You need to use the
else if
instead of just the if. What is happening is that you are going through all the conditsions when you do not have to:
if (cmbitemcode.Text == "101")
{
txtitemname.Text = "Samosa";
}
else if (cmbitemcode.Text == "102")
{
txtitemname.Text = "Cake";
}
else if (cmbitemcode.Text == "103")
{
txtitemname.Text = "Biscuits";
}