C# 委托

刘亦菲是我老婆 發表於 2025-3-30 17:00:00

原文链接：https://www.cnblogs.com/ysmc/p/18800819
<h1>C# 委托（Delegate）</h1>
<h2>介绍（摘至网络）</h2>
在 C# 中，委托（Delegate）是一种类型安全的函数指针，它允许将方法作为参数传递给其他方法
C# 中的委托（Delegate）类似于 C 或 C++ 中函数的指针。委托（Delegate） 是存有对某个方法的引用的一种引用类型变量，引用可在运行时被改变
委托在 C# 中非常常见，用于事件处理、回调函数、LINQ 等操作
所有的委托（Delegate）都派生自 System.Delegate 类
<h2>正文</h2>
委托，相信小伙伴们都知道这个玩意，但是很多小伙伴不知道这个东西在开发中能有什么用，我这里给小伙伴们一个方向，后续大家就可以扩散一下了
在工作中，大家应该是逃不了导出excel的，但是每有一个需求，就要写一遍重复代码，时间久了，就会自己写出一个通用的导出功能，但是只是适合简单的导出，晚上随便找个包都能解决，往往也会出现一些较为复杂的需求，例如某一个字段值根据区间给予不同的背景色，或者是需要加粗字体、给不同的字体颜色等等。
我这里就举一个例子，班级的学生成绩导出，不及格（60以下）的科目成绩给予红色背景色，一班为重点班不到80分也记为红色背景色
<h2>例子</h2>
首先我们写一个学生成绩类 student.cs
<div class="cnblogs_code">
<pre> 1 public class StudentGrade
 2 {
 3 /// <summary>
 4 /// 班级
 5 /// </summary>
 6 
 7
 8 public string? Class { get; set; }
 9
10 /// <summary>
11 /// 学号
12 /// </summary>
13 
14
15 public string? StudentId { get; set; }
16
17 /// <summary>
18 /// 科目
19 /// </summary>
20 
21
22 public string? Subject { get; set; }
23
24 /// <summary>
25 /// 成绩
26 /// </summary>
27 
28
29 public double Grade { get; set; }
30 }</pre>
</div>
然后，我们来一个导出excel的函数，我比较懒，意思意思能懂就好了
<div class="cnblogs_code">
<pre> 1 /// <summary>
 2 /// 导出数据到 Excel
 3 /// </summary>
 4 /// <typeparam name="T"></typeparam>
 5 /// <param name="data"></param>
 6 /// <param name="func"></param>
 7 /// <returns></returns>
 8 public async Task<Stream> ExportToExcelAsync<T>(IEnumerable<T> data, Func<string, T, CellStyle?>? func)
 9 {
10 using var package = new ExcelPackage();
11 var worksheet = package.Workbook.Worksheets.Add("StudentGrades");
12
13 var properties = typeof(T).GetProperties();
14 // 处理表头，获取属性上的 DisplayNameAttribute 特性
15 foreach (var property in properties)
16 {
17 var index = Array.IndexOf(properties, property);
18 var attributes = property.GetCustomAttributes(typeof(DisplayNameAttribute), false) as IEnumerable<DisplayNameAttribute>;
19 if (attributes == null || !attributes.Any())
20 {
21 worksheet.Cells[1, index + 1].Value = property.Name;
22 }
23 else
24 {
25 worksheet.Cells[1, index + 1].Value = attributes.First().DisplayName;
26 }
27 }
28
29 // 处理数据
30 for (int i = 0; i < data.Count(); i++)
31 {
32 var item = data.ElementAt(i);
33 for (int j = 0; j < properties.Length; j++)
34 {
35 var value = properties.GetValue(item);
36 if (func != null)
37 {
38 var cellStyle = func(properties.Name, item);
39 if (cellStyle != null)
40 {
41 worksheet.Cells.Style.Font.Color.SetColor(cellStyle.FontColor);
42 worksheet.Cells.Style.Font.Size = cellStyle.FontSize;
43 worksheet.Cells.Style.Fill.PatternType = OfficeOpenXml.Style.ExcelFillStyle.Solid;
44 worksheet.Cells.Style.Fill.BackgroundColor.SetColor(cellStyle.BackgroundColor);
45 }
46 }
47 worksheet.Cells.Value = value;
48 }
49 }
50
51 return await Task.FromResult(package.Stream);
52 }</pre>
</div>
单元格样式 CellStyle
<div class="cnblogs_code">
<pre> 1 public class CellStyle
 2 {
 3 /// <summary>
 4 /// 字体颜色
 5 /// </summary>
 6 public Color FontColor { get; set; }
 7
 8 /// <summary>
 9 /// 字体大小
10 /// </summary>
11 public float FontSize { get; set; }
12
13 /// <summary>
14 /// 背景色
15 /// </summary>
16 public Color BackgroundColor { get; set; }
17 }</pre>
</div>
这样，当你在调用通用导出的时候，由你自己处理每一个单元格的格式就好了，处理的字段columnName，以及整一行数据都给你了
<div class="cnblogs_code">
<pre> 1 var data = new List<StudentGrade>
 2 {
 3 new StudentGrade { Class = "一班", StudentId = "001", Subject = "语文", Grade = 90 },
 4 new StudentGrade { Class = "一班", StudentId = "001", Subject = "数学", Grade = 80 },
 5 new StudentGrade { Class = "一班", StudentId = "001", Subject = "英语", Grade = 70 },
 6 new StudentGrade { Class = "一班", StudentId = "002", Subject = "语文", Grade = 85 },
 7 new StudentGrade { Class = "一班", StudentId = "002", Subject = "数学", Grade = 75 },
 8 new StudentGrade { Class = "一班", StudentId = "002", Subject = "英语", Grade = 65 },
 9 new StudentGrade { Class = "二班", StudentId = "003", Subject = "语文", Grade = 95 },
10 new StudentGrade { Class = "二班", StudentId = "003", Subject = "数学", Grade = 85 },
11 new StudentGrade { Class = "二班", StudentId = "003", Subject = "英语", Grade = 75 },
12 new StudentGrade { Class = "二班", StudentId = "004", Subject = "语文", Grade = 100 },
13 new StudentGrade { Class = "二班", StudentId = "004", Subject = "数学", Grade = 90 },
14 new StudentGrade { Class = "二班", StudentId = "004", Subject = "英语", Grade = 80 }
15 };
16 var cellStyle = new CellStyle
17 {
18 FontColor = Color.Black,
19 FontSize = 12,
20 BackgroundColor = Color.Red
21 };
22 var stream = await ExportToExcelAsync(data, (columnName, item) =>
23 {
24 if (columnName == "Grade" && (item.Grade < 60
25 || (item.Class == "一班" && item.Grade < 80)))
26 {
27 return cellStyle;
28 }
29 return null;
30 });
31 using var fileStream = new FileStream("StudentGrades.xlsx", FileMode.Create);
32 stream.CopyTo(fileStream);</pre>
</div>
好了，感谢大佬们的观看！

</div>
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本文来自博客园，作者：一事冇诚，转载请注明原文链接：https://www.cnblogs.com/ysmc/p/18800819 
来源：https://www.cnblogs.com/ysmc/p/18800819

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C# 委托