使用 Linq 根据条件过滤对象列表

Question

I am solving an issue where I want to remove unwanted objects from a list List<RegionData> as a result of a region name condition . As an entry input I receive var data , which contains a List<CasesDto> . One item CasesDto corresponds to all region-based data received on one particular date .

These are two of my classes that I am using as an example:

public class CasesDto
{    
    public DateTime Date {get; set;}

    public List<RegionData> Region {get; set;}
}

and

public class RegionData
{
    public string RegionName {get; set;}

    public int DailyActiveCases {get; set;}

    public int DeceasedToDate {get; set;}
}

This is the current output example (I only included two dates (= two CasesDto) for easy-to-view purpose) where region is not taken into consideration:

[
    {
        "date": "2021-05-22T00:00:00",
        "region": [
            {
                "regionName": "ce",
                "dailyActiveCases": 615,
                "deceasedToDate": 568
            },
            {
                "regionName": "kk",
                "dailyActiveCases": 170,
                "deceasedToDate": 197
            },
            {
                "regionName": "kp",
                "dailyActiveCases": 278,
                "deceasedToDate": 166
            }
        ]
    },
    {
        "date": "2021-05-23T00:00:00",
        "region": [
            {
                "regionName": "ce",
                "dailyActiveCases": 613,
                "deceasedToDate": 570
            },
            {
                "regionName": "kk",
                "dailyActiveCases": 167,
                "deceasedToDate": 197
            },
            {
                "regionName": "kp",
                "dailyActiveCases": 277,
                "deceasedToDate": 166
            }
        ]
    }
]

This is the output I would want if regionName="ce" is applied:

[
    {
        "date": "2021-05-22T00:00:00",
        "region": [
            {
                "regionName": "ce",
                "dailyActiveCases": 615,
                "deceasedToDate": 568
            }
        ]
    },
    {
        "date": "2021-05-23T00:00:00",
        "region": [
            {
                "regionName": "ce",
                "dailyActiveCases": 613,
                "deceasedToDate": 570
            }
        ]
    }
]

I would like to solve this problem with lambda functions .

My attempt:

data = data.ForEach(x => x.Region.Where(t => t.RegionName == region)).ToList();

However, I receive a mismatch in return type. How can I access RegionData model and select only a region item (based on region identifier) that I want?

Answer 1

So you have an enumerable sequence of CasesDtos (plural noun), where every CasesDto (singular noun) has at least properties Date and Region .

Region is an enumerable sequence of RegionDatas . Every RegionData has several properties, among which a property RegionName .

I think you've described the following requirement:

Requirement: given a string value for regionName , and a sequence of CasesDtos , give me the Date and the RegionData of every CasesDto that has at least one RegionData with a value for property RegionName that equals regionName .

You didn't tell what you want if a CasesDto has two RegionData with the correct RegionName:

string regionName = "CA";
var casesDto = new CasesDto
{
     Date = ...
     Region = new List<RegionData>()
     {
          new RegionData()
          {
              RegionName = "CA",
              ...
          },
          new RegionData()
          {
              RegionName = "CA",
              ...
          }
    }
}

casesDto has two RegionDatas with a matching RegionName. What do you want in your end result: only the Date and one of the RegionDatas? which one? Or do you want all RegionDatas with a matching RegionName?

Anyway, the solution is:

string regionName = "CA";
IEnumerable<CasesDto> casesDtos = ...

// from every CasesDto, make one object, containing the Date, and a property 
// Region, which contains only those RegionDatas that have a value for property 
// RegionName that equals regionName
var result = casesDtos.Select(casesDto => new
{
    Date = casesDto.Date,

    // keep only those Regions that have a matching RegionName:
    Region = casesDto.Region
        .Where(region => region.RegionName == regionName)
        .ToList(),
}

// From this selection, keep only those object that have at least one Region:
.Where(casesDto => casesDto.Region.Any());
             

In words: for every CasesDto in casesDtos, make one new object with two properties: the Date of the CasesDto, and a property Region which contains the RegionDatas of this CasesDto that have a matching RegionName. From this Select result, keep only those object that have at least one element in list Region.

Answer 2

    var cases = new List<CasesDto>{
        new CasesDto{
            Date = DateTime.Now,
            Region = new List<RegionData> {
                new RegionData{
                    RegionName = "CE"
                },
                new RegionData{
                    RegionName = "DE"
                }
            }
        },
        new CasesDto{
            Date = DateTime.Now.AddDays(10),
            Region = new List<RegionData> {             
                new RegionData{
                    RegionName = "DE"
                }
            }
        }
    };
    
    
    var filteredCases = new List<CasesDto>();
    var regionName = "CE";
    cases.ForEach(c => {
        if(c.Region.Any(x => x.RegionName == regionName)){
            c.Region = c.Region.Where(x => x.RegionName == regionName).ToList();
            filteredCases.Add(c);
        }
    });

Somewhat simplified to save me some time. Using ForEach we enumerate over the list and check if we have a region with given regionnames. If so we filter the current list to only contain the correct regions and append this result to a new list.