Dynamics AX Mandatory Enum field cannot be set correctly through UI

Can anyone explain the following behaviour to me?

When a field type in an AX Table is set to an Enum, you can select any of the Enum values as a value for the field.

But if you make the开发者_如何学JAVA field Mandatory, you can no longer select the first Enum value in the list through the user interface.

Obviously this can be worked around by not making the field Mandatory. I am looking for an explanation of this bizarre behaviour.

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AX does not have a null value concept. Instead the following values are considered "not entered" by defintion:

• string: blank
• int and int64: 0 (zero)
• enum: 0 (typically the first value)
• date: 01\01\1900 (displays as blank)

For new base enums make a blank zero enum value (by convention name it None). This will make the use of mandatory fields possible for this enum type.

Also have a look on this: Mark mandatory fields on form, if not filled with valid value

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You're saying "if you make the field Mandatory, you can no longer select the first Enum value in the list through the user interface" - this is exactly what the Mandatory property does for enums: prevents you from using a zero value. E.g. if you make NoYesId mandatory you'll be able to enter only Yes because No would no longer be allowed - why would you need it on the form then?

Please also note that from a user perspective it isn't necessarily clear what enum value is zero, so if it didn't work the way it works, understanding what value is not allowed when the enum is mandatory could be tricky.