使用 IComparer 基于多个字段进行排序 [关闭]

Question

我使用下面的代码对List<DataAccessViewModel> 列表进行排序。

这是排序顺序：

1. PriorityScore
2. MName
3. CName
4. FName

它按预期工作。

public int Compare(DataAccessViewModel x, DataAccessViewModel y)
{
    if (x == null || y == null)
    {
        return 0;
    }

    return x.CompareTo(y);
}

public int CompareTo(DataAccessViewModel mod)
{
    int retval = (int)(this.PriorityScore?.CompareTo(mod.PriorityScore));
    if(retval != 0)
        return retval;
    else
    {
        retval = (this.MName ?? "zzzzzzzzzzzzz").CompareTo(mod.MName ?? "zzzzzzzzzzzzz");
        if (retval != 0)
            return retval;
        else
        {
            retval = (this.CName ?? "zzzzzzzzzzzzz").CompareTo(this.CName ?? "zzzzzzzzzzzzz");
            if (retval != 0)
                return retval;
            else
                retval = (this.FName ?? "zzzzzzzzzzzzz").CompareTo(this.FName ?? "zzzzzzzzzzzzz");
        }
    }
        
    return retval;
}

但代码对我来说看起来很笨拙。有没有更好的方法或者是这样吗？

Answer 1

当前代码中有一个问题：

public int Compare(DataAccessViewModel x, DataAccessViewModel y) 
{
    if (x == null || y == null)
    {
        return 0;
    }
    ...

由于null 等于任何值，那么所有值都相等：

   a == null, null == b => a == b

这不是您要实施的规则。我建议这样的事情

using System.Linq;

...

// static: we don't want "this"
public static int TheCompare(DataAccessViewModel x, DataAccessViewModel y) {
  // Special cases, nulls
  if (ReferenceEquals(x, y)) // if references are shared, then equal
    return 0;
  if (null == x) // let null be smaller than any other value
    return -1;
  if (null == y)
    return 1;
 
  // How we compare strings 
  static int MyCompare(string left, string right) =>
      ReferenceEquals(left, right) ? 0 
    : null == left ? 1    // null is greater than any other string
    : null == right ? -1
    : string.Compare(left, right);

  // Func<int> for lazy computation
  return new Func<int>[] {
      () => x.PriorityScore?.CompareTo(y.PriorityScore) ?? -1,
      () => MyCompare(x.MName, y.MName),
      () => MyCompare(x.CName, y.CName),  
      () => MyCompare(x.FName, y.FName), 
    }
    .Select(func => func())
    .FirstOrDefault(value => value != 0);
}

然后对于界面我们有

public int CompareTo(DataAccessViewModel other) => TheCompare(this, other);

// I doubt if you want to implement both IComparable<T> and
// IComparer<T> but you can easily do it
public int Compare(DataAccessViewModel x, DataAccessViewModel y) =>
  TheCompare(x, y);

Answer 2

你快到了：只需跳过return 后面的else 语句：

public int CompareTo(DataAccessViewModel mod)
{
    // I worry about this line -- if you're sure that PriorityScore is not null,
    // why not use this.PriorityScore.Value to make that clear?
    int retval = (int)(this.PriorityScore?.CompareTo(mod.PriorityScore));
    if (retval != 0)
        return retval;

    retval = (this.MName ?? "zzzzzzzzzzzzz").CompareTo(mod.MName ?? "zzzzzzzzzzzzz");
    if (retval != 0)
        return retval;

    retval = (this.CName ?? "zzzzzzzzzzzzz").CompareTo(this.CName ?? "zzzzzzzzzzzzz");
    if (retval != 0)
        return retval;

    return (this.FName ?? "zzzzzzzzzzzzz").CompareTo(this.FName ?? "zzzzzzzzzzzzz");
}

您还可以使用Comparer<T>.Default 正确处理可空值。这会将null 排序为等于其他null 值，但小于任何其他对象。

public int CompareTo(DataAccessViewModel mod)
{
    int retval = Comparer<int?>.Default.Compare(this.PriorityScore, mod.PriorityScore);
    if (retval != 0)
        return retval;

    retval = Comparer<string>.Default.Compare(this.MName, mod.MName);
    if (retval != 0)
        return retval;

    retval = Comparer<string>.Default.Compare(this.CName, mod.CName);
    if (retval != 0)
        return retval;

    return Comparer<string>.Default.Compare(this.FName, mod.FName);
}

Answer 3

考虑利用default behavior of ValueTuples：

IComparer（这应该和你的类不一样）

public int Compare(DataAccessViewModel x, DataAccessViewModel y)
{
    if (x == null && y == null)
    {
        return 0;
    }

    if (x == null) return -1;
    if (y == null) return 1;

    var thisCompareOrder = (x.PriorityScore, x.MName, x.CName, x.FName);
    var thatCompareOrder = (y.PriorityScore, y.MName, y.CName, y.FName);
    return thisCompareOrder.CompareTo(thatCompareOrder);
}

如果您希望您的类默认具有这些语义，请实现 IComparable。

public int CompareTo(DataAccessViewModel mod)
{
    if(mod == null) return 1;

    var thisCompareOrder = (this.PriorityScore, this.MName, this.CName, this.FName);
    var thatCompareOrder = (mod.PriorityScore, mod.MName, mod.CName, mod.FName);
    return thisCompareOrder.CompareTo(thatCompareOrder);
}

您也可以考虑将您的原始类设为record，默认情况下它具有基于值的语义。