# Writing a tag value to all UDT instances for memory tag

**URL:** <https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584>\
**Category:** Ignition\
**Tags:** perspective\
**Created:** [May 8, 2020, 5:30pm UTC](https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584 "2020-05-08T17:30:12Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![tmahaffey](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/tmahaffey/32/30949_2.png) [@tmahaffey](https://forum.inductiveautomation.com/u/tmahaffey)\
**Post date:** [May 8, 2020, 5:30pm UTC](https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584/1 "2020-05-08T17:30:12Z")

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I have a UDT with a memory string tag. When I update the value for that string, I would expect all instances of the UDT to have that same value for that. But, it looks like all the instances still have a nothing in for that tag value. Any idea why?

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**Author:** ![tmahaffey](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/tmahaffey/32/30949_2.png) [@tmahaffey](https://forum.inductiveautomation.com/u/tmahaffey)\
**Post date:** [May 11, 2020, 12:49pm UTC](https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584/2 "2020-05-11T12:49:31Z")

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I think the problem I was having here is if you already have instances of a UDT that you want to change a memory tag value for, changing the value inside the UDT definition doesn’t retroactively change the tag values for the instances.

What I did discover is that any new tags you create under that instance will have that value for the memory tag inside of the definition.

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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:** [May 11, 2020, 12:54pm UTC](https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584/3 "2020-05-11T12:54:22Z")

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This is intended behavior for a memory tag. Its purpose is to remember what is written to it. If you need a constant tag defined by the UDT, use an expression tag with a constant expression. Changing _that_ in your UDT definition will change all instances.

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<div class="post-metadata">

**Author:** ![tmahaffey](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/tmahaffey/32/30949_2.png) [@tmahaffey](https://forum.inductiveautomation.com/u/tmahaffey)\
**Post date:** [May 11, 2020, 12:54pm UTC](https://forum.inductiveautomation.com/t/writing-a-tag-value-to-all-udt-instances-for-memory-tag/34584/4 "2020-05-11T12:54:50Z")

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thank you