How to get the index value by column name in GridView in asp.net

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SQL-Server-2005

C#

ASP.NET

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hi,
i am having a gridview in asp.net
and now I want the index value by the column name
How to do it.

for example

productid   |  productname   |    Productcode  | edit  |  delete

when i write Edit it should give me its index value [3]

is this possible????

i tries to do this

protected void GridView_RowDataBound(object sender, GridViewRowEventArgs e)
      {
          string columnValue = ((Label)e.Row.FindControl("imgEdit")).Text;
      }

but it gives me error
"object reference not set to an instance of an object"





please help
Regards