My app search for Prime Numbers work incorrectly

Question

Here is my source code: (Also app code attached)

 =============================================================================

namespace IsThisPrime

{

public partial class Form1 : Form

{

public Form1()

{

InitializeComponent();

}

private void button1_Click(object sender, EventArgs e)

{

checkInput();

}

private void Form1_Load(object sender, EventArgs e)

{

textBox1.MaxLength = 19;

/* SET THE FORM FULL SCREEN

int width = Screen.PrimaryScreen.Bounds.Width;

int height = Screen.PrimaryScreen.Bounds.Height;

this.Location = new Point(0, 0);

this.Size = new Size(width, height);

*/

}

private void checkInput()

{

// Check Validation

string text1 = textBox1.Text;

long num1;

bool check = long.TryParse(text1, out num1);

if (check == false)

{

textBox2.Text = "Overloaded!";

}

else

{

mainContent();

}

}

private void mainContent()

{

// Processing part

long x = Int64.Parse(textBox1.Text);

int i = 0;

bool Flag = false;

for (i = 2; i <= x / 2; i++)

{

if ((i == x))

i = i + 1;

if ((x % i == 0))

{

Flag = true;

break;

}

}

if ((Flag == false))

{

// This num is prime number

textBox2.Text = "PRIME!";

MessageBox.Show("PRIME");

}

else

{

// This num is not prime number

textBox2.Text = i.ToString();

MessageBox.Show("NOT PRIME");

}

}

}

}

=============================================================================

 The problem is when I check 1000000016531 ( A prime number), it takes a long time caculating and show that 1000000016531  isn't a prime number because 1000000016531 % (-1) == 0 

It can't be like that because i starts from 2, so how can it be like this i = -1

Anyone knows the reason? I really appriciated if you read this :(