Second Set variable policy in Azure APIM throws error.

rahul b 20

Hi everyone, From the send request policy response payload , I am trying to use the two set variable policy to get two different variables and set it in here in the policy. So for the first set variable policy I am able to extract it but not the second one azure is throwing exception.

"Expression evaluation failed. Object reference not set to an instance of an object.", 

        "Object reference not set to an instance of an object."

Please help me here.

JananiRamesh-MSFT 26,151 Reputation points

2023-09-22T18:27:36.3033333+00:00

Hi rahul b Thanks for reaching out. The error message "Object reference not set to an instance of an object" typically indicates that the policy is trying to access a null object or a property of an object that does not exist. This can happen if the policy is not correctly configured or if the response payload does not contain the expected data.

Ensure that the variable you’re trying to set in the second set-variable policy actually exists in the response payload of the send-request policy. You can use a tool like Fiddler or Postman to inspect the response payload and verify that it contains the expected data.

Make sure that the expression used to set the variable is correct and returns a valid value.

Also, please note that only certain types are allowed as variable values in Azure API Management expressions. If you’re trying to set a variable to an unsupported type, you might encounter errors

https://learn.microsoft.com/en-us/azure/api-management/set-variable-policy#allowed-types

Let me know if you have any further queries, I would be happy to help.
rahul b 20 Reputation points

2023-09-23T07:41:56.42+00:00
Hi @JananiRamesh-MSFT Thanks for replying me back. So let me explain the scenario in more detail. Lets assume the send-request policy executes and give the response back as below:
{

"result": [

{ "Key1" : "Value1", "Key2" : "Value2" }

]

}
So from this payload I am trying to access the value 1 and value 2 by using set variable policy.

<set-variable name="name1" value="@((String)((IResponse)context.Variables["response-variable-name"]).Body.As<JObject>()["result"][0]["Key1"])" />

<set-variable name="name2" value="@((String)((IResponse)context.Variables["response-variable-name"]).Body.As<JObject>()["result"][0]["Key2"])" />

So from the first set variable policy I am able to access the value1.
but when again i use the set variable policy to access value2 azure throws an exception.

Please help me here.

"Expression evaluation failed. Object reference not set to an instance of an object.", "Object reference not set to an instance of an object."

Accepted answer

Thomas Meads 1,586 Reputation points

2023-09-24T11:20:29.95+00:00
When accessing the Body element through context.Request.Body or context.Response.Body by default the body is cleared from the object. In your case it seems that you are accessing this through a variable however I suspect this is still the issue you are experiencing.

To fix this simply add preserveContent: true to your .As<> method:

<set-variable name="name1" value="@((String)((IResponse)context.Variables["response-variable-name"]).Body.As<JObject>(preserveContent: true)["result"][0]["Key1"])" /> <set-variable name="name2" value="@((String)((IResponse)context.Variables["response-variable-name"]).Body.As<JObject>(preserveContent: true)["result"][0]["Key2"])" />

For more info please see the notes on this article: https://learn.microsoft.com/en-us/azure/api-management/set-body-policy
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1 person found this answer helpful.
rahul b 20 Reputation points

2023-09-25T05:18:00.3333333+00:00

Thanks Thomas, it works fine now.
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