Distinct（）如何在对象列表中查找唯一元素

Question

There is a very simple class:

public class LinkInformation
{
    public LinkInformation(string link, string text, string group)
    {
        this.Link = link;
        this.Text = text;
        this.Group = group;
    }

    public string Link { get; set; }
    public string Text { get; set; }
    public string Group { get; set; }

    public override string ToString()
    {
        return Link.PadRight(70) + Text.PadRight(40) + Group;
    }
}

And I create a list of objects of this class, containing multiple duplicates.

So, I tried using Distinct() to get a list of unique values.

But it does not work, so I implemented

IComparable<LinkInformation>

    int IComparable<LinkInformation>.CompareTo(LinkInformation other)
    {
        return this.ToString().CompareTo(other.ToString());
    }

and then...

IEqualityComparer<LinkInformation>

    public bool Equals(LinkInformation x, LinkInformation y)
    {
        return x.ToString().CompareTo(y.ToString()) == 0;
    }

    public int GetHashCode(LinkInformation obj)
    {
        int hash = 17;
        // Suitable nullity checks etc, of course :)
        hash = hash * 23 + obj.Link.GetHashCode();
        hash = hash * 23 + obj.Text.GetHashCode();
        hash = hash * 23 + obj.Group.GetHashCode();
        return hash;
    }

The code using the Distinct is:

    static void Main(string[] args)
    {
        string[] filePath = {   @"C:\temp\html\1.html",
                                @"C:\temp\html\2.html",
                                @"C:\temp\html\3.html",
                                @"C:\temp\html\4.html",
                                @"C:\temp\html\5.html"};

        int index = 0;

        foreach (var path in filePath)
        {
            var parser = new HtmlParser();

            var list = parser.Parse(path);

            var unique = list.Distinct();

            foreach (var elem in unique)
            {
                var full = new FileInfo(path).Name;
                var file = full.Substring(0, full.Length - 5);
                Console.WriteLine((++index).ToString().PadRight(5) + file.PadRight(20) + elem);
            }
        }

        Console.ReadKey();
    }

What has to be done to get Distinct() working?

Answer 1

You need to actually pass the IEqualityComparer that you've created to Disctinct when you call it. It has two overloads, one accepting no parameters and one accepting an IEqualityComparer . If you don't provide a comparer the default is used, and the default comparer doesn't compare the objects as you want them to be compared.

Answer 2

If you want to return distinct elements from sequences of objects of some custom data type, you have to implement the IEquatable generic interface in the class.

here is a sample implementation:

public class Product : IEquatable<Product>
{
    public string Name { get; set; }
    public int Code { get; set; }

    public bool Equals(Product other)
    {

        //Check whether the compared object is null.
        if (Object.ReferenceEquals(other, null)) return false;

        //Check whether the compared object references the same data.
        if (Object.ReferenceEquals(this, other)) return true;

        //Check whether the products' properties are equal.
        return Code.Equals(other.Code) && Name.Equals(other.Name);
    }

    // If Equals() returns true for a pair of objects 
    // then GetHashCode() must return the same value for these objects.

    public override int GetHashCode()
    {

        //Get hash code for the Name field if it is not null.
        int hashProductName = Name == null ? 0 : Name.GetHashCode();

        //Get hash code for the Code field.
        int hashProductCode = Code.GetHashCode();

        //Calculate the hash code for the product.
        return hashProductName ^ hashProductCode;
    }
}

And this is how you do the actual distinct:

Product[] products = { new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "orange", Code = 4 }, 
                       new Product { Name = "apple", Code = 9 }, 
                       new Product { Name = "lemon", Code = 12 } };

//Exclude duplicates.

IEnumerable<Product> noduplicates =
    products.Distinct();

Answer 3

If you are happy with defining the "distinctness" by a single property, you can do

list
    .GroupBy(x => x.Text)
    .Select(x => x.First())

to get a list of "unique" items.

No need to mess around with IEqualityComparer et al.

Answer 4

Without using Distinct nor the comparer, how about:

list.GroupBy(x => x.ToString()).Select(x => x.First())

I know this solution is not the answer for the exact question, but I think is valid to be open for other solutions.