public class OpacityForms
{
System.Windows.Forms.Form f1;
System.Windows.Forms.Label l1;
public OpacityForms(System.Windows.Forms.Form f,System.Windows.Forms.Label l)
{
this.f1=f;
this.l1=l;
}
public void formsOpacity()
{
if(f1.Opacity>0||f1.Opacity<1)
{
f1.Opacity=(f1.Opacity*100+1)/100;
double p = f1.Opacity*100;
l1.Text=p.ToString()+"%";
}
else
{
f1.Opacity=1;
l1.Text="100%";
}
}
}

在窗体的Timer控件的Tick事件中写：

private void timer1_Tick(object sender,EventArgs e)
{
if(this.label1.Text=="100%")
{
timer1.Enabed=false;
return;
}
OpacityForms ff=new OpacityForms(this,this.label1);
ff.formsOpacity();
}