[英]Get distinct list objects from list using LinQ
我有物品清單。
class Item{
public int Year { get; set; }
public int QuarterIndex { get; set; }
}
如何將列表轉換為不同的列表?
資源:
List<Item> items = new List<Item>(){
new Item(){ Year = 2013, QuarterIndex = 1},
new Item(){ Year = 2013, QuarterIndex = 2},
new Item(){ Year = 2013, QuarterIndex = 3},
new Item(){ Year = 2013, QuarterIndex = 1}
};
結果:
target = new List<Item>(){
new Item(){ Year = 2013, QuarterIndex = 1},
new Item(){ Year = 2013, QuarterIndex = 2},
new Item(){ Year = 2013, QuarterIndex = 3}
};
這是一種簡單但可能效率較低的方法,無需修改類本身即可工作:
items = items.GroupBy(i => new { i.Year, i.QuarterIndex })
.Select(g => g.First())
.ToList();
另一種方法是實現可用於Distinct
(以及Enumerable
類中的其他方法)的自定義IEqualityComparer<Item>
:
public class ItemComparer : IEqualityComparer<Item>
{
public bool Equals(Item lhs, Item rhs)
{
if(lhs == null || rhs == null) return false;
return lhs.Year == rhs.Year && lhs.QuarterIndex == rhs.QuarterIndex;
}
public int GetHashCode(Item item)
{
if(item == null) return 0;
unchecked
{
int hash = 23;
hash = (hash * 31) + item.Year;
hash = (hash * 31) + item.QuarterIndex;
return hash;
}
}
}
現在這有效:
items = items.Distinct(new ItemComparer()).ToList();
如果您可以/想要修改原始類,則可以重寫Equals
+ GetHashCode
:
public class Item
{
public int Year { get; set; }
public int QuarterIndex { get; set; }
public override bool Equals(object otherItem)
{
Item other = otherItem as Item;
if (other == null) return false;
return this.Equals(other);
}
public bool Equals(Item otherItem)
{
if(otherItem == null) return false;
return Year == otherItem.Year && QuarterIndex == otherItem.QuarterIndex;
}
public override int GetHashCode()
{
unchecked
{
int hash = 23;
hash = (hash * 31) + Year;
hash = (hash * 31) + QuarterIndex;
return hash;
}
}
}
然后Distinct
可以“自動”工作:
items = items.Distinct().ToList();
此代碼可以幫助您,
objlist.Where(w => w.ColumnName != "ColumnValue").GroupBy(g => new { g.Value, g.Name }).
Select(s=> new ClassName(s.Key.Value, s.Key.Name)).ToList()
快樂編碼:)
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