>How can I refer to controls in itemTemplate?
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>in CustomValidator1_ServerValidate, I need to refer to TextBox1 to do validation.
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Try something like:

// Replace grdTemplate with the name of your grid, assuming it's even on a grid.
TextBox text = this.grdTemplate.Rows[this.grdTemplate.EditIndex].Cells[0].FindControl("TextBox1") as TextBox;