The RowDataBound event is fired when every row in the GridView is DataBound so there's no need to further iterate through all of the rows. Do something more like this: 

Protected Sub GridViewC_Definition_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridViewC_Definition.RowDataBound
	Dim gvr As GridViewRow = e.Row


	If gvr.RowType = DataControlRowType.DataRow Then
		Dim GridViewTemplateC_ValuesDropDownList = CType(gvr.FindControl("DropDownListC_Values"), DropDownList)
		GetC_Attributes.SelectParameters("C_ID").DefaultValue = gvr.Cells(0).Text
		GridViewTemplateC_ValuesDropDownList.DataBind()
	End If
End Sub