Unable to add custom user property even if it does not exist - vsto

Question

I have implemented VSTO plug-in for Outlook 2016 / 2019. I have added one custom property. On production, for one user (this user has tried on different machines as well), whenever we try to add our custom property we get exception.
I am using following code snippet :--
UserProperty securedFlag = mailItem.UserProperties.Find("Custom.Secured", true);
int currentValue = -1;
if (securedFlag == null)
{
try
{
securedFlag = mailItem.UserProperties.Add("Custom.Secured", OlUserPropertyType.olInteger, false, OlFormatInteger.olFormatIntegerPlain);
}
catch(System.Exception ex)
{
DarkAddInEventLog.WriteException(ex, "Secure");
}
}
When we execute above code, it is throwing following exception :--
"A custom field with this name but a different data type already exists. Enter a different name."
I have also tried to search same property in non-custom properties by passing false (in second argument of find API) which also throws exception that this property does not exist. Therefore it seems confirmed that this property does not exist before.
Now I have 2 doubts :--
1- If this property does not exist, then why outlook is throwing this error ?
2- Same plug-in is working with other users but only one user is facing this issue. Is it related to some mailbox configuration ?

Answer 1

The error means a property with the same name has already been used in the same mailbox, it might not even be your property, but since all custom properties use the same GUID (PS_PUBLIC_STRINGS), you can run into this problem with duplicate names.
Your only option is to either use a different name (and make sure it is not generic enough to conflict with other properties created by some other apps - e.g. you can prefix all your properties with a unique prefix, such as "MyCompanyProp.") or avoid using UserProperties collection completely and set your custom properties using PropertyAccessor specifying the full DASL name of the named properties with your own custom GUID.