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Passing Details From One Window to Another Window

I have a selection of TextBoxes that a user fills in when they wish to note that they have had contact with another person. Most of the TextBoxes are imply filled in by typing into them. However, for one of them I would like the user to be able to select from a list of People that appears when they click on a button.
This is where I am having problems. So far I have just made a DataGrid appear and handled it's SelectionChanged method to fill in the TextBoxes text property. This has worked fine, however now there is not enough space on the current page to show an entire DataGrid with all the people they can select from.
I've decided to show the People in a separate, smaller Window that appears when the user clicks a Button. The issue I have is that when the user selects the Person they wish to mark the contact for in the new Window, I have no idea how I can notify the original Window that a Person has been selected, close the new smaller Window and fill in the appropriate TextBox on the original Window.
What would be the most intuitive way to fill in the TextBox on the original Window, based on the selection on the Window that opens?

I would use delegates,which call a function of the original window and parse the changed variable with it. So you know when the user clicked something and you can directly react to this "event".
Link:
https://msdn.microsoft.com/en-us/library/ms173171.aspx

If you use a framework like Galasoft's MVVM Light (http://www.galasoft.ch/), they have a messenger system just for this purpose. It allows you to "broadcast" messages that can be "received" by any other part of the application

This is when considering using Domain, Model, Presentation (Winforms/WPF version of MVC formatting) to do your app.
You can have each form as its own class, well they are their own class. Create each form class but add some public members to it if the controls are private. Have them have "get" properties only and to return the values of whatever controls or variables are in that form. Your main form will be the controlling form. All forms will be handled by the main form so when you open it, it is a class the main form can access.
Now, if I remember (been doing more MVC and not any Winforms lately) I believe if you use the ShowDialog() method it will freeze the main thread so when you close out the main form you can continue and read in public members you have in your forms class you opened. Synchronous I believe it runs as. If you use just Show() the thread will keep on trucking, asynchronous. With asynchronous you may then have to use a main form in your startup code so there is always a window there but subscribe to the close event of your forms and have a method that can grab those public members out. Be sure to instantiate the extra forms at the root of the main class so it doesn't fall out of scope when it exists the method that calls it. You may even be able to make the method that calls is a async call and have an await before the command that runs the Show method on the form.
Summary, treat each form as its own class but add public members that can read the values from the controls and/or variables you want. Read that data from the class when it closes via an event or synchronously when the thread closes out from the form closing. The form closing doesn't discard the object, just the visualization of the form.
Oh, if you are passing info from the main form to a child for you are opening, either add a constructor for that form class that takes your input as a model or values to fill in the appropriate variables or forms before showing it or create a public property you can put your values you want to send in before showing the class.
Remember, everything is a class, once you look at it as such and treat it as such, the answer will come. :-)
I should warn, I am a long winded explainer.
At work putting all this down from memory so some errors may exist. Let me know if there are.

I think the problem is to access the controls of the main window, isn`t it?
You can define an event of changing user`s choise and access MainWindow control by using the following construction:
((MainWindow)Application.Current.MainWindow).MyTextBox

C# WinForms ShowDialog() call arbitrary method

I'm working on a console application that creates a form to alert users of some given state - at a later stage, the code base will become a class library.
For now, however, I need to show the form (ShowDialog would be the best method here, I guess) THEN call an arbitrary method before the form closes.
As an example, I need to show the form, set the text value of a label control, wait for n number of seconds, then change the value of the label, then close the form. I know that this sounds a little trivial, but I'm trying to proof-of-concept the design.
I've taken a look around and it doesn't look like this is possible, as ShowDialog() requires me to close the form before I can continue through code listing in the calling method/class.
Here's what I have so far:
PopUpForm myForm = new PopUpForm(string messageToDisplay);
myForm.ShowDialog();
//call myForm.someMethod() here, before the form closes
//dispose of the form, now that we've no use for it
myform.Dispose();
//target method in PopUpform class
public void someMethod()
{
lblText.Text = "Waiting for some reason";
//wait n number of seconds
lblText.Text = "Finished waiting. Form will now close";
//it doesn't matter if the form closes before the user can see this.
}
It looks like ShowDialog() doesn't support this sort of behaviour. I'm looking into BackgroundWorker threads, but was wondering if anyone has any advice on this, or have encountered this before.

If you want to show the form, then continue working, then close it - you can do so via Form.Show() instead of Form.ShowDialog():
using (var myForm = new PopUpForm(messageToDisplay))
{
myForm.Show(); // Show the form
DoWork(); // Do your work...
myForm.Close(); // Close it when you're done...
}
However, if this is purely a console application (and doesn't have a message pump), then this will likely not work properly.
Other options would be to provide a timer within your Form to have it close, or pass a delegate into the Form to run your method on Show, after which it could close itself.
at a later stage, the code base will become a class library.
When you do this, you'll likely want to come up with a different mechanism to provide notifications. Coupling your library to a specific UI technology is a bad idea. It would likely be better to have your library just provide events or other notification, and allow the user to provide the UI/notification to the user.

If this code ie destiend to end up in a code library, I recommend against having it display any forms, ever, of its own volition. The library should generate events, or in some cases exceptions, that can be caught by the invoking application to allow it to display the form. If certain details requiring presentation to the user are internal to the library, expose a DisplayEventData() method from the library.

The ShowDialog-method creates a "modal" window, and usually blocks the UI until you close it (either by clicking OK or Cancel). You would need to create a WinForm yourself, can be a simple one though and create a message-pump for it. You can run your own form by calling
Application.Run(YourForm);
You would need to hold your console-thread with a mutex for example, to keep it from continuing, while the form is open.
The form offers all the methods you know from WinForms like Close, where you could tell your console-thread to continue.
See this on MSDN.

Showing Multiple Instances Of Same Form?

I'm having some serious issues with a WinForm application that I'm working on.
Currently, I'm using Form1.ShowDialog(); to display a form. This code is contained in a background worker that looks for changes in a database. Using Form1.ShowDialog(); only allows 1 form to open at a time, even if there are multiple changes to the database. What I want to have happen is for multiple forms to open at once if there is more than one change in my database.
When I use Form1.Show();, the application blows up. For some reason, the Show() method makes the forms not display properly (all the elements in the form are missing).
Is there anything I can do to make my code work the way I want it to?
Edit: here's a code snippet
//result is a linq result
foreach (var row in result)
{
Form1 Form = new Form1();
Form.ShowDialog();
}

After a first look, I can tell you this:
Showdialog can't work the way you intend: this very method makes the owner inactive until the dialog is closed. In your case, the loop will pause at the first showdialog, then resume when you close the form, opening a new one and so on until the loop is finished.
As for the "show" problem, creating empty forms, I need more information. The rest of the code and the exception(s) you're getting.

Two points from the top of my head:
1) To open more then one form , use non modal (modeless) method (i think
the show() method). see for example http://msdn.microsoft.com/en-us/library/39wcs2dh.aspx
2) I am not sure you can call UI related method from a non UI thread. You might want to send an event to your UI thread from the worker thread and the UI thread will call the show method

How can you tell if you're on the Main UI thread? (In CF)

Now unfortunately due to the fact that WinCE Usb Device Arrival / Removal exposes itself via WindowsMessages I have to ensure that a certain (non-UI) component is not created on a background thread. I would like to assert this via an exception but am lacking the code for the assertion.
This component creates a MessageWindow* and uses it to receive usb arrived/removed messages. The issue is if someone creates this component on a background thread (not necessarily; IsBackground = true) when the thread exits the window will be destroyed.
Any ideas?
*as an aside I still don't know why Form doesn't inherit from this class
Update
I think my version 1 wasn't very clear. So this is v2.
When you create a MessageWindow or a Form for that matter on a thread, when that thread exits the Window/Form is destroyed.
My component is creating a "hidden" message window to intercept some important events, ergo I do not wish for it to be destroyed. Therefore I must somehow ensure that the code that creates the form is running on the "Main UI" thread.
If possible i'd like to avoid passing down a reference to the "main" form to this component as it is (architecturally speaking) supposed to be miles away from the UI.
Update
Moving logging question to a separate Q.

Ok, I understand that you don't want for your component to "know" about the main window -- makes sense.
How about this: How about if you make sure that you always instance your component on the main thread? You component will create it's listener window on the constructor's thread.
If you do that, then you just need to make sure that you call the constructor from the main thread. I'm making some assumptions about your code, but I'm guessing that you must have some class in your architecture that knows about both the UI and your component. Create your component there, using a callback, and the main form's InvokeRequired/Invoke methods.

In forms, you use the InvokeRequired property.

Why not create the non-UI component on a background thread and when you go to update any UI component just look to see if invokeRequired then get back on the main thread to actually do the update.
You should have nothing really tying up the main event thread, IMO.

You can use it in this way:
void MyCallback()
{
if (form1.InvokeRequired) { // form1 is any existing gui control
form1.Invoke(new Action<>(MyCallBack));
return;
}
// your logic here
}

Hey there: I had an idea about your problem. This is just a random thought, and I don't know for sure whether it will work (I have not tested, nor even compiled this -- it just hit me):
What if you get the window handle of the main window of your app, then build a Control around it (I'm assuming that you have a gdi-based app, like Winforms)?
this code might not compile, but it's close (it would go into your component -- note that it would make your component require a gdi windows/winform app, as opposed to a console or WPF app).
If you do try it, I'd love to hear whether it worked for you.
using System.Diagnostics;
using System.Windows.Forms;
void Init()
{
// get handle to the main window
intPtr mainWindowHandle = Process.GetCurrentProcess().MainWindowHandle;
Control mainWindow = Control.FromHandle(mainWindowHandle);
if(mainWindow.InvokeRequired)
mainWindow.Invoke(SetupMessageWindow);
else
SetupMessageWindow();
}
void SetupMessageWindow()
{
// do your thing...
}

Load a form without showing it

Short version: I want to trigger the Form_Load() event without making the form visible. This doesn't work because Show() ignores the current value of the Visible property:
tasksForm.Visible = false;
tasksForm.Show();
Long version: I have a WinForms application with two forms: main and tasks. The main form is always displayed. The user can either click a button to open the tasks form, or click some buttons that just run a task directly without opening the tasks form.
When a user asks to run a task directly, I'd like to just call some public methods on the tasks form without showing it. Unfortunately, the task logic depends on stuff that happens in the Form_Load() event. The only way I can find to trigger Form_Load() is to call Show(). The best I've been able to do is to show the form in the minimized state:
tasksForm.WindowState = FormWindowState.Minimized;
tasksForm.Show();
I suppose the cleanest solution would be to pull the tasks logic out of the tasks form and into a controller class. Then I can use that class from the main form and from the tasks form, and only load the tasks form when I need it visible for the user. However, if it's an easy thing to load the form without displaying it, that would be a smaller change.

Perhaps it should be noted here that you can cause the form's window to be created without showing the form. I think there could be legitimate situations for wanting to do this.
Anyway, good design or not, you can do that like this:
MyForm f = new MyForm();
IntPtr dummy = f.Handle; // forces the form Control to be created
I don't think this will cause Form_Load() to be called, but you will be able to call f.Invoke() at this point (which is what I was trying to do when I stumbled upon this SO question).

It sounds to me like you need to sit down and re-think your approach here. I cannot imagine a single reason your public methods need to be in a form if you are not going to show it. Just make a new class.

I totally agree with Rich B, you need to look at where you are placing your application logic rather than trying to cludge the WinForms mechanisms. All of those operations and data that your Tasks form is exposing should really be in a separate class say some kind of Application Controller or something held by your main form and then used by your tasks form to read and display data when needed but doesn't need a form to be instantiated to exist.
It probably seems a pain to rework it, but you'll be improving the structure of the app and making it more maintainable etc.

From MSDN:
Form.Load
Occurs before a form is displayed for the first time.
Meaning the only thing that would cause the form to load, is when it is displayed.
Form.Show(); and Form.Visible = true; are the exact same thing. Basically, behind the scenes, Show checks for various conditions, then sets Visible to true. So obviously, setting visible to false (which it already is) before showing the form is meaningless.
But let's forget the technicalities. I completely agree with Rich B and Shaun Austin - the logic shouldn't be in that form anyway.

Sometimes this would be useful without it being bad design. Sometimes it could be the start of a migration from native to managed.
If you were migrating a c++ app to .NET for example, you may simply make yourwhole app a child window of the .NET form or panel, and gradually migrate over to the .NET by getting rid of your c++ app menu, status bar, toolbar and mapping teh .NEt ones to your app using platform invoke etc...
Your C++ app may take a while to load, but the .NET form doesn't..in which you may like to hide the .NEt form until your c++ app has initialised itself.
I'd set opacity=0 and visible=false to false after calling show, then when your c++ app loads, then reverse.

If you make the method public, then you could access it directly.... however, there could be some unexpected side effects when you call it. But making it public and calling it directly will not draw the screen or open the form.

Move mandatory initialization code for the form class out of the Load event handler into the constructor. For a Form class, instantiation of an instance (via the constructor), form loading and form visibility are three different things, and don't need to happen at the same time (although they do obviously need to happen in that order).

None of the answers solved the original question, so, add the below, call .Show() to load the form without showing it, then call .ShowForm() to allow it to be visible if you want to after:
private volatile bool _formVisible;
protected override void SetVisibleCore(bool value)
{
base.SetVisibleCore(_formVisible);
}
public void ShowForm()
{
_formVisible = true;
if (InvokeRequired)
{
Invoke((Action) Show);
}
else
{
Show();
}
}