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ProgrammingAutomator/Integrator with COM/Office

What are late binding and early binding in Visual Basic, what are the advantages and disadvantages of each approach, and how to use them correctly?

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Answer

Early Binding is a method where the type of the object is known at compile time. We specify a concrete type when declaring a variable:

Dim app As New Excel.Application ' Early Binding

Advantages:

  • IntelliSense support
  • Type checking at compile time (fewer runtime errors)
  • Better performance

Late Binding is when the object type is determined only at runtime. It is usually used through Object:

Dim app As Object
Set app = CreateObject("Excel.Application")
app.Visible = True

Advantages:

  • No need for an explicit reference to an external library
  • Flexibility when working with different versions of COM objects

Disadvantages of late binding:

  • No IntelliSense
  • Error checking only at runtime
  • Lower performance

Trick Question

Does it always make sense to use late binding for working with COM objects?

Answer: No, only if you are unsure about the required version of the library being present on all target machines. If version control is safe and all dependencies are known — early binding is faster, more reliable, and preferable. Unjustified use of late binding deteriorates performance and the handling of static typing.

Examples of real mistakes due to ignorance of the subtleties of the topic

Story

Automating reporting through Excel: due to late binding, a method name error was discovered only at the client site when the macro "broke" (typo in app.WorkbooksOpen instead of app.Workbooks.Open). If early binding had been used, the error would have appeared immediately.

Story

When migrating from an old to a new version of the Word library, the application started throwing exceptions — new fields were not supported. Early binding blocked the update; we had to switch to late binding, but some functions stopped working due to signature mismatches.

Story

A ready library was delivered to production, but the required COM dependency was forgotten. In early binding, the project did not start at all (registration error), while in late binding — it worked, but "crashed" on every object call, requiring additional logging.