.GW_CHILD);
SendMessage(hWnd, WM_COMMAND, toggleDesktopCommand, IntPtr.Zero);
}
public Form1()
{
InitializeComponent();
}
private void button1_Click_1(object sender, EventArgs e)
{
ToggleDesktopIcons();
}
}
But my problem is I have only one button in form. If I press the button the hide and show desktop items will happen on the same button.
I need to separate those show and Hide. Which means i need to create one more button in a form, so totally i have 2 buttons now. If I press first button I need to hide desktop items. If I press second button I need to show Desktop Items. How can I do this?

You can store internal state inside Form1 class, for example some bool property.
If you click on Show button and bool property indicates, that your item is already shown - do nothing, else ToggleDesktopIcons. The same for hide button.

See the answer in the this question.
The answer contains an IsVisible method that allows you to see if the icons are hidden or shown.
In the show button: call IsVisible, if it returns false, execute ToggleDesktopIcons, otherwise return.
In the hide button: call IsVisible, if it returns true, execute ToggleDesktopIcons, otherwise return.
You can also use the result of calling IsVisible to decide if you want to enable/disable the show and hide buttons.

Is there a way to have my app push its icon to other apps?

I have a WPF app that starts another application, I'd like for my application to change the Icon of this second app. I am able to use GetWindowText and SetWindowText to change the title. Is it possible to do this for the Icon as well?
update
I have no control of the second app.

To change the window title of another application:
Definitions of Win32 API functions and constants:
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
public static extern bool SetWindowText(IntPtr hwnd, String lpString);
[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hwnd, int message, int wParam, IntPtr lParam);
private const int WM_SETICON = 0x80;
private const int ICON_SMALL = 0;
private const int ICON_BIG = 1;
Usage:
Process process = Process.Start("notepad");
// If you have just started a process and want to use its main window handle,
// consider using the WaitForInputIdle method to allow the process to finish starting,
// ensuring that the main window handle has been created.
// Otherwise, an exception will be thrown.
process.WaitForInputIdle();
SetWindowText(process.MainWindowHandle, "Hello!");
Icon icon = new Icon(#"C:\Icon\File\Path.ico");
SendMessage(process.MainWindowHandle, WM_SETICON, ICON_BIG, icon.Handle);

In Windows Forms you would use
Icon ico = Icon.ExtractAssociatedIcon(#"C:\WINDOWS\system32\notepad.exe");
this.Icon = ico;
So im guessing for WPF it would be similar.

RadioButton click on another form

We have a legacy program with a GUI that we want to use under control of a C# program to compute some values. We can successfully enter values in the numerical input controls, press the compute button, and read the produced answers from text display boxes.
But we can't seem to control a pair of radio buttons .
Calling CheckRadioButton() returns a code of success, but the control does not change state.
Sending a message of BM_CLICK does not change the state.
Attempts at sending WM_LBUTTONDOWN and WM_LBUTTONUP events haven't changed the state.
Has anyone been successful at "remote control" of radio buttons?
Portions of code to illustrate what we are doing:
[DllImport("user32.dll", EntryPoint="SendMessage")]
public static extern int SendMessageStr(int hWnd, uint Msg, int wParam, string lParam);
[DllImport("user32.dll")]
public static extern int SendMessage(int hWnd, uint Msg, long wParam, long lParam);
[DllImport("user32.dll", EntryPoint="FindWindow", SetLastError=true)]
static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);
[DllImport("user32.dll", EntryPoint="CheckRadioButton")]
public static extern bool CheckRadioButton(IntPtr hwnd, int firstID, int lastID, int checkedID);
static IntPtr GetControlById(IntPtr parentHwnd, int controlId) {
IntPtr child = new IntPtr(0);
child = GetWindow(parentHwnd, GetWindow_Cmd.GW_CHILD);
while (GetWindowLong(child.ToInt32(), GWL_ID) != controlId) {
child = GetWindow(child, GetWindow_Cmd.GW_HWNDNEXT);
if (child == IntPtr.Zero) return IntPtr.Zero;
}
return child;
}
// find the handle of the parent window
IntPtr ParenthWnd = new IntPtr(0);
ParenthWnd = FindWindowByCaption(IntPtr.Zero, "Legacy Window Title");
// set "N" to 10
IntPtr hwndN = GetControlById(ParenthWnd, 17);
SendMessageStr(hwndN.ToInt32(), WM_SETTEXT, 0, "10");
// press "compute" button (seems to need to be pressed twice(?))
int hwndButton = GetControlById(ParenthWnd, 6).ToInt32();
SendMessage(hwndButton, BM_CLICK, 0, 0);
SendMessage(hwndButton, BM_CLICK, 0, 0);
// following code runs succesfully, but doesn't toggle the radio buttons
bool result = CheckRadioButton(ParenthWnd, 12, 13, 12);

Send the BM_SETCHECK message. Be sure to use a tool like Spy++ to see the messages.

in this case i used another message BM_SETSTATE
SendMessage((IntPtr)hWnd, Win32Api.BM_SETSTATE, (IntPtr)newState, IntPtr.Zero);

Using C#, how do I set tab positions in a multiline textbox?

Is there a graceful way to set custom tab sizes/positions in a multiline textbox in C#?

You need to send the EM_SETTABSTOPS message, like this:
static class NativeMethods {
[DllImport("user32.dll")]
public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, ref int lParam);
}
static void SetTabs(TextBox box) {
//EM_SETTABSTOPS - http://msdn.microsoft.com/en-us/library/bb761663%28VS.85%29.aspx
int lParam = 16; //Set tab size to 4 spaces
NativeMethods.SendMessage(box.Handle, 0x00CB, new IntPtr(1), ref lParam);
box.Invalidate();
}

Apart from by vb 2013 the friendly people at microsoft have decided you no longer need the windows handle and you can no longer get at it.