使用 LINQ，其中一个 object 属性的值为 x，我如何在不同的属性中返回该值

Question

I have an object, which I'm trying to query against instantiations of using LINQ. Where one property of an object has a given value, I need to return the value of another property

My constructor is as below:

public Job(string organisationType, string contractingOrganisationType, int levelNumber, string 
jobName, int jobNumberOnLevel, string jobExecutor, int stepCount, int customInputCount, int 
customOutputCount)
{
OrganisationType = organisationType;
ContractingOrganisationType = contractingOrganisationType;
LevelNumber = levelNumber;
JobName = jobName;
JobNumberOnLevel = jobNumberOnLevel;
JobExecutor = jobExecutor;
StepCount = stepCount;
CustomInputCount = customInputCount;
CustomOutputCount = customOutputCount;
}

A couple of mock instances look as below:

List<Job> JobList = new List<Job>();
JobList.Add(new Job("Owner" , null , 0, "ProjectBriefCreation" , 1, "ProjectOwner" , 5, 2, 2));
JobList.Add(new Job("GeneralContractor" , "Owner" , 1, "ProjectManagement" , 1, "ContractsManager" , 
7, 2, 2));
JobList.Add(new Job("DesignContractor" , "Owner" , 1, "DesignManagement" , 2, 
"DesignContractsManager", 7, 2, 2));
JobList.Add(new Job("ArchitecturalPractice" , "DesignContractor" , 2, "BuildingDesign" , 1, 
"LeadArchitect" , 7, 2, 2));
JobList.Add(new Job("StructuralEngineeringPractice", "DesignContractor" , 2, "StructuralDesign" , 2, 
"StructuralEngineer" , 7, 2, 2));
JobList.Add(new Job("Carpentry" , "GeneralContractor", 2, "Drywalling" , 3, "Carpenter" , 5, 2, 2));
JobList.Add(new Job("PlasteringAndPainting" , "GeneralContractor", 2, "Plastering" , 4, "Plasterer" , 
6, 2, 2));
JobList.Add(new Job("PlasteringAndPainting" , "GeneralContractor", 2, "Painting" , 5, "Painter" , 8, 
2, 2));

My query so far looks like this (though not working obviously):

int jobStepNodeCountForJob = JobList.Where(j => j.LevelNumber == 1).Where(j => j.JobNumberOnLevel == 
2).Select(;

Any help would be appreciated

Answer 1

To return eg the StepCount for all Job whose LevelNumber is 1 and JobNumberOnLevel is 2:

IEnumerable<int> jobStepCounts = JobList
  .Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
  .Select(j => j.StepCount);

BEcause there is more than one potential match, you'll get a collection of items out; you can't store multiple of those in an int

If you want multiple properties, you could project a new collection of anonymous types, for example:

var jobStepCountAndJobNames = JobList
  .Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
  .Select(j => new { j.StepCount, j.JobName });

It's easier to var it than try and declare an exact type for these anonymous things.

---

To return the first job StepCount that matches the criteria:

int jobStepCount = JobList
  .FirstOrDefault(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2).StepCount;

Note that this can crash if there is no matching job; it will return a null that then crashes upon access with a null reference exception. You can do something like:

int? jobStepCount = JobList
  .FirstOrDefault(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)?.StepCount;

The ?. before stepcount will not try to access stepcount on the null that is returned if no job matches. It will just immediately return the null and the nullable int? will have no value

---

If you don't want a single result, but instead an average, sum, or other aggregation of all the stepcounts, look at something like:

int jobStepCountAverage = JobList
  .Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
  .Average(j => j.StepCount);

Answer 2

I dunno what property you want to trap, here i suppose you want the enumerate List of JobName:

var jobStepNodeCountForJob = JobList.Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel ==2).Select(j => j.JobName);

Answer 3

You want to both select the chosen property and since it can be many matching nodes choose how to reduce them to a single number. I think this is what you want

int jobStepNodeCountForJob = JobList
    .Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel == 2)
    .Select(j => j.StepCount)
    .Sum();

I'd download LINQpad or use a similar tool to get a feeling for linq.

Answer 4

public void Example()
{
    // Two examples:
    // 1. You want the FIRST value of some int property from your object (CustomInputCount int this example)
    // Output: '2' 
    // Please note: When using FirstOrDefault() if no result is coming from the condition you will get a null value 
    int jobStepNodeCountForJob = JobList.Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel == 2)
    .Select(x => x.CustomInputCount).FirstOrDefault();

    // 2. You want the all values of some int property from your object (CustomInputCount int this example) as a list
    // Output (List<int>) -> {2,2,2,2,2,2}        
    List<int> jobStepNodeCountForJob_List = JobList.Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel == 2)
    .Select(x => x.CustomInputCount).ToList();
}

Answer 5

var jobExecutor= JobList.FirstOrDefault(j => j.LevelNumber == 1 && j.JobNumberOnLevel ==2)?.JobExecutor;

It will find firs Job in the list that fits the condition in brackets and assign it's JobExecutor field to jobExecutor variable or null if such job not found.