如何在 C# 中使用 LINQ 从另一个列表生成唯一的项目列表

Question

我有一个类似 {One,Two,Three,One,Four,One,Five,Two,One} 的项目列表，我需要一个查询来获取该列表并仅基于唯一项目生成一个列表，因此该列表返回将是{一，二，三，四，五}。

Answer 1

使用Distinct运算符：

var unique = list.Distinct();

Answer 2

The Distinct operator. There's an example at MSDN.

http://msdn.microsoft.com/en-us/library/bb348436.aspx

Answer 3

        var x = new string[] {"One", "Two", "Three", "One", "Four", "One", "Five", "Two", "One"}.ToList();
        var distinct = x.Distinct();

Answer 4

It's worth noting Distinct() will use the default means of determining equality, which might not suit you if your list contains complex objects rather than primitives.

There is an overload that allows you to specify an IEqualityComparer for providing custom equality logic.

More details on how Distinct determines if two items are equal: http://msdn.microsoft.com/en-us/library/ms224763.aspx

Answer 5

use distinct

    List<string> l = new List<string>
    {
        "One","Two","Three","One","Four","One","Five","Two","One"
    };

    var rootcategories2 = l.Distinct();

Answer 6

Aside from Distinct, as others have mentioned, you can also use a HashSet:

List<string> distinct = new HashSet<string>(list).ToList();

If you're using LINQ, though, go with Distinct.

Answer 7

Thanks for all your answers, I guess I mis-posed my question slightly. What I really have is a complex class, on which I want the comparison (for Distinct) based on a particular member of the class.

class ComplexClass
{
    public string Name{ get; set; }
    public string DisplayName{ get; set; }
    public int ComplexID{ get; set; }
}

List<ComplexClass> complexClassList = new List<ComplexClass>();

complexClassList.Add(new ComplexClass(){Name="1", DisplayName="One", ComplexID=1});
complexClassList.Add(new ComplexClass(){Name="2", DisplayName="Two", ComplexID=2});
complexClassList.Add(new ComplexClass(){Name="3", DisplayName="One", ComplexID=1});

// This doesn't produce a distinct list, since the comparison is Default
List<ComplexClass) uniqueList = complexClassList.Distinct();

class ComplexClassNameComparer : IEquatable<ComplexClass>
{
    public override bool Equals(ComplexClass x, ComplexClass y)
    {
        return (x.To.DisplayName == y.To.DisplayName);
    }

    public override int GetHashCode(ComplexClass obj)
    {
        return obj.DisplayName.GetHashCode();
    }
}

// This does produce a distinct list, since the comparison is specific
List<ComplexClass) uniqueList = Enumerable.Distinct(complexClassList , new ComplexClassNameComparer());

Answer 8

I had a similar issue. I have a list of importItems with customerName as one of the properties and wanted to generate a list of unique customers from that original list.

I pass the original list to GenCustomers()...using linq I created a unique list of itemCustomers. I then traverse that list to create a new Customer list.

public static List<Customer> GenCustomers(List<ImportItem> importItems)
{
    List<Customer> customers = new List<Customer>();

    var itemCustomers = importItems.Select(o => new { o.CustomerName }).Distinct();

    foreach (var item in itemCustomers)
    {
        customers.Add(new Customer() { CompanyName = item.CustomerName });
    }

    return customers;
}