我开始真的喜欢扩展方法... 我正在想是否有人在这里发现了真正令人惊叹或聪明的例子。
我今天写的一个例子：
由于其他用户的评论而编辑：
public static IEnumerable<int> To(this int fromNumber, int toNumber) {
while (fromNumber < toNumber) {
yield return fromNumber;
fromNumber++;
}
}
这样可以将 for 循环写成 foreach 循环：
foreach (int x in 0.To(16)) {
Console.WriteLine(Math.Pow(2, x).ToString());
}
我迫不及待地想看到其他的例子！祝你愉快！
这是我最近一直在使用的一个：
public static IDisposable Tag(this HtmlHelper html, string tagName)
{
if (html == null)
throw new ArgumentNullException("html");
Action<string> a = tag => html.Write(String.Format(tag, tagName));
a("<{0}>");
return new Memento(() => a("</{0}>"));
}
使用方法如下：
using (Html.Tag("ul"))
{
this.Model.ForEach(item => using(Html.Tag("li")) Html.Write(item));
using(Html.Tag("li")) Html.Write("new");
}
Memento 是一个方便的类：
public sealed class Memento : IDisposable
{
private bool Disposed { get; set; }
private Action Action { get; set; }
public Memento(Action action)
{
if (action == null)
throw new ArgumentNullException("action");
Action = action;
}
void IDisposable.Dispose()
{
if (Disposed)
throw new ObjectDisposedException("Memento");
Disposed = true;
Action();
}
}
并完成依赖项：
public static void Write(this HtmlHelper html, string content)
{
if (html == null)
throw new ArgumentNullException("html");
html.ViewContext.HttpContext.Response.Write(content);
}
public static String ToCSV(this DataTable dataTable)
{
return dataTable.ToCSV(null, COMMA, true);
}
public static String ToCSV(this DataTable dataTable, String qualifier)
{
return dataTable.ToCSV(qualifier, COMMA, true);
}
private static String ToCSV(this DataTable dataTable, String qualifier, String delimiter, Boolean includeColumnNames)
{
if (dataTable == null) return null;
if (qualifier == delimiter)
{
throw new InvalidOperationException(
"The qualifier and the delimiter are identical. This will cause the CSV to have collisions that might result in data being parsed incorrectly by another program.");
}
var sbCSV = new StringBuilder();
var delimiterToUse = delimiter ?? COMMA;
if (includeColumnNames)
sbCSV.AppendLine(dataTable.Columns.GetHeaderLine(qualifier, delimiterToUse));
foreach (DataRow row in dataTable.Rows)
{
sbCSV.AppendLine(row.ToCSVLine(qualifier, delimiterToUse));
}
return sbCSV.Length > 0 ? sbCSV.ToString() : null;
}
private static String ToCSVLine(this DataRow dataRow, String qualifier, String delimiter)
{
var colCount = dataRow.Table.Columns.Count;
var rowValues = new String[colCount];
for (var i = 0; i < colCount; i++)
{
rowValues[i] = dataRow[i].Qualify(qualifier);
}
return String.Join(delimiter, rowValues);
}
private static String GetHeaderLine(this DataColumnCollection columns, String qualifier, String delimiter)
{
var colCount = columns.Count;
var colNames = new String[colCount];
for (var i = 0; i < colCount; i++)
{
colNames[i] = columns[i].ColumnName.Qualify(qualifier);
}
return String.Join(delimiter, colNames);
}
private static String Qualify(this Object target, String qualifier)
{
return qualifier + target + qualifier;
}
最终，你可以这样称呼它：
someDataTable.ToCSV(); //Plain old CSV
someDataTable.ToCSV("\""); //Double quote qualifier
someDataTable.ToCSV("\"", "\t"); //Tab delimited
我不喜欢INotifyPropertyChanged接口要求将属性名称作为字符串传递。我希望有一种强类型的方法，在编译时检查我只为存在的属性提高和处理属性更改。我使用以下代码来实现：
public static class INotifyPropertyChangedExtensions
{
public static string ToPropertyName<T>(this Expression<Func<T>> @this)
{
var @return = string.Empty;
if (@this != null)
{
var memberExpression = @this.Body as MemberExpression;
if (memberExpression != null)
{
@return = memberExpression.Member.Name;
}
}
return @return;
}
}
INotifyPropertyChanged的类中，我会包含这个辅助方法：protected void NotifySetProperty<T>(ref T field, T value,
Expression<Func<T>> propertyExpression)
{
if (field == null ? value != null : !field.Equals(value))
{
field = value;
this.NotifyPropertyChanged(propertyExpression.ToPropertyName());
}
}
最终，我可以做到这样的事情：
private string _name;
public string Name
{
get { return _name; }
set { this.NotifySetProperty(ref _name, value, () => this.Name); }
}
它是强类型的，我只为实际更改值的属性引发事件。
private static readonly PropertyChangedEventArgs _namePropertyChangedEventArgs = new PropertyChangedEventArgs("Name"); 并将它们传递给 OnPropertyChanged。相比于字符串（需要查找所有的引用），这样做更方便跟踪；而且节省内存开销，因为每个类型只会创建一次，而不是每个实例每次更改都要创建。 - Sam Harwell虽然这不是很聪明的做法，但我修改了----OrDefault方法，以便您可以在行内指定默认项，而无需在代码中稍后检查null：
public static T SingleOrDefault<T> ( this IEnumerable<T> source,
Func<T, bool> action, T theDefault )
{
T item = source.SingleOrDefault<T>(action);
if (item != null)
return item;
return theDefault;
}
这是一种非常简单但确实有助于清理空检查的方法。最好在您的UI期望X项列表（例如锦标赛系统或游戏玩家插槽）并且您想要显示“空座位”时使用。
用法：
return jediList.SingleOrDefault(
j => j.LightsaberColor == "Orange",
new Jedi() { LightsaberColor = "Orange", Name = "DarthNobody");
我喜欢使用的两个方法是我编写的InsertWhere<T>和RemoveWhere<T>扩展方法。在WPF和Silverlight中使用ObservableCollections时，我经常需要修改有序列表而不必重新创建它们。这些方法允许我根据提供的Func进行插入和删除，因此不需要重新调用.OrderBy()。
/// <summary>
/// Removes all items from the provided <paramref name="list"/> that match the<paramref name="predicate"/> expression.
/// </summary>
/// <typeparam name="T">The class type of the list items.</typeparam>
/// <param name="list">The list to remove items from.</param>
/// <param name="predicate">The predicate expression to test against.</param>
public static void RemoveWhere<T>(this IList<T> list, Func<T, bool> predicate)
{
T[] copy = new T[] { };
Array.Resize(ref copy, list.Count);
list.CopyTo(copy, 0);
for (int i = copy.Length - 1; i >= 0; i--)
{
if (predicate(copy[i]))
{
list.RemoveAt(i);
}
}
}
/// <summary>
/// Inserts an Item into a list at the first place that the <paramref name="predicate"/> expression fails. If it is true in all cases, then the item is appended to the end of the list.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="list"></param>
/// <param name="obj"></param>
/// <param name="predicate">The sepcified function that determines when the <paramref name="obj"/> should be added. </param>
public static void InsertWhere<T>(this IList<T> list, T obj, Func<T, bool> predicate)
{
for (int i = 0; i < list.Count; i++)
{
// When the function first fails it inserts the obj paramiter.
// For example, in a list myList of ordered Int32's {1,2,3,4,5,10,12}
// Calling myList.InsertWhere( 8, x => 8 > x) inserts 8 once the list item becomes greater then or equal to it.
if(!predicate(list[i]))
{
list.Insert(i, obj);
return;
}
}
list.Add(obj);
}
编辑：
Talljoe 对我匆忙构建的 RemoveWhere/RemoveAll 进行了一些重大改进。新版本只需约 50 毫秒（如果可以调用 List.RemoveAll，则不到 10 毫秒），即可删除约 300 万项中的每三个，而 RemoveWhere 则需要多秒钟（我等得有点累了）。以下是他极大地改进后的版本，再次感谢！
public static void RemoveAll<T>(this IList<T> instance, Predicate<T> predicate)
{
if (instance == null)
throw new ArgumentNullException("instance");
if (predicate == null)
throw new ArgumentNullException("predicate");
if (instance is T[])
throw new NotSupportedException();
var list = instance as List<T>;
if (list != null)
{
list.RemoveAll(predicate);
return;
}
int writeIndex = 0;
for (int readIndex = 0; readIndex < instance.Count; readIndex++)
{
var item = instance[readIndex];
if (predicate(item)) continue;
if (readIndex != writeIndex)
{
instance[writeIndex] = item;
}
++writeIndex;
}
if (writeIndex != instance.Count)
{
for (int deleteIndex = instance.Count - 1; deleteIndex >= writeIndex; --deleteIndex)
{
instance.RemoveAt(deleteIndex);
}
}
}
这是我随便写的一个方法，所以请随意挑剔。它需要一个有序整数列表，并返回连续区间的字符串列表。例如：
1,2,3,7,10,11,12 --> "1-3","7","10-12"
这个静态类中的函数：
public static IEnumerable<string> IntRanges(this IEnumerable<int> numbers)
{
int rangeStart = 0;
int previous = 0;
if (!numbers.Any())
yield break;
rangeStart = previous = numbers.FirstOrDefault();
foreach (int n in numbers.Skip(1))
{
if (n - previous > 1) // sequence break - yield a sequence
{
if (previous > rangeStart)
{
yield return string.Format("{0}-{1}", rangeStart, previous);
}
else
{
yield return rangeStart.ToString();
}
rangeStart = n;
}
previous = n;
}
if (previous > rangeStart)
{
yield return string.Format("{0}-{1}", rangeStart, previous);
}
else
{
yield return rangeStart.ToString();
}
}
使用示例：
this.WeekDescription = string.Join(",", from.WeekPattern.WeekPatternToInts().IntRanges().ToArray());
这段代码用于将来自一款值得上 DailyWTF 的课程表应用程序的数据进行转换。WeekPattern 是存储在字符串中的比特掩码 "0011011100..."。WeekPatternToInts() 将其转换为 IEnumerable<int>，在本例中为 [3,4,6,7,8]，最终变成了 "3-4,6-8"。它提供了一个简洁的描述，说明一门课程在哪些学术周范围内开设。
我有各种.Debugify扩展方法，可以将对象转储到日志文件中。例如，这是我的Dictionary debugify（我也有针对List、Datatable、param array等的扩展方法）：
public static string Debugify<TKey, TValue>(this Dictionary<TKey, TValue> dictionary) {
string Result = "";
if (dictionary.Count > 0) {
StringBuilder ResultBuilder = new StringBuilder();
int Counter = 0;
foreach (KeyValuePair<TKey, TValue> Entry in dictionary) {
Counter++;
ResultBuilder.AppendFormat("{0}: {1}, ", Entry.Key, Entry.Value);
if (Counter % 10 == 0) ResultBuilder.AppendLine();
}
Result = ResultBuilder.ToString();
}
return Result;
}
这里有一个用于 DbParameterCollection 的示例（可以用于将数据库调用转储到日志文件中）：
public static string Debugify(this DbParameterCollection parameters) {
List<string> ParameterValuesList = new List<string>();
foreach (DbParameter Parameter in parameters) {
string ParameterName, ParameterValue;
ParameterName = Parameter.ParameterName;
if (Parameter.Direction == ParameterDirection.ReturnValue)
continue;
if (Parameter.Value == null || Parameter.Value.Equals(DBNull.Value))
ParameterValue = "NULL";
else
{
switch (Parameter.DbType)
{
case DbType.String:
case DbType.Date:
case DbType.DateTime:
case DbType.Guid:
case DbType.Xml:
ParameterValue
= "'" + Parameter
.Value
.ToString()
.Replace(Environment.NewLine, "")
.Left(80, "...") + "'"; // Left... is another nice one
break;
default:
ParameterValue = Parameter.Value.ToString();
break;
}
if (Parameter.Direction != ParameterDirection.Input)
ParameterValue += " " + Parameter.Direction.ToString();
}
ParameterValuesList.Add(string.Format("{0}={1}", ParameterName, ParameterValue));
}
return string.Join(", ", ParameterValuesList.ToArray());
}
示例结果：
Log.DebugFormat("EXEC {0} {1}", procName, params.Debugify);
// EXEC spProcedure @intID=5, @nvName='Michael Haren', @intRefID=11 OUTPUT
请注意，如果您在数据库调用之后调用此函数，则输出参数也将被填充。我在包括存储过程名称的行上调用此函数，以便可以将调用复制/粘贴到 SSMS 进行调试。
这些使我的日志文件更加美观，易于生成，而不会中断我的代码。
一对将基于36进制的字符串转换为整数的扩展方法:
public static int ToBase10(this string base36)
{
if (string.IsNullOrEmpty(base36))
return 0;
int value = 0;
foreach (var c in base36.Trim())
{
value = value * 36 + c.ToBase10();
}
return value;
}
public static int ToBase10(this char c)
{
if (c >= '0' && c <= '9')
return c - '0';
c = char.ToUpper(c);
if (c >= 'A' && c <= 'Z')
return c - 'A' + 10;
return 0;
}
有些天才决定将数字编码为字符串存储在数据库中，因为十进制数占用太多空间。十六进制更好，但不使用字符G-Z。所以显然需要将基数16扩展到基数36！
只需一个指令即可从DbConnection创建DbCommand：
public static DbCommand CreateCommand(this DbConnection connection, string commandText)
{
DbCommand command = connection.CreateCommand();
command.CommandText = commandText;
return command;
}
向 DbCommand 添加参数:
public static DbParameter AddParameter(this DbCommand command, string name, DbType dbType)
{
DbParameter p = AddParameter(command, name, dbType, 0, ParameterDirection.Input);
return p;
}
public static DbParameter AddParameter(this DbCommand command, string name, DbType dbType, object value)
{
DbParameter p = AddParameter(command, name, dbType, 0, ParameterDirection.Input);
p.Value = value;
return p;
}
public static DbParameter AddParameter(this DbCommand command, string name, DbType dbType, int size)
{
return AddParameter(command, name, dbType, size, ParameterDirection.Input);
}
public static DbParameter AddParameter(this DbCommand command, string name, DbType dbType, int size, ParameterDirection direction)
{
DbParameter parameter = command.CreateParameter();
parameter.ParameterName = name;
parameter.DbType = dbType;
parameter.Direction = direction;
parameter.Size = size;
command.Parameters.Add(parameter);
return parameter;
}
通过名称而不是索引访问 DbDataReader 字段：
public static DateTime GetDateTime(this DbDataReader reader, string name)
{
int i = reader.GetOrdinal(name);
return reader.GetDateTime(i);
}
public static decimal GetDecimal(this DbDataReader reader, string name)
{
int i = reader.GetOrdinal(name);
return reader.GetDecimal(i);
}
public static double GetDouble(this DbDataReader reader, string name)
{
int i = reader.GetOrdinal(name);
return reader.GetDouble(i);
}
public static string GetString(this DbDataReader reader, string name)
{
int i = reader.GetOrdinal(name);
return reader.GetString(i);
}
...
还有一个（与此无关的）扩展方法，允许我在WinForms窗体和控件上执行DragMove操作（就像在WPF中），请看这里。
public static class EventExtension
{
public static void RaiseEvent<T>(this EventHandler<T> handler, object obj, T args) where T : EventArgs
{
EventHandler<T> theHandler = handler;
if (theHandler != null)
{
theHandler(obj, args);
}
}
}
for(int x=0; x<=16; ++x)对有经验的程序员来说更易读。但是，闭区间往往较少使用。 - Tom Hawtin - tackline