c# - Error while calculating percentage -

unable cast object of type 'system.web.ui.webcontrols.textbox' type 'system.iconvertible'.

i getting error while converting. have 2 text boxes in 1 have amount , in have percentage value should come in third text box.

public partial class caltxt : system.web.ui.page {    double amt1;     double exc;     public void page_load(object sender, eventargs e)     {      }      public void txtamt_textchanged(object sender, eventargs e)     {           amt1 = math.round(convert.todouble (txtamt));          //decimal percentagerate = convert.todecimal(this.txtamt.text);      }       protected void txtexc_textchanged(object sender, eventargs e)     {          double exc = math.round (convert.todouble(this.txtexc.text));           // decimal    temp = (math.round(convert.todecimal(txtexc.text) / convert.todecimal(txtamt.text)*100));        // txttotalexc.text = temp.tostring();     }      protected void txttotalexc_textchanged(object sender, eventargs e)     {         double totat = math.round (convert.todouble(exc) /100)* convert.todouble(amt1); //+ convert.toint32(this.txtexc.text);         this.txttotalexc.text = totat.tostring();     } } 

you should convert textbox.textproperty double instead of textbox

replace

amt1 = math.round(convert.todouble (txtamt)); 

with

amt1 = math.round(convert.todouble(txtamt.text));