how to validate  

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C#5.0

In gridview as follows

           Compid   Companyname Address
Select       1        trom

Gridviewselected index change code as follows

protected void gvcomdet_SelectedIndexChanged(object sender, EventArgs e)
  {
      txt_compid.text = gvcomdet.SelectedRow.Cells[1].text.tostring();
      txt_name.text = gvcomdet.SelectedRow.Cells[2].text.tostring();
      txt_address.text = gvcomdet.SelectedRow.cells[3].text.tostring()
  }

When i click the select in gridview in textbox gridview selected row will be displayed as follows

Compid          1     (txt_compid)
Companyname     trom  (txt_name)
Address         <code>" "</code>

In address textbox txt_address shows  
i want to validate " " should not be shown in txt_address textbox it will be replaced with empty value

for that how can i validate in asp.net using c#.