在C＃中，如何按该对象的多个字段对对象集合进行排序？

Question

I have a list of project objects and I want to sort the collection by using a few different fields in the project object.

IEnumerable<Project> list = GetProjectList();

public class Project
{
      public DateTime? Date;
      public string ImportanceLevel;
}  

I first want to

• Sort by the date field (sooner dates should be at the top of the list versus dates farther out in the future) and those should show up before any item without a date.
• For the items that don't have Dates set them i want to sort by ImportanceLevel (which can be one of 3 strings (Low, Medium or High) where High would go first and then Medium and then Low

I appreciate this would be better if these were integer values but unfortunately they are stored as strings right now).

Answer 1

One option is

var sorted = list.OrderBy(l => l.Date.HasValue ? l.Date : DateTime.MaxValue)
    .ThenBy(l => l.ImportanceLevel == "High" ? 1 : 
                    (l.ImportanceLevel == "Medium" ? 2 : 3));

Here, it will do what you want, also it'll sort the projects with same date, by importance.

Or,

var sorted2 = list.OrderBy(l => l.Date.HasValue ? 
                  int.Parse(l.Date.Value.ToString("yyyyMMdd")) :
                  (l.ImportanceLevel == "High" ? 
                      100000001 :
                      (l.ImportanceLevel == "Medium" ? 100000002 : 100000003)));

Where, it'll not sort the projects which have date, by importance.

Answer 2

//Order by the projects with Date
var projectsWithDate = list.Where(d => d.Date != null)
           .OrderBy(s => s.Date).ToList();

// Projects without Dates

var withoutdate = list.Where(d => d.Date == null)
    .OrderBy(g =>
    {
        if (g.ImportanceLevel == "High")
            return 1;
        if (g.ImportanceLevel == "Medium")
            return 2;
        if (g.ImportanceLevel == "Low")
            return 3;

        return 0;
    })
    .ToList();

//Add the second list to first.
projectsWithDate.AddRange(withoutdate);

// Now you can use projectsWithDate collection.

Answer 3

This will work

var orderedList = list
                .OrderBy(p => p.Date)
                .ThenByDescending(p =>
                {
                    if (p.ImportanceLevel == "Low")
                        return 1;
                    if (p.ImportanceLevel == "Medium")
                        return 2;
                    if (p.ImportanceLevel == "Hight")
                        return 3;
                    return 0;
                });

Answer 4

    public class Project    
    {
        public DateTime? Date;
        public string ImportanceLevel;
    }

    var sortedProejcts =
            projects.OrderByDescending(p => p.Date.HasValue)
            .ThenBy(p => Math.Abs(DateTime.Now.CompareTo(p.Date ?? DateTime.MaxValue)))
            .ThenByDescending(
                p =>
                {
                    switch (p.ImportanceLevel)
                    {
                        case "Low":
                            return 1;
                        case "Medium":
                            return 2;
                        case "High":
                            return 3;
                        default:
                            return 0;
                    }
                });