IComparable CompareTo()，如何比较多个对象属性？

Question

我正在尝试比较对象的多个属性，但我的代码只能比较 degree 属性。在 Visual Studio 中调试时，我的代码似乎完全缺少 else 语句。如果有任何提示，我将不胜感激。

class Student : IComparable 
{
    private string fName;
    private string lName;
    private string deg;
    private int gra;

    public Student(string firstName, string lastName, string degree, int grade)
    {
        fName = firstName;
        lName = lastName;
        deg = degree;
        gra = grade;
    }

    public override string ToString()
    {
        string var = lName + ", " + fName + " (" + deg + ") Grade: " + gra;
        return var;
    }

    public int CompareTo(object obj)
    {
        Student newStudent = obj as Student;

        if (this.deg.CompareTo(newStudent.deg) == 1)
        {
            return 1;
        }
        else if (this.deg.CompareTo(newStudent.deg) != 1)
        {
            return -1;
        }
        else //this is what my code is ignoring and not ordering by firstname as well
        {
            if (this.fName == newStudent.fName)
            {
                return 0;
            }
            else if (this.fName != newStudent.fName)
            {
                return -1;
            }
            else
            {
                return 0;
            }
        }
    }
}

Answer 1

您的问题的第一个答案（为什么else 部分永远不会被执行）已经回答，也就是说，不存在1 或not 1 条件的第三种可能性。

如果您尝试按不同的属性排序，例如，如果我们首先要按度数排序，然后按fName 那么我们就可以实现IComparer-

class Student : IComparer<Student>{

    /*
    codes
   */


    public int Compare(Student student1, Student student2)
   {
          if(student1.deg.Equals(student2.deg))  //if both degrees are same
          {
              return string.Compare(student1.fName , student2.fName); // then compare fName
          }
          else
              return string.Compare(student1.deg , student2.deg);


   }
}

Answer 2

如果this.deg.CompareTo(newStudent.deg) == 1，则返回1，如果this.deg.CompareTo(newStudent.deg) != 1，则返回-1。由于比较要么等于 1，要么不等于 1，因此您永远无法达到其余的 else。

因此您的代码应如下所示：

partial class Student : IComparable, IComparable<Student>
{
    public int CompareTo(object obj)
    {
        if (obj != null && obj.GetType() != GetType())
        {
            // From https://msdn.microsoft.com/en-us/library/system.icomparable.compareto(v=vs.110).aspx#Remarks
            // The parameter, obj, must be the same type as the class or value type that implements this interface; otherwise, an ArgumentException is thrown.
            throw new ArgumentException(string.Format("Object must be of type {0}", GetType()));
        }
        return CompareTo((Student)obj);
    }

    public int CompareTo(Student newStudent)
    {
        if (object.ReferenceEquals(this, newStudent))
            return 0;
        else if (newStudent == null)
            // From https://msdn.microsoft.com/en-us/library/43hc6wht(v=vs.110).aspx#Remarks
            // By definition, any object compares greater than null, and two null references compare equal to each other.
            return 1;

        var cmp = this.deg.CompareTo(newStudent.deg);
        if (cmp != 0)
            return cmp;

        cmp = this.fName.CompareTo(newStudent.fName);
        if (cmp != 0)
            return cmp;

        // Compare additional members as required, return the first nonzero member comparison.

        // Finally return 0 if all member comparisons returned 0.
        return 0;
    }
}

注意事项：

• 如果您要实施IComparable，您还应该实施IComparable<Student>，原因在IComparable and IComparable<T> 中解释。

• 由于IComparable 由其值可以排序或排序的类型实现，如IComparable<T>.CompareTo Method (T): Notes to Implementers 中所述，您应该确保您的比较方法是reflexive, antisymmetric and transitive。通过连续调用Student 成员的CompareTo 方法并返回第一个非零值，您可以确保情况确实如此。

Answer 3

在您的代码中

if (this.deg.CompareTo(newStudent.deg) == 1)
{
    // do something
}
else if (this.deg.CompareTo(newStudent.deg) != 1)
{
    // do something
}
else 
{
    // do something
}

else 语句永远不会到达，因为结果可以等于或不等于 1。 而且您只检查“度”值。例如，您可以像这样检查它们是否相等：

public int CompareTo(object obj)
{
    if (obj == null)
    {
        return -1;
    }
    Student newStudent = obj as Student;
    // are equal
    if (deg.CompareTo(newStudent.deg) == 0 && 
        gra.CompareTo(newStudent.gra) == 0 &&
        lName.CompareTo(newStudent.lName) == 0 &&
        fName.CompareTo(newStudent.fName) == 0)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}