>In a Grid object I can reference each columns by their idx number, what i was asking to myself is : how can I retrieve this number from a column ?
>I found properties like columnorder but it could change.

You can simply add that... in grid's .init,

for i=1 to this.columncount
   this.columns[i].addproperty("colindex",i)
endfor

Then for any column object you send as a parameter, this column object's index can be retrieved from its .colindex property. I've tried this and I'm seeing the proper values in the debugger.