也许您应该考虑使用HashSet。

从MSDN链接:

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        HashSet<int> evenNumbers = new HashSet<int>();
        HashSet<int> oddNumbers = new HashSet<int>();

        for (int i = 0; i < 5; i++)
        {
            // Populate numbers with just even numbers.
            evenNumbers.Add(i * 2);

            // Populate oddNumbers with just odd numbers.
            oddNumbers.Add((i * 2) + 1);
        }

        Console.Write("evenNumbers contains {0} elements: ", evenNumbers.Count);
        DisplaySet(evenNumbers);

        Console.Write("oddNumbers contains {0} elements: ", oddNumbers.Count);
        DisplaySet(oddNumbers);

        // Create a new HashSet populated with even numbers.
        HashSet<int> numbers = new HashSet<int>(evenNumbers);
        Console.WriteLine("numbers UnionWith oddNumbers...");
        numbers.UnionWith(oddNumbers);

        Console.Write("numbers contains {0} elements: ", numbers.Count);
        DisplaySet(numbers);
    }

    private static void DisplaySet(HashSet<int> set)
    {
        Console.Write("{");
        foreach (int i in set)
        {
            Console.Write(" {0}", i);
        }
        Console.WriteLine(" }");
    }
}

/* This example produces output similar to the following:
 * evenNumbers contains 5 elements: { 0 2 4 6 8 }
 * oddNumbers contains 5 elements: { 1 3 5 7 9 }
 * numbers UnionWith oddNumbers...
 * numbers contains 10 elements: { 0 2 4 6 8 1 3 5 7 9 }
 */

使用HashSet可以很容易地做到这一点。

List<int> listWithDuplicates = new List<int> { 1, 2, 1, 2, 3, 4, 5 };
HashSet<int> hashWithoutDuplicates = new HashSet<int> ( listWithDuplicates );
List<int> listWithoutDuplicates = hashWithoutDuplicates.ToList();

在Java中(我认为c#或多或少是相同的):

list = new ArrayList<T>(new HashSet<T>(list))

如果你真的想改变原来的列表:

List<T> noDupes = new ArrayList<T>(new HashSet<T>(list));
list.clear();
list.addAll(noDupes);

为了保持顺序，只需将HashSet替换为LinkedHashSet。

我喜欢使用这个命令:

List<Store> myStoreList = Service.GetStoreListbyProvince(provinceId)
                                                 .GroupBy(s => s.City)
                                                 .Select(grp => grp.FirstOrDefault())
                                                 .OrderBy(s => s.City)
                                                 .ToList();

我的列表中有这些字段:Id、StoreName、City、PostalCode 我想在一个有重复值的下拉列表中显示城市。 解决方案:按城市分组，然后选择列表中的第一个城市。

如果你有两个类Product和Customer，我们想从它们的列表中删除重复的项

public class Product
{
    public int Id { get; set; }
    public string ProductName { get; set; }
}

public class Customer
{
    public int Id { get; set; }
    public string CustomerName { get; set; }

}

您必须在下面的表单中定义一个泛型类

public class ItemEqualityComparer<T> : IEqualityComparer<T> where T : class
{
    private readonly PropertyInfo _propertyInfo;

    public ItemEqualityComparer(string keyItem)
    {
        _propertyInfo = typeof(T).GetProperty(keyItem, BindingFlags.GetProperty | BindingFlags.Instance | BindingFlags.Public);
    }

    public bool Equals(T x, T y)
    {
        var xValue = _propertyInfo?.GetValue(x, null);
        var yValue = _propertyInfo?.GetValue(y, null);
        return xValue != null && yValue != null && xValue.Equals(yValue);
    }

    public int GetHashCode(T obj)
    {
        var propertyValue = _propertyInfo.GetValue(obj, null);
        return propertyValue == null ? 0 : propertyValue.GetHashCode();
    }
}

然后，你可以删除列表中重复的项目。

var products = new List<Product>
            {
                new Product{ProductName = "product 1" ,Id = 1,},
                new Product{ProductName = "product 2" ,Id = 2,},
                new Product{ProductName = "product 2" ,Id = 4,},
                new Product{ProductName = "product 2" ,Id = 4,},
            };
var productList = products.Distinct(new ItemEqualityComparer<Product>(nameof(Product.Id))).ToList();

var customers = new List<Customer>
            {
                new Customer{CustomerName = "Customer 1" ,Id = 5,},
                new Customer{CustomerName = "Customer 2" ,Id = 5,},
                new Customer{CustomerName = "Customer 2" ,Id = 5,},
                new Customer{CustomerName = "Customer 2" ,Id = 5,},
            };
var customerList = customers.Distinct(new ItemEqualityComparer<Customer>(nameof(Customer.Id))).ToList();

这段代码通过Id删除重复项，如果你想通过其他属性删除重复项，你可以更改名称(YourClass.DuplicateProperty)和名称(Customer.CustomerName)，然后通过CustomerName属性删除重复项。

我认为最简单的方法是:

创建一个新列表并添加唯一的项目。

例子:

        class MyList{
    int id;
    string date;
    string email;
    }
    
    List<MyList> ml = new Mylist();

ml.Add(new MyList(){
id = 1;
date = "2020/09/06";
email = "zarezadeh@gmailcom"
});

ml.Add(new MyList(){
id = 2;
date = "2020/09/01";
email = "zarezadeh@gmailcom"
});

 List<MyList> New_ml = new Mylist();

foreach (var item in ml)
                {
                    if (New_ml.Where(w => w.email == item.email).SingleOrDefault() == null)
                    {
                        New_ml.Add(new MyList()
                        {
                          id = item.id,
     date = item.date,
               email = item.email
                        });
                    }
                }