与 lambda 不同（）？

IEnumerable<Customer> filteredList = originalList
  .GroupBy(customer => customer.CustomerId)
  .Select(group => group.First());

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在我看来，您想从 MoreLINQ 获得 DistinctBy。然后你可以写：

var distinctValues = myCustomerList.DistinctBy(c => c.CustomerId);

这是 DistinctBy 的简化版本（没有无效检查，也没有指定您自己的密钥比较器的选项）：

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
     (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
    HashSet<TKey> knownKeys = new HashSet<TKey>();
    foreach (TSource element in source)
    {
        if (knownKeys.Add(keySelector(element)))
        {
            yield return element;
        }
    }
}

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收拾东西。我认为大多数像我一样来到这里的人都想要最简单的解决方案，而无需使用任何库并具有最佳性能。

（对我来说，我认为按方法接受的分组在性能方面是过度的。）

这是一个使用 IEqualityComparer 接口的简单扩展方法，它也适用于空值。

用法：

var filtered = taskList.DistinctBy(t => t.TaskExternalId).ToArray();

扩展方法代码

public static class LinqExtensions
{
    public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> items, Func<T, TKey> property)
    {
        GeneralPropertyComparer<T, TKey> comparer = new GeneralPropertyComparer<T,TKey>(property);
        return items.Distinct(comparer);
    }   
}
public class GeneralPropertyComparer<T,TKey> : IEqualityComparer<T>
{
    private Func<T, TKey> expr { get; set; }
    public GeneralPropertyComparer (Func<T, TKey> expr)
    {
        this.expr = expr;
    }
    public bool Equals(T left, T right)
    {
        var leftProp = expr.Invoke(left);
        var rightProp = expr.Invoke(right);
        if (leftProp == null && rightProp == null)
            return true;
        else if (leftProp == null ^ rightProp == null)
            return false;
        else
            return leftProp.Equals(rightProp);
    }
    public int GetHashCode(T obj)
    {
        var prop = expr.Invoke(obj);
        return (prop==null)? 0:prop.GetHashCode();
    }
}

---

速记解决方案

myCustomerList.GroupBy(c => c.CustomerId, (key, c) => c.FirstOrDefault());

---

不，没有这样的扩展方法重载。过去我发现这让自己很沮丧，因此我通常会编写一个帮助类来处理这个问题。目标是将 Func<T,T,bool> 转换为 IEqualityComparer<T,T>。

例子

public class EqualityFactory {
  private sealed class Impl<T> : IEqualityComparer<T,T> {
    private Func<T,T,bool> m_del;
    private IEqualityComparer<T> m_comp;
    public Impl(Func<T,T,bool> del) { 
      m_del = del;
      m_comp = EqualityComparer<T>.Default;
    }
    public bool Equals(T left, T right) {
      return m_del(left, right);
    } 
    public int GetHashCode(T value) {
      return m_comp.GetHashCode(value);
    }
  }
  public static IEqualityComparer<T,T> Create<T>(Func<T,T,bool> del) {
    return new Impl<T>(del);
  }
}

这允许您编写以下内容

var distinctValues = myCustomerList
  .Distinct(EqualityFactory.Create((c1, c2) => c1.CustomerId == c2.CustomerId));

---

这是一个简单的扩展方法，可以满足我的需要......

public static class EnumerableExtensions
{
    public static IEnumerable<TKey> Distinct<T, TKey>(this IEnumerable<T> source, Func<T, TKey> selector)
    {
        return source.GroupBy(selector).Select(x => x.Key);
    }
}

很遗憾他们没有在框架中加入这样的独特方法，但是嘿嘿。

---

这会做你想要的，但我不知道性能：

var distinctValues =
    from cust in myCustomerList
    group cust by cust.CustomerId
    into gcust
    select gcust.First();

至少它不冗长。

---

从 .Net 6（Preview 7）或更高版本开始，有一个新的内置方法 Enumerable.DistinctBy 可以实现此目的。

var distinctValues = myCustomerList.DistinctBy(c => c.CustomerId);

// With IEqualityComparer
var distinctValues = myCustomerList.DistinctBy(c => c.CustomerId, someEqualityComparer);

---

我用过的东西对我很有效。

/// <summary>
/// A class to wrap the IEqualityComparer interface into matching functions for simple implementation
/// </summary>
/// <typeparam name="T">The type of object to be compared</typeparam>
public class MyIEqualityComparer<T> : IEqualityComparer<T>
{
    /// <summary>
    /// Create a new comparer based on the given Equals and GetHashCode methods
    /// </summary>
    /// <param name="equals">The method to compute equals of two T instances</param>
    /// <param name="getHashCode">The method to compute a hashcode for a T instance</param>
    public MyIEqualityComparer(Func<T, T, bool> equals, Func<T, int> getHashCode)
    {
        if (equals == null)
            throw new ArgumentNullException("equals", "Equals parameter is required for all MyIEqualityComparer instances");
        EqualsMethod = equals;
        GetHashCodeMethod = getHashCode;
    }
    /// <summary>
    /// Gets the method used to compute equals
    /// </summary>
    public Func<T, T, bool> EqualsMethod { get; private set; }
    /// <summary>
    /// Gets the method used to compute a hash code
    /// </summary>
    public Func<T, int> GetHashCodeMethod { get; private set; }

    bool IEqualityComparer<T>.Equals(T x, T y)
    {
        return EqualsMethod(x, y);
    }

    int IEqualityComparer<T>.GetHashCode(T obj)
    {
        if (GetHashCodeMethod == null)
            return obj.GetHashCode();
        return GetHashCodeMethod(obj);
    }
}

---

我在这里看到的所有解决方案都依赖于选择一个已经相当的字段。但是，如果需要以不同的方式进行比较，this solution here 似乎通常可以工作，例如：

somedoubles.Distinct(new LambdaComparer<double>((x, y) => Math.Abs(x - y) < double.Epsilon)).Count()

---

采取另一种方式：

var distinctValues = myCustomerList.
Select(x => x._myCaustomerProperty).Distinct();

序列返回不同的元素，通过属性 '_myCaustomerProperty' 比较它们。

---

您可以使用 LambdaEqualityComparer：

var distinctValues
    = myCustomerList.Distinct(new LambdaEqualityComparer<OurType>((c1, c2) => c1.CustomerId == c2.CustomerId));


public class LambdaEqualityComparer<T> : IEqualityComparer<T>
    {
        public LambdaEqualityComparer(Func<T, T, bool> equalsFunction)
        {
            _equalsFunction = equalsFunction;
        }

        public bool Equals(T x, T y)
        {
            return _equalsFunction(x, y);
        }

        public int GetHashCode(T obj)
        {
            return obj.GetHashCode();
        }

        private readonly Func<T, T, bool> _equalsFunction;
    }

---

您可以使用 InlineComparer

public class InlineComparer<T> : IEqualityComparer<T>
{
    //private readonly Func<T, T, bool> equalsMethod;
    //private readonly Func<T, int> getHashCodeMethod;
    public Func<T, T, bool> EqualsMethod { get; private set; }
    public Func<T, int> GetHashCodeMethod { get; private set; }

    public InlineComparer(Func<T, T, bool> equals, Func<T, int> hashCode)
    {
        if (equals == null) throw new ArgumentNullException("equals", "Equals parameter is required for all InlineComparer instances");
        EqualsMethod = equals;
        GetHashCodeMethod = hashCode;
    }

    public bool Equals(T x, T y)
    {
        return EqualsMethod(x, y);
    }

    public int GetHashCode(T obj)
    {
        if (GetHashCodeMethod == null) return obj.GetHashCode();
        return GetHashCodeMethod(obj);
    }
}

使用示例：

  var comparer = new InlineComparer<DetalleLog>((i1, i2) => i1.PeticionEV == i2.PeticionEV && i1.Etiqueta == i2.Etiqueta, i => i.PeticionEV.GetHashCode() + i.Etiqueta.GetHashCode());
  var peticionesEV = listaLogs.Distinct(comparer).ToList();
  Assert.IsNotNull(peticionesEV);
  Assert.AreNotEqual(0, peticionesEV.Count);

来源：https://stackoverflow.com/a/5969691/206730
Using IEqualityComparer for Union
Can I specify my explicit type comparator inline?

---

如果 Distinct() 没有产生独特的结果，试试这个：

var filteredWC = tblWorkCenter.GroupBy(cc => cc.WCID_I).Select(grp => grp.First()).Select(cc => new Model.WorkCenter { WCID = cc.WCID_I }).OrderBy(cc => cc.WCID); 

ObservableCollection<Model.WorkCenter> WorkCenter = new ObservableCollection<Model.WorkCenter>(filteredWC);

---

一个棘手的方法是使用 Aggregate() 扩展，使用字典作为累加器，key-property 值作为键：

var customers = new List<Customer>();

var distincts = customers.Aggregate(new Dictionary<int, Customer>(), 
                                    (d, e) => { d[e.CustomerId] = e; return d; },
                                    d => d.Values);

GroupBy-style 解决方案正在使用 ToLookup()：

var distincts = customers.ToLookup(c => c.CustomerId).Select(g => g.First());

---

IEnumerable lambda 扩展：

public static class ListExtensions
{        
    public static IEnumerable<T> Distinct<T>(this IEnumerable<T> list, Func<T, int> hashCode)
    {
        Dictionary<int, T> hashCodeDic = new Dictionary<int, T>();

        list.ToList().ForEach(t => 
            {   
                var key = hashCode(t);
                if (!hashCodeDic.ContainsKey(key))
                    hashCodeDic.Add(key, t);
            });

        return hashCodeDic.Select(kvp => kvp.Value);
    }
}

用法：

class Employee
{
    public string Name { get; set; }
    public int EmployeeID { get; set; }
}

//Add 5 employees to List
List<Employee> lst = new List<Employee>();

Employee e = new Employee { Name = "Shantanu", EmployeeID = 123456 };
lst.Add(e);
lst.Add(e);

Employee e1 = new Employee { Name = "Adam Warren", EmployeeID = 823456 };
lst.Add(e1);
//Add a space in the Name
Employee e2 = new Employee { Name = "Adam  Warren", EmployeeID = 823456 };
lst.Add(e2);
//Name is different case
Employee e3 = new Employee { Name = "adam warren", EmployeeID = 823456 };
lst.Add(e3);            

//Distinct (without IEqalityComparer<T>) - Returns 4 employees
var lstDistinct1 = lst.Distinct();

//Lambda Extension - Return 2 employees
var lstDistinct = lst.Distinct(employee => employee.EmployeeID.GetHashCode() ^ employee.Name.ToUpper().Replace(" ", "").GetHashCode()); 

---

我假设您有一个 IEnumerable，并且在您的示例委托中，您希望 c1 和 c2 引用此列表中的两个元素？

我相信您可以通过 self join var distinctResults = from c1 in myList join c2 in myList on 来实现这一点

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Microsoft System.Interactive package 具有采用键选择器 lambda 的 Distinct 版本。这实际上与 Jon Skeet 的解决方案相同，但它可能有助于人们了解并查看图书馆的其余部分。

---

以下是您的操作方法：

public static class Extensions
{
    public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
                                                    Func<T, V> f, 
                                                    Func<IGrouping<V,T>,T> h=null)
    {
        if (h==null) h=(x => x.First());
        return query.GroupBy(f).Select(h);
    }
}

此方法允许您通过指定一个参数（如 .MyDistinct(d => d.Name)）来使用它，但它也允许您指定一个有条件作为第二个参数，如下所示：

var myQuery = (from x in _myObject select x).MyDistinct(d => d.Name,
        x => x.FirstOrDefault(y=>y.Name.Contains("1") || y.Name.Contains("2"))
        );

NB 这也允许您指定其他函数，例如 .LastOrDefault(...)。

如果您只想公开条件，则可以通过将其实现为更简单：

public static IEnumerable<T> MyDistinct2<T, V>(this IEnumerable<T> query,
                                                Func<T, V> f,
                                                Func<T,bool> h=null
                                                )
{
    if (h == null) h = (y => true);
    return query.GroupBy(f).Select(x=>x.FirstOrDefault(h));
}

在这种情况下，查询将如下所示：

var myQuery2 = (from x in _myObject select x).MyDistinct2(d => d.Name,
                    y => y.Name.Contains("1") || y.Name.Contains("2")
                    );

NB 在这里，表达式更简单，但请注意 .MyDistinct2 隐式使用 .FirstOrDefault(...)。

注意：上面的例子是使用下面的演示类

class MyObject
{
    public string Name;
    public string Code;
}

private MyObject[] _myObject = {
    new MyObject() { Name = "Test1", Code = "T"},
    new MyObject() { Name = "Test2", Code = "Q"},
    new MyObject() { Name = "Test2", Code = "T"},
    new MyObject() { Name = "Test5", Code = "Q"}
};

---

我发现这是最简单的解决方案。

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
        (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        return source.GroupBy(keySelector).Select(x => x.FirstOrDefault());
    }

---