使用LinQ从列表中获取不同的列表对象

Question

I am having list of Item.

class Item{
      public int Year { get; set; }
      public int QuarterIndex { get; set; }
}

How to convert List to distinct List?

Source:

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}
};

Result:

target = new List<Item>(){
 new Item(){ Year = 2013, QuarterIndex = 1},
 new Item(){ Year = 2013, QuarterIndex = 2},
 new Item(){ Year = 2013, QuarterIndex = 3}
};

Answer 1

Here's a simple but potentially less efficient way which works without modifying the class itself:

items = items.GroupBy(i => new { i.Year, i.QuarterIndex })
    .Select(g => g.First())
    .ToList();

Another way is to implement a custom IEqualityComparer<Item> which you can use for Distinct (and other methods in Enumerable class):

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;
        }
    }
}

Now this works:

items = items.Distinct(new ItemComparer()).ToList();

If you can/want to modify the original class you can override 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;
        }
    }
}

Then Distinct works "automagically":

items = items.Distinct().ToList();

Answer 2

This code helps you,

objlist.Where(w => w.ColumnName != "ColumnValue").GroupBy(g => new { g.Value, g.Name }).
                 Select(s=> new ClassName(s.Key.Value, s.Key.Name)).ToList()

Happy Coding :)