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Before we look at UDF's in detail I should point out that UDF's do not have the same flexibility as a standard Procedure. a UDF cannot alter the structure of a Worksheet, such as change the Worksheet name, turn off gridlines, protect the Worksheet etc. They cannot change a physical characteristic of a cell, including the one that houses the UDF. So we cannot use a UDF to change the font colour, background colour etc of any cell. They cannot be used to try and change any part of another cell in any way at all. This means a UDF cannot place a value into any other cell except the cell housing the UDF. A UDF cannot use many of Excels built in features such as AutoFilters, AdvancedFilters, Find, Replace to name but a few!  
 
 
Для примера попробуйте выполнить пару макросов.  
 
Function UDF(rng As Range)              
   Call ttt  
   UDF = 5  
End Function  
 
Sub ttt()  
   Cells(1, 1) = 3  
End Sub