C# 撰寫擴展CLASS - DistinctBy

C# 撰寫擴展CLASS - DistinctBy

void Main()
{
    List<ClassA> list = new List<UserQuery.ClassA>();
    list.Add(new ClassA() { strA = "A", strB = "A1" });
    list.Add(new ClassA() { strA = "B", strB = "B1" });
    list.Add(new ClassA() { strA = "B", strB = "C1" });

    var a = list.DistinctBy(x => x.strA).ToList();
    // strA , strB
    // A, A1
    // B, B1
    
    var b = list.DistinctBy(x => x.strB).ToList();
    // strA , strB
    // A, A1
    // B, B1
    // B, C1
	
}

public class ClassA
{
	public string strA {get;set;}
	public string strB {get;set;}
}

//撰寫一個擴展CLASS
public static class DistinctByClass
{
	//方法套用在列舉上
	public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
	{
		HashSet<TKey> seenKeys = new HashSet<TKey>();
		foreach (TSource element in source)
		{
			if (seenKeys.Add(keySelector(element)))
			{
				yield return element;
			}
		}
	}
}

DistinctBy可以直接使用在 list 上, 是因為 DistinctByClass 是一個靜態的擴展CLASS, 由方法DinstinctBy來看, 可以接受 IEnumerable< TSource > 。