Enable or disable button on list page form x++

Go to list page interaction class  —>  selection change method

public void selectionChanged()
    {
        INTC_MaterialRequest        materialRequest;

        super();

        materialRequest =   this.listPage().activeRecord(queryDataSourceStr(INTC_MaterialRequest, INTC_MaterialRequest));

        if(materialRequest.INTC_MaterialRequestStatus ==  INTC_MaterialRequestStatus::Approved)
        {
            this.listPage().actionPaneControlEnabled(formcontrolstr(INTC_MaterialRequestListPage, INTC_CreatePurchReq), true);
        }
        else
        {
            this.listPage().actionPaneControlEnabled(formcontrolstr(INTC_MaterialRequestListPage, INTC_CreatePurchReq), false);
        }
    }