# UDT Input Parameter be an Expression?

**URL:** <https://forum.inductiveautomation.com/t/udt-input-parameter-be-an-expression/115245>\
**Category:** Ignition\
**Tags:** perspective, scripting, ignition81\
**Created:** [April 24, 2026, 7:01pm UTC](https://forum.inductiveautomation.com/t/udt-input-parameter-be-an-expression/115245 "2026-04-24T19:01:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![drewdin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/drewdin/32/3363_2.png) [@drewdin](https://forum.inductiveautomation.com/u/drewdin)\
**Post date:** [April 24, 2026, 7:01pm UTC](https://forum.inductiveautomation.com/t/udt-input-parameter-be-an-expression/115245/1 "2026-04-24T19:01:05Z")

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I have a UDT that I want to have an input parameter be an expression. The goal is to have a three tag expression from another tag provider be the input. Is this possible and is it bad practice to do it this way? I don't see an easy way to make the input an expression and I was curious how you have done it or would do it?

This tag dictates the steps it should take next, I was hoping to keep things in one place by having an input parameter of an expression and a proceed tag be true or false based on the expression.

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**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [April 24, 2026, 7:07pm UTC](https://forum.inductiveautomation.com/t/udt-input-parameter-be-an-expression/115245/2 "2026-04-24T19:07:02Z")

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> [@drewdin](#):
>
> Is this possible

It is possible with scripts. Receive tag events at your driving expression tag and write to the UDT instance parameters.

> [@drewdin](#):
>
> is it bad practice

I've argued elsewhere in the forum that it is a bad practice. You are triggering **tag reconfiguration** based on live data. There are a few applications where it is difficult to avoid (redundant driver paths, for example), but every tag reconfig is a brief halt and opportunity for data loss/races.

Tags should not be reconfigured at runtime from live data. Full stop.

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<div class="post-metadata">

**Author:** ![drewdin](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/drewdin/32/3363_2.png) [@drewdin](https://forum.inductiveautomation.com/u/drewdin)\
**Post date:** [April 24, 2026, 7:29pm UTC](https://forum.inductiveautomation.com/t/udt-input-parameter-be-an-expression/115245/3 "2026-04-24T19:29:22Z")

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Thanks or the input, i was thinking it was no different that a standard expression tag with the exception that its a UDT. I was just trying to prevent from writing scripts between two tag providers, the thought was bring everything into one UDT and then I know my paths and what my values are regardless of the tag provider and tag path i might need to read.

I'll rethink the process and see what i come up with. Thanks!