Question

我遇到了Except（）方法的麻烦。它不是返回差异，而是返回原始集合。

我已尝试在Account类中实现IEquatable和IEqualityComparer。我也尝试为Account创建一个单独的IEqualityComparer类。

当从main调用Except（）方法时，它似乎不会调用我的自定义Equals（）方法，但是当我尝试使用Count（）时，它确实调用了自定义的GetHashCode（）方法！

我确定我在某个地方犯了一个微不足道的错误，我希望一双新眼睛可以帮助我。

主：

IEnumerable<Account> everyPartnerID = 
    from partner in dataContext.Partners
    select new Account { IDPartner = partner.ID, Name = partner.Name };


IEnumerable<Account> hasAccountPartnerID = 
    from partner in dataContext.Partners
    from account in dataContext.Accounts
    where
        !partner.ID.Equals(Guid.Empty) &&
        account.IDPartner.Equals(partner.ID) &&
        account.Username.Equals("Special")
    select new Account { IDPartner = partner.ID, Name = partner.Name };

IEnumerable<Account> noAccountPartnerID = 
    everyPartnerID.Except(
        hasAccountPartnerID, 
        new LambdaComparer<Account>((x, y) => x.IDPartner.Equals(y.IDPartner)));

帐户：

    public class Account : IEquatable<Account>
    {
        public Guid IDPartner{ get; set; }
        public string Name{ get; set; }

/*        #region IEquatable<Account> Members

        public bool Equals(Account other)
        {
            return this.IDPartner.Equals(other.IDPartner);
        }

        #endregion*/
    }

LambdaComparer：

   public class LambdaComparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, T, bool> _lambdaComparer;
        private readonly Func<T, int> _lambdaHash;

        public LambdaComparer(Func<T, T, bool> lambdaComparer) :
            this(lambdaComparer, o => o.GetHashCode())
        {
        }

        public LambdaComparer(Func<T, T, bool> lambdaComparer, Func<T, int> lambdaHash)
        {
            if (lambdaComparer == null)
                throw new ArgumentNullException("lambdaComparer");
            if (lambdaHash == null)
                throw new ArgumentNullException("lambdaHash");

            _lambdaComparer = lambdaComparer;
            _lambdaHash = lambdaHash;
        }

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

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

Answer 1

基本上，当您传入一个函数时，您的LambdaComparer类就会被破坏，因为如果您不提供任何其他功能，它将使用“身份哈希码”提供程序。哈希代码由Except使用，这就是造成问题的原因。

这里有三个选项：

实施您自己的ExceptBy方法，然后最好将其贡献给包含此类内容的MoreLINQ。
使用IEqualityComparer<T>的其他实现。我有一个ProjectionEqualityComparer课程，您可以在MiscUtil中使用 - 或者您可以使用代码as posted in another question。

将lambda表达式传递到您的LambdaComparer代码中以用于哈希：

new LambdaComparer<Account>((x, y) => x.IDPartner.Equals(y.IDPartner)),
                            x => x.IDPartner.GetHashCode());

Answer 2

当只提供如下所示的相等参数时，您还可以快速修复LambdaComparer：

    public LambdaComparer(Func<T, T, bool> lambdaComparer) :
        this(lambdaComparer, o => 1)
    {
    }

Answer 3

请看这里，如何通过linq.Except及其他方式使用和实现IEqualityComparer。

https://www.dreamincode.net/forums/topic/352582-linq-by-example-3-methods-using-iequalitycomparer/

public class Department {
public string Code { get; set; }
public string Name { get; set; }

}

公共类DepartmentComparer：IEqualityComparer {

// equal if their Codes are equal
public bool Equals(Department x, Department y) {
    // reference the same objects?
    if (Object.ReferenceEquals(x, y)) return true;

    // is either null?
    if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
        return false;

    return x.Code == y.Code;
}

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

    // if null default to 0
    if (Object.ReferenceEquals(dept, null)) return 0;

    return dept.Code.GetHashCode();
}

}

IEnumerable<Department> deptExcept = departments.Except(departments2, 
    new DepartmentComparer());

foreach (Department dept in deptExcept) {
    Console.WriteLine("{0} {1}", dept.Code, dept.Name);
}
// departments not in departments2: AC, Accounts.

IEnumerable.Except（）和自定义比较器

3 个答案: