API 管理策略表达式API Management policy expressions

• 07/10/2020
• • 

本文讨论 C# 7 中的策略表达式语法。This article discusses policy expressions syntax in C# 7. 每个表达式都可以访问隐式提供的上下文变量以及允许的 .NET Framework 类型子集。Each expression has access to the implicitly provided context variable and an allowed subset of .NET Framework types.

更多相关信息：For more information:

• 了解如何向后端服务提供上下文信息。See how to supply context information to your backend service. 使用设置查询字符串参数和设置 HTTP 标头策略来提供此信息。Use the Set query string parameter and Set HTTP header policies to supply this information.
• 了解如何使用验证 JWT 策略根据令牌声明预先授予操作访问权限。See how to use the Validate JWT policy to pre-authorize access to operations based on token claims.
• 了解如何使用 API 检查器跟踪查看策略求值方法和这些求值的结果。See how to use an API Inspector trace to see how policies are evaluated and the results of those evaluations.
• 了解如何对从缓存获取和存储到缓存策略使用表达式，以便配置 API 管理响应缓存。See how to use expressions with the Get from cache and Store to cache policies to configure API Management response caching. 设置持续时间，使之匹配由后端服务的 Cache-Control 指令指定的后端服务响应缓存。Set a duration that matches the response caching of the backend service as specified by the backed service's Cache-Control directive.
• 了解如何进行内容筛选。See how to perform content filtering. 删除使用控制流和设置正文策略从后端接收的响应中的数据元素。Remove data elements from the response received from the backend using the Control flow and Set body policies.
• 要下载策略语句，请参阅 api-management-samples/policies GitHub 存储库。To download the policy statements, see the api-management-samples/policies GitHub repo.

语法Syntax

单一语句表达式括在 @(expression) 中，其中 expression 是格式正确的 C# 表达式语句。Single statement expressions are enclosed in @(expression), where expression is a well-formed C# expression statement.

多语句表达式括在 @{expression} 中。Multi-statement expressions are enclosed in @{expression}. 多语句表达式中的所有代码路径必须以 return 语句结尾。All code paths within multi-statement expressions must end with a return statement.

示例Examples

@(true)

@((1+1).ToString())

@("Hi There".Length)

@(Regex.Match(context.Response.Headers.GetValueOrDefault("Cache-Control",""), @"max-age=(?<maxAge>\d+)").Groups["maxAge"]?.Value)

@(context.Variables.ContainsKey("maxAge") ? int.Parse((string)context.Variables["maxAge"]) : 3600)

@{
  string[] value;
  if (context.Request.Headers.TryGetValue("Authorization", out value))
  {
      if(value != null && value.Length > 0)
      {
          return Encoding.UTF8.GetString(Convert.FromBase64String(value[0]));
      }
  }
  return null;

}

用法Usage

在任何 API 管理策略中，表达式都可用作属性值或文本值（除非策略引用另行指定）。Expressions can be used as attribute values or text values in any API Management policies (unless the policy reference specifies otherwise).

策略表达式中允许的 .NET Framework 类型.NET Framework types allowed in policy expressions

下表列出了策略表达式中允许的 .NET Framework 类型及其成员。The following table lists the .NET Framework types and their members that are allowed in policy expressions.

上下文变量Context variable

在每个策略表达式中均可隐式使用名为 context 的变量。A variable named context is implicitly available in every policy expression. 其成员提供与 \request 相关的信息。Its members provide information pertinent to the \request. 所有 context 成员均为只读的。All of the context members are read-only.

后续步骤Next steps

有关如何使用策略的详细信息，请参阅：For more information working with policies, see:

• API 管理中的策略Policies in API Management
• 转换 APITransform APIs
• 策略参考，获取策略语句及其设置的完整列表Policy Reference for a full list of policy statements and their settings
• 策略示例Policy samples