Linq IEnumerable 去重

去重

1. IEqualityComparer
   实现 IEqualityComparer接口

public class ProductIdComparer : IEqualityComparer<Product>
{
    public bool Equals(Product x, Product y)
    {
        if (x == null)
            return y == null;
        return x.Id == y.Id;
    }


    public int GetHashCode(Product obj)
    {
        if (obj == null)
            return 0;
        return obj.Id.GetHashCode();
    }
}

2. GroupBy
   使用GroupBy方式去重

List<Product> distinctProduct = allProduct
 .GroupBy(p => new {p.Id, p.Name} )
 .Select(g => g.First())
 .ToList();

3. Distinct
   对需要Distinct的字段进行分组，取组内的第一条记录这样结果就是Distinct的数据了。

方法的使用

• 针对ID，和Name进行Distinct

var query = allProduct.DistinctBy(p => new { p.Id, p.Name });

• 仅仅针对ID进行distinct：

var query = allProduct.DistinctBy(p => p.Id);

4. hash 去重

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;
       }
   }
}

Linq groupby

1. 分组合并查询 查询表达式 … group… by…

var groupPlanDetails = from projectPlanDetail in planDetails
                                   group projectPlanDetail by projectPlanDetail.ProjectId;
            var periPlanDetails =groupPlanDetails.Select(t => new PeriPlanDetailInfo(){
                EmpIds = t.FirstOrDefault().EmpIds,
                Patrollers = t.FirstOrDefault().Patrollers,
                PeriStartTime = t.FirstOrDefault().PeriStartTime,
                PeriEndTime = t.FirstOrDefault().PeriEndTime,
                ProjectId = t.FirstOrDefault().ProjectId,
                ProjectName = t.FirstOrDefault().ProjectName,
                Type = t.FirstOrDefault().Type,
                State = t.Where(s => s.State==2).Count()>0?2: (t.Where(s => s.State == 0).Count() == 0 ? 1 : 0),
                Count = t.Sum(s=>s.Count),
                PeriFrequency = t.Sum(s=>s.PeriFrequency),
            });

Linq数据分组官方文档