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Things can be on layer '0'.
The point is it has a special purpose for block creation. Nothing else should be on layer 0, because that's just bad drawing organization.
Any entity placed on layer 0 in a block definition will assume the layer properties of whatever layer the block reference is placed on. That is, any properties that were not explicitly set to the entity.
So, I can create some widget. Place it on layer 0, color bylayer, linetype bylayer...
draw a centerline on it. on Layer 0, color red, linetype centerline...
attach some gadget to it (I don't know... a J-Box for wiring maybe) layer 0, color ByBlock, linetype ByBlock.
Now make all that into a block and place several instances on different layers with different color and linetype settings.
Then take a couple of the blocks and change the color or linetype of the block itself.
The result is:
the centerline will always be color red, linetype centerline in all instances.
the widget will always follow the layer settings in all instances
the gadget will follow the settings given to the block reference (which can be bylayer making it the same as the widget, or specific color/Ltype, making the widget and the gadget appear differently.
The point is it has a special purpose for block creation. Nothing else should be on layer 0, because that's just bad drawing organization.
Any entity placed on layer 0 in a block definition will assume the layer properties of whatever layer the block reference is placed on. That is, any properties that were not explicitly set to the entity.
So, I can create some widget. Place it on layer 0, color bylayer, linetype bylayer...
draw a centerline on it. on Layer 0, color red, linetype centerline...
attach some gadget to it (I don't know... a J-Box for wiring maybe) layer 0, color ByBlock, linetype ByBlock.
Now make all that into a block and place several instances on different layers with different color and linetype settings.
Then take a couple of the blocks and change the color or linetype of the block itself.
The result is:
the centerline will always be color red, linetype centerline in all instances.
the widget will always follow the layer settings in all instances
the gadget will follow the settings given to the block reference (which can be bylayer making it the same as the widget, or specific color/Ltype, making the widget and the gadget appear differently.
Dave O. 
Based on your question in the beginning about the ability to do this as a side database, combined with the fact that you indicated a very large number of drawings needing searched, I worked up an example of how to do it, as a side database.
It would be much simpler if you opened the drawings into the editor, because you could use a selection filter to get all entities on layer 0, however, if you tried to do it that way on 5000 drawings it might take about a week. (depending somewhat on the drawing content)
Since you can't use the editor methods on a side database, you have to loop through both the ModelSpace and the PaperSpace opening every entity you find to check it's layer property.
I had a routine for copying block definitions from a side database to the current database, so I hacked up a copy of it (attached) to make a simple text report (In your MyDocuments Folder) of all entities found to be on layer 0 (with one exception, next paragraph). You pass it a string collection of filenames to process.
Tony, If you have any interest, you may note that my code here does not use the IsUsed property of the layer to short circuit the process. I did try to do that, thinking that at least that way if a drawing is clean, you don't have to loop the whole MS and PS to find out, but I found that it always returned true. (At first, I was not aware of the need for calling GenerateUsageData first, but even after it still always returned true) I believe through my limited efforts to test this routine, I determined that the root PaperSpace Viewport (the one that exists even of there are no mview created viewports) is reporting itself on layer 0, therefore IsUsed may work fine for other layers but could never be false for layer 0.
Of course this simple text report does you little good, but you would just insert whatever code you desire at the points where I am writing to the report, and there is one more detail which may need some work. I think if the drawing has multiple layouts there is more work to be done to catch all the paperspaces.
Anyone got any input on that? I used the SymbolUtilityServices.GetBlockPaperSpaceID(db) method to get the paperspace entities. Is that a 'Parent' PaperSpace to the multiple 'child' layout PaperSpaces, or will it represent the most recently activated Layout?
It would be much simpler if you opened the drawings into the editor, because you could use a selection filter to get all entities on layer 0, however, if you tried to do it that way on 5000 drawings it might take about a week. (depending somewhat on the drawing content)
Since you can't use the editor methods on a side database, you have to loop through both the ModelSpace and the PaperSpace opening every entity you find to check it's layer property.
I had a routine for copying block definitions from a side database to the current database, so I hacked up a copy of it (attached) to make a simple text report (In your MyDocuments Folder) of all entities found to be on layer 0 (with one exception, next paragraph). You pass it a string collection of filenames to process.
Tony, If you have any interest, you may note that my code here does not use the IsUsed property of the layer to short circuit the process. I did try to do that, thinking that at least that way if a drawing is clean, you don't have to loop the whole MS and PS to find out, but I found that it always returned true. (At first, I was not aware of the need for calling GenerateUsageData first, but even after it still always returned true) I believe through my limited efforts to test this routine, I determined that the root PaperSpace Viewport (the one that exists even of there are no mview created viewports) is reporting itself on layer 0, therefore IsUsed may work fine for other layers but could never be false for layer 0.
Of course this simple text report does you little good, but you would just insert whatever code you desire at the points where I am writing to the report, and there is one more detail which may need some work. I think if the drawing has multiple layouts there is more work to be done to catch all the paperspaces.
Anyone got any input on that? I used the SymbolUtilityServices.GetBlockPaperSpaceID(db) method to get the paperspace entities. Is that a 'Parent' PaperSpace to the multiple 'child' layout PaperSpaces, or will it represent the most recently activated Layout?
Dave O.
That's crap. It does exactly what I said it does.
It will find all Graphical references to Layer 0 (the specific problem presented) other than my question about multiple layouts.
It does use Using as much as is neccessary, and has plenty of exception handling.
I missed putting the DB.closeinput and dispose into a finally... sue me. If it had actually failed once I would have caught that.
And for Tim's benefit I'll mention that in the process of trying to figure out what was up with the viewports I accidentally made it only report the viewports from paperspace, so needs an Else in the If.
It will find all Graphical references to Layer 0 (the specific problem presented) other than my question about multiple layouts.
It does use Using as much as is neccessary, and has plenty of exception handling.
I missed putting the DB.closeinput and dispose into a finally... sue me. If it had actually failed once I would have caught that.
And for Tim's benefit I'll mention that in the process of trying to figure out what was up with the viewports I accidentally made it only report the viewports from paperspace, so needs an Else in the If.
Dave O.
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