Question

我需要根据ListView标题的点击对ListView上的内容进行排序。我正在使用C ++ / CLI，因此我得到了this Microsoft article并使用C ++ / CLI对其进行了编码。

这是我的结果：

ListViewColumnSorter.h

/// <summary>
/// This class is an implementation of the 'IComparer' interface.
/// </summary>
#pragma once

public ref class ListViewColumnSorter : System::Collections::IComparer
{
    public:

        ListViewColumnSorter();
        virtual int Compare(System::Object^ x, System::Object^ y);

    private:
        /// <summary>
        /// Specifies the column to be sorted
        /// </summary>
        int ColumnToSort;

        /// <summary>
        /// Specifies the order in which to sort (i.e. 'Ascending').
        /// </summary>
        System::Windows::Forms::SortOrder OrderOfSort;

        /// <summary>
        /// Case insensitive comparer object
        /// </summary>
        System::Collections::CaseInsensitiveComparer^ ObjectCompare;

        property int SortColumn;
        property System::Windows::Forms::SortOrder SortOrder;
};

ListViewColumnSorter.cpp

/// <summary>
/// This class is an implementation of the 'IComparer' interface.
/// </summary>

#include "ListViewColumnSorter.h"

ListViewColumnSorter::ListViewColumnSorter()
{
    // Initialize the column to '0'
    ColumnToSort = 0;

    // Initialize the sort order to 'none'
    OrderOfSort = System::Windows::Forms::SortOrder::None;

    // Initialize the CaseInsensitiveComparer object
    ObjectCompare = gcnew System::Collections::CaseInsensitiveComparer();
}

/// <summary>
/// This method is inherited from the IComparer interface.  It compares the two objects passed using a case insensitive comparison.
/// </summary>
/// <param name="x">First object to be compared</param>
/// <param name="y">Second object to be compared</param>
/// <returns>The result of the comparison. "0" if equal, negative if 'x' is less than 'y' and positive if 'x' is greater than 'y'</returns>
int ListViewColumnSorter::Compare(System::Object^ x, System::Object^ y)
{
    int compareResult;
    System::Windows::Forms::ListViewItem^ listviewX, listviewY;

    // Cast the objects to be compared to ListViewItem objects
    listviewX = (System::Windows::Forms::ListViewItem^) x; <--- ERROR HERE
    listviewY = (System::Windows::Forms::ListViewItem^) y; <--- ERROR HERE

    // Compare the two items
    compareResult = ObjectCompare->Compare(listviewX->SubItems[ColumnToSort].Text,
                                           listviewY->SubItems[ColumnToSort].Text);

    // Calculate correct return value based on object comparison
    if (OrderOfSort == System::Windows::Forms:: SortOrder::Ascending)
    {
        // Ascending sort is selected, return normal result of compare operation
        return compareResult;
    }
    else if (OrderOfSort == System::Windows::Forms::SortOrder::Descending)
    {
        // Descending sort is selected, return negative result of compare operation
        return (-compareResult);
    }
    else
    {
        // Return '0' to indicate they are equal
        return 0;
    }
}

/// <summary>
/// Gets or sets the number of the column to which to apply the sorting operation (Defaults to '0').
/// </summary>
void ListViewColumnSorter::SortColumn::set(int value)
{
    ColumnToSort = value;
}

int ListViewColumnSorter::SortColumn::get()
{
    return ColumnToSort;
}

/// <summary>
/// Gets or sets the order of sorting to apply (for example, 'Ascending' or 'Descending').
/// </summary>
void ListViewColumnSorter::SortOrder::set(System::Windows::Forms::SortOrder value)
{
    OrderOfSort = value;
}

System::Windows::Forms::SortOrder ListViewColumnSorter::SortOrder::get()
{
    return OrderOfSort;
}

我在编译此代码时遇到问题，在函数Compare上，我无法将ListViewItem转换为listviewX和listviewY以获取其字符串（检查代码标记）：

表达式必须具有类型

解决该问题所需的建议。

Answer 1

您是否正在寻找从Object^投射到ListViewItem^的正确方法？

投射到托管类型的常用方法是使用dynamic_cast或static_cast。

我会使用dynamic_cast<ListViewItem^>(x)，但这需要您检查nullptr。您也可以使用static_cast<ListViewItem^>(x)，如果它不是正确类型，则会抛出异常。

另外，请注意，没有什么可以阻止您在C＃中实现ListViewColumnSorter类，并从C ++ / CLI引用它。

单击标题时排序ListView

1 个答案: