Asp.net: How to retrive the DataGrid checkbox value

hi friends,

I need One solution ,help me .
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CheckBox gold=(CheckBox)e.Item.FindControl("ckGold");

CheckBox silver=((CheckBox)e.Item.FindControl("ckSilver"));

string txt=((TextBox)e.Item.FindControl("txt1")).Text;;

Response.Write(((CheckBox)e.Item.FindControl("ckGold")).Text);

if(gold.Checked==true)

silver.Checked=false;

else if(silver.Checked==true)

gold.Checked=false;

else

Page.RegisterStartupScript("Statrup","");

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Above code to be displayed one error:'Object Reference to be set an instance of an object'

What's the solution this?

Thank u