Great question! I’ve run into this exact pattern in legacy Office interop code too, so let me unpack the reasons behind it:

• Historical .NET Framework Limitations
  Back in older .NET versions (like .NET Framework 1.x and early 2.0), C# didn’t support optional parameters natively. When working with COM interop (like Office APIs), every parameter in the COM method signature was treated as required. To signal that you wanted to use the COM method’s default value for an optional parameter, you had to explicitly pass Missing.Value—there was no option to just omit the parameter. This was a workaround to bridge the gap between .NET’s strict parameter handling and COM’s optional parameters.

• Language Differences & Legacy Code Habits
  VB.NET has supported optional parameters since its early days, so VB developers could always omit optional interop parameters. But C# developers couldn’t until C# 4.0 (released with .NET Framework 4.0) added support for optional parameters and named arguments. A lot of older Office interop examples were written for C# before 4.0, so they stuck with Missing.Value. Even though modern C# allows omitting the parameter, the old pattern lingered in codebases and documentation.

• Explicit Readability
  Some developers prefer using Missing.Value intentionally to make it clear: "I’m choosing to use the default value here, not forgetting to pass a parameter." This can be helpful in long parameter lists where skipping a parameter might look like an oversight at first glance. It’s a style choice to make the code’s intent more explicit.

• Rare Edge Case Consistency
  For the vast majority of Office interop methods (like your Workbooks.Add example), omitting the parameter and passing Missing.Value behave identically. However, in a tiny number of niche COM methods, there might be subtle differences in how the runtime handles omitted parameters vs. explicitly passing Missing.Value (related to how COM marshals arguments). That said, these cases are extremely rare, and your test confirms the two approaches work the same for Workbooks.Add.

So in short: Missing.Value is a holdover from older C# versions, but modern code can safely omit the optional parameter without breaking functionality. The choice now mostly comes down to readability and maintaining consistency with existing codebases.

内容的提问来源于stack exchange，提问作者Balinth