Error - The field 'FieldName' on the extension 'ExtensionName.Extension' already exists. This extension cannot be applied to the base element.

In a recent D365O development, I came across this error when moving fields from a model to another model.

The field 'FieldName' on the extension 'ExtensionName.Extension' already exists. This extension cannot be applied to the base element.

The scenario is that while doing testing on development, some fields have been moved to the standard element (overlayered it by "Customize") [Eg. This is in ModelA]. When the test is done, these field are then moved back to the extension [Eg. This is in ModelB]. Unfortunately after moved it back to the extension, this error occur during the build of the solution.

This is what has been derived:

• When the fields are moved to the base element, a build is done and the assembly is updated
• After the testing is done, the change on the base element is deleted (deleting the whole overlayered element)
• Then used the "Get latest" to restore the extension (it is where the fields are originally created)
• Then build solution again and error occur
• It is suspected that after the change in base element is deleted, the reference to the assembly still there. Hence, when building the solution of the extension, it found that the same fields already exist in the base element.

Solution

1. In Visual Studio, go to Dynamics 365 > Build models
2. Select only the model where the base element sit (Eg. Application suite)
3. Build it
4. Then build the solution again