Select a random number of textboxes

Try this 

public partial class Form1 : Form
{

int[] index ;
int textCount = 0,textChoice = 0;
Random random1 = new Random();
Random random = new Random();
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{

}

private void button1_Click(object sender, EventArgs e)
{
textCount = 0;
textChoice = 0;
clearBoxes();
textChoice = random1.Next(1, textCount);
index = new int[textChoice];

for (int i = 0; i < index.Length; i++)
{
index[i] = 0;
}

int j;

for (int i = 0; i < index.Length; i++)
{
j = getRandom();
index[i] = j;

}

Array.Sort(index);
int count = 0,listCount = 0;
foreach (Control tb in groupBox1.Controls)
{
if (tb is TextBox)
{
if (listCount > index.Length - 1)
{
break;
}
if (count == index[listCount])
{
tb.BackColor = Color.Green;
listCount++;

}

count++;
}
}


}
public int getRandom()
{

int i = random.Next(1, textCount);
if (index.Contains(i))
{
i = getRandom();
}
return i;
}

public void clearBoxes()
{
foreach (Control tb in groupBox1.Controls)
{
if (tb is TextBox)
{
tb.BackColor = Color.White;
textCount++;
}
}
}


}
}

Sir, I would like it to be a random number...(i.e. Random rand = new Random();)  So, you would hit a button and a random number of textboxes (1 to 300) backcolor will change.   

Okay
So 1 to 300 textboxes
we still need to know how many of the 300 to change the backcolor of

Sir,

The user will imput the number...it will be between 10 and 300...  So, the random number will be somewhere in that range. 

Any specific random number eg
groupbox has 6 textboxes. Change any three to a different backcolor