c# - "is not null here"

Question

我有一个方法不断给我警告“这里不为空”。

        StockHolding stockHoldingFiltered = new StockHolding();
        List<StockHolding> stockHoldingsFiltered = new List<StockHolding>();
        IReadOnlyList<StockHolding> stockHoldings = this.GetAll();

        stockHoldingsFiltered = stockHoldings.Where(stckhold =>
            stckhold.PortfolioId.Equals(portfolioId) &&
            stckhold.StockSymbol.Equals(stockSymbol)).ToList();

        if(stockHoldingsFiltered != null)
            stockHoldingFiltered = stockHoldingsFiltered.FirstOrDefault();

        return stockHoldingFiltered;

警告:此处“stockHoldingFiltered”不为空。CS8600:将 null 文字或可能的 null 值转换为不可为 null 的类型。

我无法让这个警告消失。有没有人遇到过这个问题并能够解决这个警告？

Answer 1

此问题与您的 if 语句有关。您正在对 ToList() 进行空检查.该列表不会为空，但它可以为空。下面是您的代码，用于演示这一点。

void Main()
{
    int portfolioId = 1;
    string stockSymbol = "USD";

    StockHolding stockHoldingFiltered = new StockHolding();
    List<StockHolding> stockHoldingsFiltered = new List<StockHolding>();
    IReadOnlyList<StockHolding> stockHoldings = StockHolding.GetAll();

    stockHoldingsFiltered = stockHoldings.Where(stckhold =>
        stckhold.PortfolioId.Equals(portfolioId) &&
        stckhold.StockSymbol.Equals(stockSymbol)).ToList();

    if (stockHoldingsFiltered != null) // this cannot be null as its returning a list. It might be empty but not null.
    {
        Console.WriteLine("I WASN't NULL, but has no records. Line below will fail.");
        //stockHoldingFiltered = stockHoldingsFiltered.FirstOrDefault();
    }
    
    if(stockHoldingsFiltered.Count > 0)
    {
        Console.WriteLine("I wasnt null but had 0 records. Line below is safe");
        stockHoldingFiltered = stockHoldingsFiltered.FirstOrDefault();
    }

    //return stockHoldingFiltered;
}

// You can define other methods, fields, classes and namespaces here
public class StockHolding
{
    public int PortfolioId { get; set; }
    public string StockSymbol { get; set; }
    
    public static IReadOnlyList<StockHolding> GetAll()
    {
        List<StockHolding> someList = new List<StockHolding>();
        
        return someList;
        
    }
}

输出:I WASN't NULL, but has no records. Line below will fail.

所以不是 if (stockHoldingsFiltered != null)尝试使用 if(stockHoldingsFiltered.Count > 0)

该警告将消失。