byte tgelen1, tgelen2;
private void Form1_Load(object sender, EventArgs e)
{
textBox1.MaxLength = 1;
textBox2.MaxLength = 1;
}

private void textBox1_TextChanged(object sender, EventArgs e)
{

}

private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
tgelen1 = (byte)e.KeyValue;
}

private void textBox2_KeyDown(object sender, KeyEventArgs e)
{
tgelen2 = (byte)e.KeyValue;
}

private void alfabeyazdir_Click(object sender, EventArgs e)
{
for (byte i = tgelen1; i <= tgelen2; i++)
{
listBox1.Items.Add((char)i);

}
}