Getting an Integer from a property in REST request body

As {#Project#polnr} is noticed, assuming that a custom Project property value is being used.

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the only way to set properties is AFAIK setPropertyValue(string, string).

Yes, they always store the values in string format.

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Is there a way to get an integer into the rest request body? I've looked everywhere and it looks like it's impossible, but it seems like such a small and easy thing.

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Yes, it is indeed possible.

Here is the sample JSON payload which demonstrate how to use both string, and number respectively using the same project property named "ABC" with the value 10

{
	"value" : "${#Project#ABC}"
	"number" : ${= (context.testCase.testSuite.project.getPropertyValue('ABC') as int)}
}

And here is the snapshot of RAW request for the above.

So, you can use in the similar fashion, just use your property.

May be, "number" : ${#Project#ABC} might also works (note no quotes around the value), please check it out.