How to use the "@(...)" expression in Azure API Policy?

The "@(...)" expression in Azure API Policy can be used to include a single C# expression as the attribute value or text value in most policy statements.

When a "@(...)" expression is included in a policy statement, the C# expression will be evaluated first and the result of the evaluation will be used in the policy statement.

For example, the following "set-variable" policy statement uses a "@(...)" expression as an attribute value to set a new variable to be used later:

<set-variable name="isMobile" value="@(Context.Request.Headers["User-Agent"].Contains("iPhone"))" />  

The following "trace" policy statements uses a "@(...)" expression as a text content to verify some properties from the built-in "Context" object:

<trace source="MyDebug">@("I am here!")</trace>
<trace source="MyDebug">@(context.Request.Url.Path)</trace>
<trace source="MyDebug">@(context.Request.MatchedParameters["id"])</trace>

Note that the C# expression inside "@(...)" must be XML friendly. For example, the "<" sign should be replaced with &lt;. But Azure takes the "<" sign too without any trouble:

<trace source="MyDebug">@( 1 &lt; 2 )</trace>
<trace source="MyDebug">@( 1 < 2 )</trace>

Also note that the "@(...)" expression can not be mixed with text as attribute values or text contents. The following examples are wrong uses of "@(...)" expressions:

<set-variable name="message" value="Hello @("world!")" />  
<trace source="MyDebug">Hello @("world!")</trace>
<trace source="MyDebug">@("Hello") world!</trace>

For more information on "@(...)" expressions, see API Management policy expressions Website.

⇒ Use '@(...)' Expressions in XML Attributes

⇐ 'trace' Policy Statement

⇑ Using Azure API Policy

⇑⇑ Microsoft Azure API Management Tutorials