VSTO - Office Interop API Extensions

Available in .NET Framework 3.5.
These are a set of additional interop assemblies that can be referenced to make working with COM Interop easier.
These are installed as part of the VSTO Power Tools.
The Office Interop API Extensions are libraries that make working with optional parameters, variant types and parameterized properties easier when writing C# VSTO..

VSTO - C# Method Overloads

Eliminates the use of System.Type.Missing
Eliminates the use of Variant

VSTO - C# Optional Parameters

Creates a single argument class that contains all the necessary parameters.
Provides you with an equivalent to optional parameters
This functionality was added in Visual Studio 2010.
For more details, refer to Optional Parameters

VSTO - C# Parameteried Properties

Uses extension methods to remove the _get and _set methods.
This functionality was added in Visual Studio 2010.