I am afraid that you can not use the ItemType property of GridViewColumn in AutoGeneratingColumn. However, you can use the GridView's Items property. In fact, the GridViewColumn.ItemType property is using a similar approach:
/// Gets the type of the item that the field belongs to.
>The type of the item.</
public virtual Type ItemType
if (this.DataControl != null)
However, ElementType is internal, so you can't use it directly.
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