Introduction

Yes, you read the title right, this blog is about evaluating IF condition. You might be wondering what about IF, even novice developer with no experience knows about it.

Allow me to explain a specific scenario that helps us understand it’s behavior in Logic Apps, it might blow your mind.

Some of us come from years of development experience, and at times we like to skill up ourselves to various other technologies, which leaves us with a mindset based on our past development experience and programming habits, which we gained over the years. When clients requirements are approached based on these backgrounds, we expect the code to work with the certain flow and these are where rules are broken while using IF condition in Azure Logic Apps.

Understanding JSON expression

JSON expression evaluates string to JSON object using syntax as shown below

json({"Person":{"Name": "Simpson"}})  evaluates to var name = Person.Name as Simpson

But, the same json(null), throws an error (important), avoid where possible.

Understanding IF condition

IF don’t need any special introduction, we know how it works. As we know, it has two code blocks that are evaluated based on the condition and falls to one of the blocks. As applies to Logic Apps and below is the syntax for it.

@if("condition","true","false")

To understand IF better, let’s also look into @equals(), it is a simple expression that returns true or false based on the given input and provided comparing value.

Example 1

Below is just an example, please ignore simple equality condition.
@if(equals(1,1),"true1","false1")
Result: true1

Example 2

@if(equals(1,2),"true1","false1")
Result: false1

Now, let us take our person JSON and understand it.
@if(equals(1,1),"Merge",json({"Person":{"Name": "Homer"}}) ['Name'])
Result: Merge

and similarly when the comparison is not equal

@if(equals(1,2),"Merge",json({"Person":{"Name": "Homer"}}) ['Name'])
Result: Homer

Now, recall that IF falls to one of the code blocks and returns. But in case of Azure Logic Apps, it evaluates both the code blocks and returns one code block result, that it falls into.

Here is the proof

For Example, if I do something like below, it should result as “Merge”, but it actually throws an error. According to current Logic Apps, this is the current behavior.

@if(equals(1,1),"Merge",json(null) ['Name'])
Result: error

And similarly when not equal

@if(equals(1,2),"Merge",json(null) ['Name'])
Result: error

The above examples imply that Logic App evaluates both the code blocks and returns one.

Actual error thrown is as below from real logic app

InvalidTemplate. Unable to process template language expressions in action ‘Compose’ inputs at line ‘1’ and column ‘1525’: ‘The template language function ‘json’ expects its parameter to be a string or an XML. The provided value is of type ‘Null’. Please see https://aka.ms/logicexpressions#json for usage details.’.

Category:
Azure Platform
Tags:
,

Join the conversation! 2 Comments

  1. […] Azure Logic App – Evaluating IF condition with the help of JSON expression by passing null […]

    Reply
  2. […] Azure Logic App – Evaluating IF condition with the help of JSON expression by passing null […]

    Reply

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: