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C# Windows Forms: Access Controls(Labels, Buttons) in other Classes

I need to know, how I can access lables or buttons other than in my "Form1"-Class.
My Problem:
I created for example labels, buttons via the design viewer. Now I can access
them in my Form1 Class. (testlabel.Enabled == true) just for example.
What I CAN'T do: Access those labels, buttons in another class! Let's say
I have a class "second-class" and I want to have a method there, that changes
the property of a label to
`testlabel.Enabled == false`
That's not possible, because in that "second-class" it's not visible.
So, is there an obvious easy solution to make those controls accessible in other classes?

Create a method in that (Second class) which takes that component (Label or Button or whatever you want to modify) as parameter into that method.
public void disableLabel(Label inputLabel)
{
inputLabel.Enabled == false
}
Create a method like the above.
Now in the form1 class you just to need to call that method and pass your Label into that method to Disable it.
SecondClass objSecondClass = new SecondClass();
objSecondClass.disableLabel(testlabel);

Every control in a form class is created by default with its property Modifiers set to Private
If you change it to Public you could access the control instance from another class.
However this is really a bad practice to follow. Messing with the visibility of the control is dangerous and could cause very complicated bugs to resolve.
If you really need to change something in your form class then provide a public method and call this method to change the internal functionality of the target form

VCL's CustomControl equivalent in Windows Forms

Delphi's VCL has a very useful class, CustomControl. This class is a direct base class of Control, which is equivalent to C# WF's UserControl.
For those, who are not familiar with VCL, CustomControl differs very little from Control; the main difference is that most of properties are protected; when implementing the new control, developer may decide, which ones does he want to publish and which ones shall remain hidden.
I'm developing my own control for Windows Forms and I want to hide some properties and events. For instance, I don't want to expose the MouseDown event - instead I allow capturing clicking on control's elements.
Is there an equivalent of VCL's CustomControl in Windows Forms? If not, how can I hide unwanted public properties and events in my control?
In response to answers:
This is not a matter of security, but rather a matter of code elegance. In Delphi I can derive from CustomControl, leave the OnMouseDown event protected (as in C#'s protected) and say to the control's user:
You cannot use OnMouseDown, because there is none. If you want to react to user clicking on control, simply use OnElementClicked - you'll even get detailed information about which element was clicked and what was its state.
I may disable calling the MouseDown event as Hans Passant suggested, but then I would have to include the following in the control's user's manual:
Please do not use the MouseDown event, because I've overridden the OnMouseDown method, such that it won't call the MouseDown event. This is because the control's logic is designed in such way, that you should use OnElementClicked rather than OnMouseDown. Please don't criticize the control because of MouseDown not working. Please don't report it as a bug, because it is by design. Please don't post messages in forums or create blog entries explaining how to fix the MouseDown problem by inheriting the class and manually calling the MouseDown event, because it would break the control's logic. Pleas don't... damnit, told you so!
If someone actually inherits from my control - I assume then, that he knows, what he's doing (also because one would then gain access to my control's internal logic as well). But if someone just uses my control, I would give him only these properties, events and methods, that I'm sure will work as designed.
I hope it explains my motives :)

There's a fundamental difference between hiding, what you asked for, and making it inaccessible, what I assume Delphi does. Hiding is simple, just repeat the declaration and apply attributes:
using System;
using System.ComponentModel;
using System.Windows.Forms;
class MyControl : Control {
[Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
private new event MouseEventHandler MouseDown;
}
Which prevents the event from being displayed in the Properties window, it won't show up in the IntelliSense dropdown and generates a compile error when he tries to assign the event in code anyway.
It is however not an absolute guarantee that the client programmer couldn't work around the restriction anyway. By casting or overriding your class for example. But the ultimate backdoor is implementing the IMessageFilter interface, nothing you can do about that. So this ought to be good enough if elegance is the goal.

How do you send data from a form to a class?

I have a Class named Testing and a Form called TitleScreen. In TitleScreen I have a textBox1 who's text I would like to be passed to a Class and then pass it back to my Form into a textBox2.
I know how to do only the basics in C# so if you try and make it simple as possible.

In your Class:
public class Class1
{
public static string SeparateName(string fullName)
{
string[] wordsInText = fullName.Split(' ');
return wordsInText[0];
}
}
In your Form:
private void button1_Click(object sender, System.EventArgs e)
{
textBox2.Text = Class1.SeparateName(textBox1.Text);
}
"I highly recommend that you read a book or tutorial that targets new users, otherwise there will be holes in your understanding of the language and the frameworks."

It sounds like you want to perform an operation on the textbox's value and then print the result in another textbox.
You can write a method (function) that accepts an argument of type String and perform the operation in that method. The method can then set the Text property of the textbox to the result.
If you're asking how to input code in a winforms project, you can double-click the background of the form to reach its code. (At least in Visual Studio)
If you don't know how to do the above suggestions, I highly recommend that you read a book or tutorial that targets new users, otherwise there will be holes in your understanding of the language and the frameworks.

I would suggest you want to look at the concept of data binding, whereby you bind the controls on your forms to the properties of the underlying objects (instances of your classes).
Binding removes the need to write code to cross-load the data from the class into the form and back again, instead you can then say "text box 1 is bound to this property of my class". Then, when you update the value of the textbox the data is automatically placed into the chosen property of your class instance. Typically you then have a save button that calls a save method on your class to persist the data to your data store (database or whatever).
It is perfectly reasonable to bind more than one control on your form to the same property on your underlying class, so in your example you can bind both textBox1 and textBox2 to the same property on your class. Then, once you've implemented databinding, when you change the value in textBox1, the value will automatically be reflected in textBox2, either on each keystroke or when the field is validated (typically when you move focus to another control).
This is the microsoft documentation on Winforms binding which covers everything you need: https://msdn.microsoft.com/en-us/library/ef2xyb33(v=vs.110).aspx

Blindly calling a public method (C#)

OK, weird one. I have many usercontrols with a repeater, the layout of the repeater is the same in all controls, and they all have a bindData() method publically available.
I'm wondering, can I setup another usercontrol for paging without having to specify the parent control?
I'm able to do the following:
((controls.specificuserControlClass)Parent).bindData();
Which is all fine - however I'd need to specify the specificuserControlClass into the pager and then would need it "per repeater" if you see what I mean?
So can I call Parent.bindData() blindly from the child control? I "know" that method exists (or would build checks to make sure), however Visual Studio isn't happy as it doesn't know of the method.

Why not make your controls all implement a specific interface?
public interface IBindData
{
void bindData();
}
Then, you would simply do:
((IBindData)Parent).bindData()
And it should invoke each control's method as appropriate.

ASP.NET UserControl's and DefaultEvent

Outline
OK, I have Google'd this and already expecting a big fat NO!! But I thought I should ask since I know sometimes there can be the odd little gem of knowledge lurking around in peoples heads ^_^
I am working my way through some excercises in a book for study, and this particular exercise is User Controls. I have cobbled together a control and would like to set the DefaultEvent for it (having done this for previous controls) so when I double-click it, the default event created is whatever I specify it to be.
NOTE: This is a standard User Control (.ascx), NOT a custom rendered control.
Current Code
Here is the class & event definition:
[System.ComponentModel.DefaultEvent("OKClicked")]
public partial class AddressBox : System.Web.UI.UserControl
{
public event EventHandler OKClicked;
Current Result
Now, when I double click the the control when it is on a ASPX page, the following is created:
protected void AddressBox1_Load(object sender, EventArgs e)
{
}
Not quite what I was expecting! So, my question:
Is it possible to define a DefaultEvent for a UserControl? Is it a hack? If it's [not] supported, is there a reason?
Side Note: How do we put underscores in code? I cant seem to put and escape char in?

Here is a possible answer, without testing (like martin did).
In reflector, you will see that the DefaultEventAttribute allows itself to be inherited.
In reflector, you see that the UserControl class has it's default event set to the Load event.
So the possible reason is that even though you are decorating your user control with the default event of OKClick, VS might still be thinking that the default event is load, as it's being inherited from UserControl whose default event is Load.
Just a high level guess at what might be happening.

OK, I checked this out, Inheriting from WebControl rather than UserControl.. All worked fine.
Looks like Darren Kopp takes the crown for this one! Thanks for the input!