如何编写 LINQ 查询以仅基于特定属性检索不同的记录？

Question

我有一个对象数组DataTestplans，我尝试使用所示的 LINQ 查询检索特定DataID 和ProductID 的记录，我当前的查询有Distinct()，它区分所有5 个提到的属性,如何根据属性DataID,TestPlanName,TCIndexList 和ProductID 检索不同的记录？

数据测试计划：-

[
    {
    "DataTestPlanID": 0,
    "DataID": 19148,
    "TestPlanName": "string",
    "TCIndexList": "string",
    "ProductID": 2033915
  },
    {
    "DataTestPlanID": 0,
    "DataID": 19148,
    "TestPlanName": "string",
    "TCIndexList": "string",
    "ProductID": 2033915
  },
      {
    "DataTestPlanID": 0,
    "DataID": 19149,
    "TestPlanName": "string",
    "TCIndexList": "string",
    "ProductID": -2642
  }

]

LINQ

            DataTestPlans_DataID_ProductID = DataTestPlans.Where(c => c.DataID == DataID_ProductID_Record.DataID && c.ProductID == DataID_ProductID_Record.ProductID).Distinct();

Answer 1

你可以这样做..

DataTestPlans.Where(c => c.DataID == YourInput && c.ProductID == YourInput)
             .GroupBy(x => new {x.DataID,x.TestPlanName,x.TCIndexList,x.ProductID})
             .Select(x => x.First());

Answer 2

有两种方法可以做，都在this question中突出显示，不需要IComparer。这是一个您可以使用的快速示例（我没有使用您的实际对象，因为这样更容易解释）：

class Program
{
    static void Main(string[] args)
    {
        var persons = Setup();

        //option 1, can stream, option suggested by Jon Skeet
        //https://stackoverflow.com/a/1300116/897326
        var result1 = persons.
            DistinctBy(m => new {m.FirstName, m.LastName});

        //option 2, cannot stream, but does reference to DistinctBy
        //https://stackoverflow.com/a/4158364/897326
        var result2 = persons.
            GroupBy(m => new { m.FirstName, m.LastName }).
            Select(group => group.First());
    }

    class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public string Address { get; set; }
    }

    private static List<Person> Setup()
    {
        var p1 = new Person
        {
            FirstName = "John",
            LastName = "Doe",
            Address = "USA"
        };

        var p2 = new Person
        {
            FirstName = "John",
            LastName = "Doe",
            Address = "Canada"
        };

        var p3 = new Person
        {
            FirstName = "Jane",
            LastName = "Doe",
            Address = "Australia"
        };

        var persons = new List<Person>();
        persons.Add(p1);
        persons.Add(p2);
        persons.Add(p3);

        return persons;
    }
}

public static class LinqExtensions
{
    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        HashSet<TKey> knownKeys = new HashSet<TKey>();
        foreach (TSource element in source)
        {
            if (knownKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }
}

Answer 3

您必须使用GroupBy()，这将创建一个您可以迭代的IEnumerable<IGrouping<TKey, TElement>>。您可以使用 group.First() 或通过组上的某种聚合函数访问记录。

Answer 4

你可以使用多个链式 wheres 。如果您正在进行 db 调用，请确保它是 IQueryable。下面是一个例子

List<SomeClass> c = new List<SomeClass>();
 var result = c.Where(x => x.ID == 4).Distinct().Where(y => y.Name == "foo").Distinct();

Answer 5

您可以实现IEqualityComparer 并在Distinct() 方法中使用默认值。如果您示例实现DataTestPlansComparer，您可以使用如下示例：

DataTestPlans_DataID_ProductID = DataTestPlans.Where(c => c.DataID == DataID_ProductID_Record.DataID && c.ProductID == DataID_ProductID_Record.ProductID).Distinct(new DataTestPlansComparer());

注意，您的自定义比较器应作为参数传递给Distinct() 方法。

你的情况可以是：

        public class DataTestPlanComparer : IEqualityComparer<DataTestPlan>
        {
            public bool Equals(DataTestPlan x, DataTestPlan y)
            {

                if (Object.ReferenceEquals(x, y)) return true;

                if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
                    return false;

                return x.DataID == y.DataID && x.ProductID == y.ProductID;
            }


            public int GetHashCode(DataTestPlan dataTestPlan)
            {
                if (Object.ReferenceEquals(dataTestPlan, null)) return 0;

                int hashDataTestPlanDataID = dataTestPlan.DataID == null ? 0 : dataTestPlan.DataID.GetHashCode();

                int hashDataTestPlanProductID = dataTestPlan.ProductID.GetHashCode();

                return hashDataTestPlanDataID ^ hashDataTestPlanProductID;
            }
        }

请按照MSDN 指南实施IEqualityComparer。