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In MaskedTextBox, how do I write a mask to check that the input is none empty?

As far as I can tell, this is none regex. And I can't find anywhere for quantifier. This is what I have right now, which the MaskedTextBox does not recognize the + as a quantifier
A+

You are trying to validate the masked text box too early. You can't know that it doesn't have any input until it doesn't have input. I assume that you have some sort of form that you are having a user fill in and at the end they press a button to complete the data entry. You can validate the input within that event.
If you just want to ensure non-empty then you can check the TextLength field after focus leaves the textbox, or when a button is pushed, etc..

Wrapping text in C# dialog box

I've got a dialog box that pops up with a dynamic list of numbers, and I'd like to get the box to wrap the text because at the moment it displayed up to screen width and then cuts the rest off.
I know I can use \n to declare a new line, but the list is dynamic - it could be one item, it could be 20.
Is there any way to tell the dialog box to wrap text?
Edit: clarification + example code
I'm not using MessageBox.Show() - our code uses its own defined message box class, but the guts of it calls System.Windows.Forms.Form.ShowDialog(parent). Maybe this isn't as well-behaved (i.e., doesn't wrap) as MessageBox.Show()?

Create your own simple form and add a label. Do the wrapping there... You cannot do that much things with Dialog boxes.
In this way you have much more flexibility to show your information to the user.

Are you using the System.Windows.Forms.TextBox? It has a property WordWrap that you can set to true

No other way for a standart MessageBox. Only creating your own form.

You could programmatically format the text by restricting each line to a specific number of words then inserting a \n or Envoronment.NewLine

C# winforms: move text from one textbox to another

I have a winform application with two textboxes. The textboxes are multilined and has 5 rows.
When the user enters more than 5 lines of text in the first textbox I want the text to continue in the second textbox. And if he/she deletes text from the first textbox I want the text to move back from the second to the first one...
I have tried to solve this in my code by checking how many rows the first textbox has and moved text between the two textboxes. But it doesnt work that well so I wonder if anyone got a better solution??

You could accomplish this by registering for the TextChanged events on the TextBox controls. Then in the event handler, manually inspect the Text property and set focus to the appropriate control. However, what you are describing sounds like it may lead to an inconsistent user experience.
From a UX standpoint I would suggest changing the approach. First of all do you really need to split the text in the UI, or could it be split afterward in the business layer? If you do need it split in the UI, you could have a single TextBox which allows the user to enter the full text, and below it have 2 read-only textbox's which display the 2 split segments as they type (you would also use the TextChanged event logic to do this as they type).
I hope this helps.

Have you tried checking the visible Characters in the text box? or text box character length?

Validation to have only numeric data in a textbox in a window form

I want that a user only enters numeric data into the textbox in a windows form. How can I achieve this?

The easiest way would be to use a MaskedTextBox (use the Mask property) or a NumericUpDown control. If you really need fine-grained control that these controls do not provide, handle the KeyPress and other appropriate events of the TextBox control as required.
EDIT: Clarified that KeyPress is not the only relevant event, as mentioned by ho1.

What do you want to happen when the user attempts to enter a non-numeric value? Does this matter before a data submission attempt? Without entering your code block every time a key is pressed, I think it'd be cleaner to just let the user enter non-numerics and validate either when the form is submitted and/or when the text box loses focus. Then if validation fails, notify the user of the strict numeric format. This would be less invasive to a user, rather than interrupting.
As for validation, just use either regular expressions or try to parse the text box's text as an integer.
Regular Expression
System.Text.RegularExpressions.Regex.IsMatch(e.KeyChar.ToString(), #"^\d$");
Integer Parse
int validNumbers = 0;
bool isValid = int.TryParse(myTextBox.Text, out validNumbers);

First it depends on what kind of numeric data you want to allow. (Integer, Double, ..., or something app-specific? Like an ISBN or something?)
Short:
The easiest way would be like Ani said, but if you need a more specific way, you should subcribe an matching Event. For Example TextBox.Validating, TextBox.OnLeave, TextBox.OnTextChange... depends on when you want to test the matching.
Than you can test in the Eventhandler whatever you want, even RegEx would be possible for complex alphanumeric data.
PS: You should really have a look at http://msdn.microsoft.com/en-us/library/system.windows.forms.control.validating.aspx

Send keystrokes to TextBox

I have a derived TextBox where I intercept the userinput to manipulate it. However I have to preserve the original typed input. So my idea was to hold an inner TextBox within my derived class and send the user's input to that TextBox before manipulating it.
The reason for this approach is that I do not want to take care of all this special actions like: typing something, ctrl+a, [del], type something else, [backspace] and so on ...
However I do not know how to send a single keystroke (keycode, ascii, char) to a TextBox. Maybe you have another idea without an inner TextBox at all? Thank you!

If I understand you correctly, you could just let the inner text box have keyboard focus and handle the input. Then you could handle its KeyDown/KeyUp/KeyPress events in your container class to "intercept" the input.

You probably don't want to use keyboard-level manipulation because there are ways to change the text without the keyboard (in particular, with copy, cut, and paste). Why not use the Text property to get the text from the original textbox and then just save that?