.OrderBy() / .OrderByDescending() 和 .FirstOrDefault()/.First()

Question

I have list of results from table named "product".

Id      User    ProductName     ArrivalDate
----    -----   ------------    ------------
1       James   Shoes           05/07/2016
2       Jenny   TShirt          05/01/2018
3       James   Pants           13/05/2017

i would like to sort the result by descending order based on ArrivalDate where User is "James", which mean it should return to me the third row result. However, if i do as below:

return List(spec).OrderByDescending(x=>x.ArrivalDate).FirstOrDefault();

the result i got is still the first one, appereciate if anyone could guide on this. Below is the sample code:

public class EfRepository<T> : IRepository<T>, IAsyncRepository<T> where T : BaseEntity
{
    public T GetSingleBySpec(ISpecification<T> spec)
    {
        return List(spec).FirstOrDefault();
    }
}

public class ProductSpecification : BaseSpecification<NewProducts>
{
    public ProductSpecification(string User)
    : base(b => b.User == User) //User = James
    {

    }
}

public class ProductService : IProductService
{
    public void getOrderProductDetails
    {
        var data = _productRepository.GetSingleBySpec(new ProductSpecification(user));
    }
}

Answer 1

I don't see a filter for the user and you are ordering by user. Try this.

return List(spec).Where(x => x.User == "James")
                 .OrderByDescending(y => y.ArrivalDate)
                 .FirstOrDefault();

Answer 2

如果您只想获取特定用户上次到达时间，您应该向 where 子句 FirstOrDefault 添加额外条件，生成如下查询：

  Select top 1 ....

Answer 3

It seems to me that you are trying to encapsulate some basic Linq functionality using your own methods, but you are only complicating things, at the same time you are making it limited and hard to use, instead of flexible and easy to use.

This is what I would do instead:

public void getOrderProductDetails
{
    var data = _productRepository
        .Where(x => x.User == user)
        .OrderByDescending(x => x.ArrivalDate)
        .FirstOrDefault();
    // process data ...
}

Answer 4

You can also put the "filter" ~inside the FirstOrDefault method.

I show a generic example (both ways) below.

Also note the string-compare for case insensitivity.

ICollection<Employee> employees = (blah blah blah);


Employee firstEmployeeWhereClause = employees.Where(z => z.LastName.Equals("Smith", StringComparison.OrdinalIgnoreCase)).OrderByDescending(x => x.EmployeeSurrogateKey).FirstOrDefault();


Employee secondEmployeeNoWhereClause = employees.OrderByDescending(x => x.EmployeeSurrogateKey).FirstOrDefault(z => z.LastName.Equals("Smith", StringComparison.OrdinalIgnoreCase));