Basically, you shouldn't do that.
The problem is that the usercontrol should be a "black box" - your form should not be able to tell what it's content is, except for the items it publicly exposes.
So, either your UserControl needs to provide a property to return the textbox content or (possibly better) do the validation itself.
There is a Tip here which describes this: Transfering information between two forms, Part 2: Child to Parent
] - it's form based, but the same thing applies to UserControls as well.
"I don't want to show textbox.Text in MessageBox.Show method, I am trying to create a textBox in MessageBox.Show() method, the textBox in MessageBox.Show should contain the same content of one of the textbox in user control and if i change in MessageBox method it has to change there. Is it possible?"
You can't change "standard" messageboxes - they are what they are.
But it's pretty easy to create your own, with all the functionality you want. Just create a new form, size it appropriately, and add the buttons you actually need.
Set these properties:
FormBorderStyle = FixedToolWindow
ShowInTaskBar = False
StartPosition = CenterScreen
And use the ShowDialog method to display it.