Implicit conversion is slow, Try with Explicit conversion via .NET Class Convert.
Set your Font properties on properties windows in design time of your richtextboxcontrol.
This line of code is called twice for every row in your collection, so you will have a very slow performance. So, here is the Main problem of your code.
if ((int)qs.selectsurrah(int1) == row.Surrah && (int)qs.selectsurrah(int1) < row.Surrah + 1)
Try this one:
private void button1_Click(object sender, EventArgs e)
{
int int1 = 0;
int.TryParse(textBox1.Text,out int1);
int int_from_select = qs.selectsurrah(int1);
foreach(Database24DataSet._Quran_simple___1Row row in qs.GetData())
{
if (row.Surrah.Equals(int_from_select) && int_from_select <(row.Surrah+1))
{
richTextBox1.Text = richTextBox1.Text + "\n" + row.Arabic;
Application.DoEvents();
}
}
}
Hope it helps.