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Control flow

  • Стаття

APPLIES TO: All API Management tiers

Use the choose policy to conditionally apply policy statements based on the results of the evaluation of Boolean expressions. Use the policy for control flow similar to an if-then-else or a switch construct in a programming language.

Note

Set the policy's elements and child elements in the order provided in the policy statement. Learn more about how to set or edit API Management policies.

Policy statement

<choose>
    <when condition="Boolean expression | Boolean constant">
        <!— one or more policy statements to be applied if the above condition is true  -->
    </when>
    <when condition="Boolean expression | Boolean constant">
        <!— one or more policy statements to be applied if the above condition is true  -->
    </when>
    <otherwise>
        <!— one or more policy statements to be applied if none of the above conditions are true  -->
    </otherwise>
</choose>

The choose policy must contain at least one <when/> element. The <otherwise/> element is optional. Conditions in <when/> elements are evaluated in order of their appearance within the policy. Policy statement(s) enclosed within the first <when/> element with condition attribute equals true will be applied. Policies enclosed within the <otherwise/> element, if present, will be applied if all of the <when/> element condition attributes are false.

Elements

Element Description Required
when One or more elements specifying the if or ifelse parts of the choose policy. If multiple when elements are specified, they are evaluated sequentially. Once the condition of a when element evaluates to true, no further when conditions are evaluated. Yes
otherwise The policy snippet to be evaluated if none of the when conditions evaluate to true. No

when attributes

Attribute Description Required
condition The Boolean expression or Boolean constant to be evaluated when the containing when policy statement is evaluated. Yes

Usage

Examples

Modify request and response based on user agent

The following example demonstrates a set-variable policy and two control flow policies.

The set variable policy is in the inbound section and creates an isMobile Boolean context variable that is set to true if the User-Agent request header contains the text iPad or iPhone.

The first control flow policy is also in the inbound section, and conditionally applies one of two Set query string parameter policies depending on the value of the isMobile context variable.

The second control flow policy is in the outbound section and conditionally applies the Convert XML to JSON policy when isMobile is set to true.

<policies>
    <inbound>
        <set-variable name="isMobile" value="@(context.Request.Headers.GetValueOrDefault("User-Agent","").Contains("iPad") || context.Request.Headers.GetValueOrDefault("User-Agent","").Contains("iPhone"))" />
        <base />
        <choose>
            <when condition="@(context.Variables.GetValueOrDefault<bool>("isMobile"))">
                <set-query-parameter name="mobile" exists-action="override">
                    <value>true</value>
                </set-query-parameter>
            </when>
            <otherwise>
                <set-query-parameter name="mobile" exists-action="override">
                    <value>false</value>
                </set-query-parameter>
            </otherwise>
        </choose>
    </inbound>
    <outbound>
        <base />
        <choose>
            <when condition="@(context.Variables.GetValueOrDefault<bool>("isMobile"))">
                <xml-to-json kind="direct" apply="always" consider-accept-header="false"/>
            </when>
        </choose>
    </outbound>
</policies>

Modify response based on product name

This example shows how to perform content filtering by removing data elements from the response received from the backend service when using the Starter product. The example backend response includes root-level properties similar to the OpenWeather One Call API.

<!-- Copy this snippet into the outbound section to remove a number of data elements from the response received from the backend service based on the name of the product -->
<choose>
  <when condition="@(context.Response.StatusCode == 200 && context.Product.Name.Equals("Starter"))">
    <set-body>@{
        var response = context.Response.Body.As<JObject>();
        foreach (var key in new [] {"current", "minutely", "hourly", "daily", "alerts"}) {
          response.Property (key).Remove ();
        }
        return response.ToString();
      }
    </set-body>
  </when>
</choose>

Related content

For more information about working with policies, see: