"Object reference not set to an object instance“on predicate

Question

我正在尝试查找具有或包含某些内容的字段的客户端。如果我硬编码谓词，一切都会正常，但当我传递谓词时，它会抛出一个NullReferenceException。

回购代码：

public IEnumerable<Contractor> Find(Func<Contractor, bool> predicate)
{
    var test = db.Contractors.Where(x => x.NIP.Contains("7822574676")).ToList(); //this is correct
    try
    {
        return db.Contractors.Where(predicate).ToList(); // this gives exception
    }
    catch (Exception ex)
    {
        return null;
    }  
}

服务代码：

public IEnumerable<ContractorShortDataDTO> FindByNIP(string NIP)
{
    try {
        return Database.Contractors.Find(x => x.NIP.Contains(NIP)).Select(x =>
            new ContractorShortDataDTO()
            {
                NIP = x.NIP,
                CompanyName = x.CompanyName,
                ID = x.ID
            }).AsEnumerable();
    } catch(Exception ex) {
            return null;
    }
}

这段代码有什么问题？

Answer 1

将方法中的参数类型从

Func<Contractor, bool> predicate

至

Expression<Func<Contractor, bool>> predicate

另请参阅相关问题Why would you use Expression rather than Func?