usercontrol in a placeholder

I've tried doing the following:

        var userControl = UserControls.ProductDetailUserControl();
        use开发者_JS百科rControl.ID = "id_SubProduct_"+productId;
        userControl.Visible = true;
        userControl.ValidationDependentOnControl = productOption.ClientID;
        userControl.ConditionallyVisible = productOption;
        plhDetails.Controls.Add(userControl);

throws a null exception

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Try this

 var uProductDetails = (UserControls.ProductDetailUserControl)Page.LoadControl("UserControls/ProductDetailUserControl.ascx");

    //set properties     
    plhDetails.Controls.Add(userControl);