如何编写LINQ查询以仅基于特定属性检索不同的记录？

Question

I've an array of objects DataTestplans from which I try to retrieve records for a particular DataID and ProductID using the LINQ query shown, my current query has Distinct() which distiguishes on all 5 mentioned properties,how do I retrieve distinct records based on properties DataID , TestPlanName , TCIndexList and ProductID ?

DataTestplans:-

[
    {
    "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

You could do like this..

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

There are two ways to do, both highlighted in this question , no need for IComparer. Here is a quick example you can play with (I did not use your actual object, because it's easier to explain this way):

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

You would have to use GroupBy() , which will create an IEnumerable<IGrouping<TKey, TElement>> which you can iterate over. You can access records either with group.First() or by some sort of aggregate function over the group.

Answer 4

You can use multiple chained wheres . Make sure you it is IQueryable if you are making db call. An example is below

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

Answer 5

You can implement IEqualityComparer and use that instead default in Distinct() method. If you example implement DataTestPlansComparer you can use like in following example:

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

Note, your custom comparer should be passed as parameter to Distinct() method.

In your case can be:

        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;
            }
        }

Please follow MSDN guide to implement IEqualityComparer.