Create button during runtime in C#.net?

You can't declare extra variables at execution time in C# - but you really don't want to anyway, as you wouldn't be able to access them dynamically afterwards. Just create an array:

// buttons would be declared as Button[] as a member variable
buttons = new Button[30]; 
for(int i = 0; i < buttons.Length; i++) {
    buttons[i] = new Button();
    // Button customization here...
    ...
    groupBox1.Controls.Add(buttons[i]);
}

Alternatively, use a List<Button>, which will certainly be more convenient if you don't know how many buttons you need beforehand. (See the obligatory "arrays considered somewhat harmful" blog post.)

Of course, if you don't actually need to get at the buttons later, don't bother assigning them to anything visible outside the loop:

for(int i = 0; i < 30; i++) {
    Button button = new Button();
    // Button customization here...
    ...
    groupBox1.Controls.Add(button);
}

You need to think about what information you need access to when... and how you want to access it. If you logically have a collection of buttons, you should use a collection type variable (like a list or an array).

Frankly I think it's one of the curses of the VS designers that you end up with horrible names such as "groupBox1" which carry no information beyond what's already in the type declaration, and encourage developers to think of collections of controls via individually-named variables. That's just me being grumpy though :)

Try this

for(int i = 1; i < 30; i++) {
Button button = new Button();
// Button customization here...
button.Name = "Button" + i.ToString();
groupBox1.Controls.Add(button);
}

You seem like you're almost on the right track:

// in form class
Button[] m_newButtons = new Button[30];

// in your trigger function
for(int i = 0; i < 30; ++i)
{
    m_newButtons[i] = new Button();
    // ...
    groupBox1.Controls.Add(m_newButtons[i]);
}

If you try and do this more than once you may have to remove the old buttons from the control before adding the new ones.

buttons = new Button[30]; 
for(int i = 0; i < buttons.Length; i++) {
    buttons[i] = new Button();

    groupBox1.Controls.Add(buttons[i]);
}

this code will work but button will be added one over other so set location also

buttons = new Button[30]; 
    for(int i = 0; i < buttons.Length; i++) 
    {
        buttons[i] = new Button();
        Point p=new Point(xvalue,yvalue);
       buttons[i].Location = p;       
        groupBox1.Controls.Add(buttons[i]);
    }

one thing you want to remember increment the x or y position by which you want to display it

Try this one out, I have just learned it myself:

public partial class Form1 : Form
{
 Button[] btn = new Button[12];// <--------<<<Button Array

 public Form1()
 {
  InitializeComponent();
 }   

 private void Form1_Load (object sender, EventArgs e)
 {    
    for (int i = 0; i < 12; i++)
    { 
     btn[i] = new Button ( ); 
     this.flowLayoutPanel1.Controls.Add(btn[i]);
    }
 }
 // double click on the flow layoutPannel initiates this code
 private void flowLayoutPanel1_Paint(object sender, PaintEventArgs e)
 {

 }

}