first of all don't set event desing time.
Remove selectedvaluechanged event from desing.
Make your code structure like this
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
this.combobox1.ValueChanged += new System.EventHandler(this.numericDiameter_ValueChanged);
}
so your initialization will not call event and you can able to change value
hope this helps you
happy coding
:)