how can I manipulate variables in the policy expression azure api management

Question

how can I manipulate variables in the policy expression, I want to add/ change a variable value, but the variables is in a immutable map(read-only)

e.g

<policies>  
<inbound>  
<base />  
<set-variable name="body" value="@((context.Request.Body == null) ? "" : (string)context.Request.Body.As<string>(preserveContent: true))" />  
               <set-variable name="errorMessage" value="@("")" />  
            <set-variable name="errorMessage" value="" />  
<choose>  
<when condition="@{  
  
                 //do some stuff  
                var errorMessage = "set an error here ";  
  
              //add the error to the global variable so I can access it on the response.  
                context.Variables["errorMessage"]=errorMessage;  
                      
                      
                return true;  
            }">  
<return-response>  
<set-status code="401" reason="Unauthorized" />  
<set-body>@{  
                    return new JObject(  
                    new JProperty("title", "any error"),  
                    new JProperty("status", "any status"),  
                    new JProperty("detail", (string)context.Variables.GetValueOrDefault<string>("errorMessage"))).ToString();  
                    }</set-body>  
</return-response>  
</when>  
</choose>  
</inbound>  
  
</policies>  

Answer 1

@Claudio Resende Thank you for reaching out to Microsoft Q&A. Based on your statement, you are looking to modify variable value inside the policy expression.

Unfortunately, that is not possible. Context variable members are read-only as mentioned in docs, and this cannot be changed.

However, using set-variable policy you can set the value of the variable with policy expression like below, but you might have to repeat the logic inside condition as well. Refer Set variable for more info about this policy.

<set-variable name="errorMessage" value="@{  
            //do some stuff  
            var errorMessage = "set an error here ";  
            return errorMessage;  
        }" />  
        <choose>  
            <when condition="@{  
                //do some stuff                    
                return true;  
                }">  
                <return-response>  
                    <set-status code="401" reason="Unauthorized" />  
                    <set-body>@{  
                                return new JObject(  
                                new JProperty("title", "any error"),  
                                new JProperty("status", "any status"),  
                                new JProperty("detail", (string)context.Variables.GetValueOrDefault<string>("errorMessage"))).ToString();  
                                }</set-body>  
                </return-response>  
            </when>  
        </choose>  

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