首先将public IEnumerable<Portfolio> portfolios { get; set; }更改为

public List<Portfolio> portfolios { get; set; } 

因为IEnumerable用于读取列表，并在您的代码中添加

because IEnumerable is used for reading the list and in your code add

 //2. CALCULATE PORTFOLIO ALPHA
         for (var i =0; i<portfolioListItem.Count; i++)
            {

        List<PortfolioHolding> list = new List<PortfolioHolding>();
        list=portfolioListItem;
        Portfolio port = new Portfolio();
        port.portfolioIdentifier = "current";
        port.portfolioAlpha = portfolioListItem[i].fundWeight * portfolioListItem[i].fundAlpha;
        port.portfolioHoldings = list;
        result.portfolios.Add(port);

        };