如何比较同一类的两个对象？ [复制]

Question

我想知道如何像string.Compare() 方法那样比较两个对象（属于同一类）。

有没有办法做到这一点？

Answer 1

可以检查两个相等Reference Equality和Value Equality

引用相等

引用相等意味着两个对象引用引用同一个底层对象。这可以通过简单的赋值来实现，如下例所示。

class Test
{
    public int Num { get; set; }
    public string Str { get; set; }

    static void Main()
    {
        Test a = new Test() { Num = 1, Str = "Hi" };
        Test b = new Test() { Num = 1, Str = "Hi" };

        bool areEqual = System.Object.ReferenceEquals(a, b);
        // Output will be false
        System.Console.WriteLine("ReferenceEquals(a, b) = {0}", areEqual);

        // Assign b to a.
        b = a;

        // Repeat calls with different results.
        areEqual = System.Object.ReferenceEquals(a, b);
        // Output will be true
        System.Console.WriteLine("ReferenceEquals(a, b) = {0}", areEqual);


    }
}

价值平等

值相等意味着两个对象包含相同的值或多个值。对于 int 或 bool 等原始值类型，值相等的测试很简单。

Answer 2

public class BaseEntity
{
    public int Id { get; set; }
}

public class Product : BaseEntity
{
   public string Name { get; set; }
   public decimal Price { get; set; }
}

//Generic Comparer
public class EntityComparer<T> :  IEqualityComparer<T>, IComparer<T> 
where T : BaseEntity
{
    public enum AscDesc : short
    {
        Asc, Desc
    }

    #region Const
    public string PropertyName { get; set; }
    public AscDesc SortType { get; set; }
    #endregion

    #region Ctor
    public EntityComparer(string _propertyname = "Id", AscDesc _sorttype = AscDesc.Asc)
    {
        this.PropertyName = _propertyname;
        this.SortType = _sorttype;
    }
    #endregion

    #region IComparer
    public int Compare(T x, T y)
    {
        if (typeof(T).GetProperty(PropertyName) == null)
            throw new ArgumentNullException(string.Format("{0} does not contain a property with the name \"{1}\"", typeof(T).Name, PropertyName));

        var xValue = (IComparable)x.GetType().GetProperty(this.PropertyName).GetValue(x, null);
        var yValue = (IComparable)y.GetType().GetProperty(this.PropertyName).GetValue(y, null);

        if (this.SortType == AscDesc.Asc)
            return xValue.CompareTo(yValue);

        return yValue.CompareTo(xValue);
    }
    #endregion

    #region IEqualityComparer
    public bool Equals(T x, T y)
    {
        if (typeof(T).GetProperty(PropertyName) == null)
            throw new InvalidOperationException(string.Format("{0} does not contain a property with the name -> \"{1}\"", typeof(T).Name, PropertyName));

        var valuex = x.GetType().GetProperty(PropertyName).GetValue(x, null);
        var valuey = y.GetType().GetProperty(PropertyName).GetValue(y, null);

        if (valuex == null) return valuey == null;

        return valuex.Equals(valuey);
    }

    public int GetHashCode(T obj)
    {
        var info = obj.GetType().GetProperty(PropertyName);
        object value = null;
        if (info != null)
        {
            value = info.GetValue(obj, null);
        }

        return value == null ? 0 : value.GetHashCode();
    }
    #endregion
}

 //Usage
 Product product = new Product();
 Product product2 =new Product();
 EntityComparer<Product> comparer = new EntityComparer<Product>("Propert Name Id Name whatever u want", Asc Or Desc);
 comparer.Compare(product, product2);

Answer 3

因为对象是引用类型，所以你应该使用 obj.Equals() 方法。 另请注意：

string.ReferenceEquals(str, str2);

它显然是比较参考文献。

str.Equals(str2) 

首先尝试比较参考。然后它会尝试按值进行比较。

str == str2

和 Equals 一样。

Answer 4

您需要实现 IComparable 接口。

private class sortYearAscendingHelper : IComparer
{
   int IComparer.Compare(object a, object b)
   {
      car c1=(car)a;
      car c2=(car)b;
      if (c1.year > c2.year)
         return 1;
      if (c1.year < c2.year)
         return -1;
      else
         return 0;
   }
}

原帖可以找到Here

Answer 5

你可以实现IComparable接口，就像建议的here：

public class Temperature : IComparable 
{
    // The temperature value 
    protected double temperatureF;

    public int CompareTo(object obj) {
        if (obj == null) return 1;

        Temperature otherTemperature = obj as Temperature;
        if (otherTemperature != null) 
            return this.temperatureF.CompareTo(otherTemperature.temperatureF);
        else 
           throw new ArgumentException("Object is not a Temperature");
    }
  ...

source

您将拥有CompareTo 方法，该方法将比较您班级的项目。有关IComparable 的更多信息，请访问SO。拥有CompareTo，您可以根据here 中提到的比较函数对对象列表进行排序