[英]Using LINQ, where one object property has a value of x, how do I return the value in a different property
我有一个 object,我试图查询使用 LINQ 的实例。 如果 object 的一个属性具有给定值,我需要返回另一个属性的值
我的构造函数如下:
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;
}
几个模拟实例如下所示:
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));
到目前为止,我的查询看起来像这样(尽管显然不起作用):
int jobStepNodeCountForJob = JobList.Where(j => j.LevelNumber == 1).Where(j => j.JobNumberOnLevel ==
2).Select(;
任何帮助,将不胜感激
例如,返回StepCount
为 1 且 JobNumberOnLevel 为 2 的所有 Job 的 StepCount:
IEnumerable<int> jobStepCounts = JobList
.Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
.Select(j => j.StepCount);
因为有不止一个潜在的匹配,你会得到一组物品; 你不能在一个 int 中存储多个
如果你想要多个属性,你可以投影一个新的匿名类型集合,例如:
var jobStepCountAndJobNames = JobList
.Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
.Select(j => new { j.StepCount, j.JobName });
对它进行 var 比尝试为这些匿名事物声明一个确切的类型更容易。
要返回符合条件的第一个作业 StepCount:
int jobStepCount = JobList
.FirstOrDefault(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2).StepCount;
请注意,如果没有匹配的作业,这可能会崩溃; 它将返回一个 null,然后在访问时崩溃,并出现 null 引用异常。 您可以执行以下操作:
int? jobStepCount = JobList
.FirstOrDefault(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)?.StepCount;
?.
如果没有作业匹配,则在 stepcount 之前不会尝试访问 null 上的 stepcount。 它会立即返回 null 和可为空的int?
将没有价值
如果您不想要单个结果,而是想要所有步数的平均值、总和或其他聚合,请查看以下内容:
int jobStepCountAverage = JobList
.Where(j => j.LevelNumber == 1 && j => j.JobNumberOnLevel == 2)
.Average(j => j.StepCount);
我不知道你想捕获什么属性,在这里我想你想要 JobName 的枚举列表:
var jobStepNodeCountForJob = JobList.Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel ==2).Select(j => j.JobName);
您想要 select 选择的属性,因为它可以有许多匹配节点选择如何将它们减少到一个数字。 我想这就是你想要的
int jobStepNodeCountForJob = JobList
.Where(j => j.LevelNumber == 1 && j.JobNumberOnLevel == 2)
.Select(j => j.StepCount)
.Sum();
我会下载 LINQpad 或使用类似的工具来感受 linq。
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();
}
var jobExecutor= JobList.FirstOrDefault(j => j.LevelNumber == 1 && j.JobNumberOnLevel ==2)?.JobExecutor;
它将在列表中找到符合括号中条件的第一个作业,并将其JobExecutor
字段分配给jobExecutor
变量或 null 如果找不到此类作业。
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