kevinya

下列的linq查詢結果，可以透過IEqualityComparer 的比較性質類別，來排除重複的資料
可以做到像是tsql一樣直接下distinct排除重複的資料的效果

var DataTableQuery = (from c in db.tblCase.AsEnumerable()
					  join h in db.mytable on c.CaseId equals h.CaseId
					  join de in db.mytableDtl on h.HitId equals de.HitId
					                                  
					  select new MyDataModel
					  {
						  CreatedTime = h.CreateTime.ToString("yyyy-MM-dd"),
						  Idno = de.Idno,						  
					  }                                  
					  ).Distinct(new MyDataComparer());


public class MyDataModel
{
	public string Idno { get; set; } 
	[Display(Name = "日期")]
	public string CreatedTime { get; set; }
}

實做一個 IEqualityComparer 類別的細節如下

public class MyDataComparer : IEqualityComparer<MyDataModel>
{
	public bool Equals(MyDataModel t1, MyDataModel t2)
	{
		return (t1.CreatedTime == t2.CreatedTime && t1.Idno == t2.Idno);
	}

	public int GetHashCode(MyDataModel t)
	{
		return t.ToString().GetHashCode();
	}
}					  

補充方法二:
按照下面方式也可以達成distinct的效果，雖然不如上面的方法的高度客製化，但是在實務上是簡單快速方便使用
達成distinct的方式其實就是利用group by 語法

var DataTableQuery = from c in db.tblCase.AsEnumerable()
					  join h in db.mytable on c.CaseId equals h.CaseId
					  join de in db.mytableDtl on h.HitId equals de.HitId
					                                  
					  select new MyDataModel
					  {
						  CreatedTime = h.CreateTime.ToString("yyyy-MM-dd"),
						  Idno = de.Idno,						  
					  };
					  
DataTableQuery = DataTableQuery.GroupBy(g => new { g.CreatedTime, g.Idno })
                .Select(p => new MyDataModel
                {                    
                    CreatedTime = p.Key.CreatedTime,
                    Idno = p.Key.Idno,                    
                    
                });					  
				
public class MyDataModel
{
	public string Idno { get; set; } 
	[Display(Name = "日期")]
	public string CreatedTime { get; set; }
}				

參考資料：
[C#]Linq在使用Distinct去除重複資料時如何指定所要依據的成員屬性
https://dotblogs.com.tw/larrynung/2012/09/18/74901

LINQ - DISTINCT的使用
https://blog.xuite.net/f8789/DCLoveEP/23587655-LINQ+-+DISTINCT%E7%9A%84%E4%BD%BF%E7%94%A8