原文地址
log4net.Layout.PatternLayout，是一個靈活的布局，配置模式字符串。
線程安全。該類型的 Public static 成員對多線程操作是安全的。實例成員不保證線程安全。
注意：
該類的目標是把 LoggingEvent 格式化成字符串。格式化的結果依賴于 conversion 模式。
conversion 模式與 C 語言的 PRintf 函數很像。conversion 模式是由文字文本和格式控制表達式（稱為轉換指定符）組成。
你可以自由地將任何文字文本插入到轉換模式中。
每個轉換指定符（conversion specifier）以百分號（%）開始，后面跟一個可選的格式修飾符（format modifiers）和轉換模式（conversion pattern）名稱。 轉換模式名稱指定數據的類型，例如，日志器（logger）、級別（level）、日期（date）、線程名（thread name）。格式修飾符控制諸如，字段寬度，填充，左邊和右邊對齊等事情。
下面是一個簡單的例子：
轉換模式為 "%-5level [%thread]: %message%newline"，并假設 log4net 環境設置為 PatternLayout，那么，下面語句：
[C#]
ILog log = LogManager.GetLogger(typeof(TestApp)); log.Debug("Message 1"); log.Warn("Message 2"); 將輸出，
DEBUG [main]: Message 1
WARN [main]: Message 2
需要注意的是，文字和轉換指示器之間沒有明確的分隔符。當模式解析器讀取一個轉換字符時，它知道何時已經達到了一個轉換指示器的末尾。上面的例子，在轉換指示符 ％-5level 含義是，日志事件的級別占 5 個字符的寬度，并且左邊對齊。
下表是可以識別的轉換模式名稱：
| 轉換模式名稱 | 效果 |
| a | 等價于 appdomain |
| appdomain | 用于輸出當產生日志事件時的 AppDomain 名稱。 |
| aspnet-cache | 指定 %aspnet-cache 時輸出所有 cache 項；指定 %aspnet-cache{key} 時只輸出 key 指定的項。 該模式對 Compact Framework or Client Profile 不可用。 |
| aspnet-context | 指定 %aspnet-context 時輸出所有 context 項；指定 %aspnet-context 時只輸出 key 指定的項。 該模式對 Compact Framework or Client Profile 不可用。 |
| aspnet-request | 指定 %aspnet-request 時輸出所有請求參數的項；指定 %aspnet-request{key} 時只輸出 key 指定的項。 該模式對 Compact Framework or Client Profile 不可用。 |
| aspnet-session | 指定 %aspnet-session 時輸出所有 session 項；指定 %aspnet-session{key} 時只輸出 key 指定的項。 該模式對 Compact Framework or Client Profile 不可用。 |
| c | 等價于 logger |
| C | 等價于 type |
| class | 等價于 type |
| d | 等價于 date |
| date | 以 local time zone 輸出日志事件的日期。若輸出 universal time 日期要使用 推薦使用 log4net 日期格式化器，能得到更好的輸出。通過指定字符串“ABSOLUTE”，“DATE”和“ISO8601”中的一個，分別表示 AbsoluteTimeDateFormatter, DateTimeDateFormatter 和 Iso8601DateFormatter。例如，%date{ISO8601} 或 %date{ABSOLUTE}。 這寫專用的日期格式化要比 ToString 更好。 |
| exception | Used to output the exception passed in with the log message. If an exception object is stored in the logging event it will be rendered into the pattern output with a trailing newline. If there is no exception then nothing will be output and no trailing newline will be appended. It is typical to put a newline before the exception and to have the exception as the last data in the pattern. |
| F | 等價于 file |
| file | 輸出發生日志請求的文件名。 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 See the note below on the availability of caller location information. |
| identity | 輸出當前活躍用戶的用戶名（Principal.Identity.Name）。 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 |
| l | 等價于 location |
| L | 等價于 line |
| location | Used to output location information of the caller which generated the logging event. 位置信息（location information）依賴 CLI 的實現，但通常是由調用方法的完整限定名（fully qualified name）組成，后面跟調用者源文件名和行號。 位置信息很有用。然而，它的產生相當慢。除非執行速度不是問題，否則要避免使用。 See the note below on the availability of caller location information. |
| level | 輸出日志事件的級別。 |
| line | 輸出發生日志請求時的行號。 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 See the note below on the availability of caller location information. |
| logger | 輸出日志事件的日志器。The logger conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets. If a precision specifier is given, then only the corresponding number of right most components of the logger name will be printed. By default the logger name is printed in full. 例如，若日志器名為 "a.b.c"，模式為 %logger{2}，將輸出 "b.c". |
| m | 等價于 message |
| M | 等價于 method |
| message | 輸出與日志事件相關聯的應用程序提供的信息，也就是你敲入的信息。 |
| mdc | The MDC (old name for the ThreadContext.Properties) is now part of the combined event properties. This pattern is supported for compatibility but is equivalent to property. |
| method | 輸出發生日志請求時的方法名。 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 See the note below on the availability of caller location information. |
| n | 等價于 newline |
| newline | 輸出換行。換行是平臺依賴的，各個平臺可能不同。 This conversion pattern offers the same performance as using non-portable line separator strings such as "/n", or "/r/n". Thus, it is the preferred way of specifying a line separator. |
| ndc | 輸出與生產日志事件線程有關的 NDC（nested diagnostic context）。 |
| p | 等價于 level |
| P | 等價于 property |
| properties | 等價于 property |
| property | Used to output the an event specific property. The key to lookup must be specified within braces and directly following the pattern specifier, e.g. %property{user} would include the value from the property that is keyed by the string 'user'. Each property value that is to be included in the log must be specified separately. Properties are added to events by loggers or appenders. By default the If no key is specified, e.g. %property then all the keys and their values are printed in a comma separated list. The properties of an event are combined from a number of different contexts. These are listed below in the order in which they are searched.
|
| r | 等價于 timestamp |
| stacktrace | Used to output the stack trace of the logging event The stack trace level specifier may be enclosed between braces. For example, %stacktrace{level}. If no stack trace level specifier is given then 1 is assumed Output uses the format: type3.MethodCall3 > type2.MethodCall2 > type1.MethodCall1 This pattern is not available for Compact Framework assemblies. |
| stacktracedetail | Used to output the stack trace of the logging event The stack trace level specifier may be enclosed between braces. For example, %stacktracedetail{level}. If no stack trace level specifier is given then 1 is assumed Output uses the format: type3.MethodCall3(type param,...) > type2.MethodCall2(type param,...) > type1.MethodCall1(type param,...) This pattern is not available for Compact Framework assemblies. |
| t | 等價于 thread |
| timestamp | Used to output the number of milliseconds elapsed since the start of the application until the creation of the logging event. |
| thread | 輸出產生日志事件的線程名稱。如果沒有可用的線程名稱，則使用數字。 |
| type | Used to output the fully qualified type name of the caller issuing the logging request. This conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets. If a precision specifier is given, then only the corresponding number of right most components of the class name will be printed. By default the class name is output in fully qualified form. For example, for the class name "log4net.Layout.PatternLayout", the pattern %type{1} will output "PatternLayout". 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 See the note below on the availability of caller location information. |
| u | 等價于 identity |
| username | 輸出當前活躍用戶的 WindowsIdentity。 警告：產生調用位置信息相當慢。除非執行速度不是問題，否則要避免使用它。 |
| utcdate | Used to output the date of the logging event in universal time. The date conversion specifier may be followed by a date format specifier enclosed between braces. For example, %utcdate{HH:mm:ss,fff} or %utcdate{dd MMM yyyy HH:mm:ss,fff}. If no date format specifier is given then ISO8601 format is assumed (Iso8601DateFormatter). The date format specifier admits the same syntax as the time pattern string of the ToString. For better results it is recommended to use the log4net date formatters. These can be specified using one of the strings "ABSOLUTE", "DATE" and "ISO8601" for specifying AbsoluteTimeDateFormatter, DateTimeDateFormatter and respectively Iso8601DateFormatter. For example, %utcdate{ISO8601} or %utcdate{ABSOLUTE}. These dedicated date formatters perform significantly better than ToString. |
| w | 等價于 username |
| x | 等價于 ndc |
| X | 等價于 mdc |
| % | 連續兩個百分號 %% 會輸出一個百分號。 |
The single letter patterns are deprecated in favor of the longer more descriptive pattern names.
By default the relevant information is output as is. However, with the aid of format modifiers it is possible to change the minimum field width, the maximum field width and justification.
The optional format modifier is placed between the percent sign and the conversion pattern name.
The first optional format modifier is the left justification flag which is just the minus (-) character. Then comes the optional minimum field width modifier. This is a decimal constant that represents the minimum number of characters to output. If the data item requires fewer characters, it is padded on either the left or the right until the minimum width is reached. The default is to pad on the left (right justify) but you can specify right padding with the left justification flag. The padding character is space. If the data item is larger than the minimum field width, the field is expanded to accommodate the data. The value is never truncated.
This behavior can be changed using the maximum field width modifier which is designated by a period followed by a decimal constant. If the data item is longer than the maximum field, then the extra characters are removed from the beginning of the data item and not from the end. For example, it the maximum field width is eight and the data item is ten characters long, then the first two characters of the data item are dropped. This behavior deviates from the printf function in C where truncation is done from the end.
Below are various format modifier examples for the logger conversion specifier.
| 格式修飾符 | 左邊對齊 | 最小寬度 | 最大寬度 | 備注 |
| %20logger | false | 20 | 無 | 若日志器的名稱小于 20 個字符，則左邊補空格。 |
| %-20logger | true | 20 | 無 | 若日志器的名稱小于 20 個字符，則右邊補空格。 |
| %.30logger | NA | 無 | 30 | 若日志器的名稱大于 30 個字符，則截取。 |
| %20.30logger | false | 20 | 30 | 若日志器的名稱小于 20 個字符，則左邊補空格。但是，若日志器名稱的長度大于 30 個字符，則截取。 |
| %-20.30logger | true | 20 | 30 | 若日志器的名稱小于 20 個字符，則右邊補空格。但是，若日志器名稱的長度大于 30 個字符，則截取。 |
關于調用者位置信息（caller location information）
模式 %type、%file、%line、%method、%location、%class、%C、%F、%L、%l 和 %M 都會產生調用者位置信息。位置信息使用 System.Diagnostics.StackTrace 類來產生調用堆棧。調用者信息是從這個堆棧提取的。
注意
.NET Compact Framework 1.0 不支持
System.Diagnostics.StackTrace類，因此，調用者位置信息也是不可用的。
注意
The
System.Diagnostics.StackTraceclass has this to say about Release builds:"StackTrace information will be most informative with Debug build configurations. By default, Debug builds include debug symbols, while Release builds do not. The debug symbols contain most of the file, method name, line number, and column information used in constructing StackFrame and StackTrace objects. StackTrace might not report as many method calls as expected, due to code transformations that occur during optimization."
This means that in a Release build the caller information may be incomplete or may not exist at all! Therefore caller location information cannot be relied upon in a Release build.
Additional pattern converters may be registered with a specific PatternLayout instance using the AddConverter method.
This is a more detailed pattern.
%timestamp [%thread] %level %logger %ndc - %message%newline
A similar pattern except that the relative time is right padded if less than 6 digits, thread name is right padded if less than 15 characters and truncated if longer and the logger name is left padded if shorter than 30 characters and truncated if longer.
%-6timestamp [%15.15thread] %-5level %30.30logger %ndc - %message%newline
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