[英]how do I write LINQ query to retrieve distinct records based on only specific properties?
我有一個對象DataTestplans
數組,我嘗試使用所示的LINQ查詢從其中檢索特定DataID
和ProductID
記錄,我當前的查詢具有Distinct()
,它Distinct()
了所有提到的5個屬性,如何根據以下內容檢索不同的記錄屬性DataID
, TestPlanName
, TCIndexList
和ProductID
?
數據測試計划:
[
{
"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();
你可以這樣
DataTestPlans.Where(c => c.DataID == YourInput && c.ProductID == YourInput)
.GroupBy(x => new {x.DataID,x.TestPlanName,x.TCIndexList,x.ProductID})
.Select(x => x.First());
有兩種方法可以完成此問題 , 這兩種方法都不需要IComparer。 這是一個可以使用的快速示例(我沒有使用您的實際對象,因為這樣更容易解釋):
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;
}
}
}
}
您將必須使用GroupBy()
,這將創建一個IEnumerable<IGrouping<TKey, TElement>>
,您可以對其進行迭代。 您可以使用group.First()或通過組上的某種聚合函數來訪問記錄。
您可以使用多個鏈接的wheres。 如果要進行數據庫調用,請確保它是IQueryable。 下面是一個例子
List<SomeClass> c = new List<SomeClass>();
var result = c.Where(x => x.ID == 4).Distinct().Where(y => y.Name == "foo").Distinct();
您可以實現IEqualityComparer
並在Distinct()
方法中使用默認值。 如果您示例實現DataTestPlansComparer
,則可以在以下示例中使用:
DataTestPlans_DataID_ProductID = DataTestPlans.Where(c => c.DataID == DataID_ProductID_Record.DataID && c.ProductID == DataID_ProductID_Record.ProductID).Distinct(new DataTestPlansComparer());
注意,您的自定義比較器應作為參數傳遞給Distinct()
方法。
您的情況可以是:
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;
}
}
請按照MSDN指南實施IEqualityComparer。
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