Question

我有一个 MongoDB 数据库，我存储了所有图片，当我检索它们时，我存储了一些双打，这不是很好，但无论如何我想只显示不同的元素。

 @foreach (Foto f in fotos.Distinct(new IEqualityComparer<Foto> { )

但是Foto类有一个名为smallurl的属性，我想通过这个属性只显示不同的元素。那么如何编写自定义 IEqualityComparer 。

Answer 1

var listOfUrls = fotos.Select(f => f.smallurl).Distinct();

编辑专门回答您的问题

从搜索c＃IEqualityComparer http://msdn.microsoft.com/en-us/library/ms132151.aspx

的MSDN文档中实际复制过来

class FotoEqualityComparer : IEqualityComparer<Foto>
{
    public bool Equals(Foto f1, Foto f2)
    {
        return f1.smallurl == f2.smallurl;
    }
    public int GetHashCode(Foto f)
    {
         return f.smallurl.GetHashCode();
    }
}

@foreach (Foto f in fotos.Distinct(new FotoEqualityComparer() )

Answer 2

实际上很容易。只需为您的方法提供一个独特的选择器，如下所示：

    public static IEnumerable<TSource> DistinctBy<TSource, TResult>(this IEnumerable<TSource> enumerable, Func<TSource, TResult> keySelector)
    {
        Dictionary<TResult, TSource> seenItems = new Dictionary<TResult, TSource>();

        foreach (var item in enumerable)
        {
            var key = keySelector(item);

            if (!seenItems.ContainsKey(key))
            {
                seenItems.Add(key, item);
                yield return item;
            }
        }
    }

或者，您可以创建另一个以实现IEquality比较器的通用实现：

    public static IEnumerable<TSource> DistinctBy<TSource>(this IEnumerable<TSource> enumerable, Func<TSource, TSource, bool> equalitySelector, Func<TSource, int> hashCodeSelector)
    {
        return enumerable.Distinct(new GenericEqualitySelector<TSource>(equalitySelector, hashCodeSelector));
    }

    class GenericEqualitySelector<TSource> : IEqualityComparer<TSource>
    {
        public Func<TSource, TSource, bool> _equalityComparer = null;
        public Func<TSource, int> _hashSelector = null;

        public GenericEqualitySelector(Func<TSource, TSource, bool> selector, Func<TSource, int> hashSelector)
        {
            _equalityComparer = selector;
            _hashSelector = hashSelector;
        }

        public bool Equals(TSource x, TSource y)
        {
            return _equalityComparer(x, y);
        }

        public int GetHashCode(TSource obj)
        {
            return _hashSelector(obj);
        }
    }

Answer 3

从MSDN修改

public class MyEqualityComparer : IEqualityComparer<Foto>
{
    public bool Equals(Foto x, Foto y)
    {

        //Check whether the compared objects reference the same data. 
        if (Object.ReferenceEquals(x, y)) return true;

        //Check whether any of the compared objects is null. 
        if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
            return false;

        //Check whether the foto's properties are equal. 
        return x.smallurl == y.smallurl ;
    }

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

    public int GetHashCode(Foto foto)
    {
        //Check whether the object is null 
        if (Object.ReferenceEquals(foto, null)) return 0;

        //Get hash code for the foto.smallurl field if it is not null. 
        return foto.smallurl == null ? 0 : foto.smallurl.GetHashCode();
    }
}

Answer 4

创建自己的：

public class FotoEqualityComparer : IEqualityComparer<Foto>
{
    public bool Equals(Foto x, Foto y)
    {
         return x.smallurl.Equals(y.smallurl);   
    }

    public int GetHashCode(Foto foto)
    {
         return foto.smallurl.GetHashCode();
    }
}

并像这样使用它：

fotos.Distinct(new FotoEqualityComparer());

修改

.Distinct（）没有内联lambda重载，因为当两个对象比较相等时，它们必须具有相同的GetHashCode返回值（否则Distinct内部使用的哈希表将无法正常运行）。

但是如果你想要它在一行中，那么你也可以进行分组以获得相同的结果：

fotos.GroupBy(f => f.smallurl).Select(g => g.First());

Answer 5

使用GroupBy的代码更简单：

@foreach (Foto f in fotos.GroupBy(f => f.smallurl).Select(g => g.First()))

Answer 6

您应该创建自己的EqulityComparer：

    class FotoEqualityComparer : IEqualityComparer<Foto>
    {

    public bool Equals(Foto b1, Foto b2)
    {
            if (b1.smallurl == b2.smallurl)                
                return true;                
            else        
                return false;        
    }


    public int GetHashCode(Foto bx)
    {
            int hCode = bx.smallurl ;
            return hCode.GetHashCode();
    }

    }

如何从我的C＃集合中返回不同的元素？

6 个答案: