Hi

I think in this scenario, we would better write a COM Addin to set the
click call back function. Because when we automation Excel to tell the
Excel to add a context menu item, while setting the call back function in
the Winform's member function. Since Excel will try to call the event
handler, while the event handler function is in another process.
So I think you may try to write a COM Addin so that the event handler in
the C# Com-Addin will be in the same process of Excel(The Addin assembly
will be loaded into Excel process)
Here is a sample add-in code for your reference.
How To Build an Office COM Add-in by Using Visual C# .NET
http://support.microsoft.com/?id=302901

Best regards,

Peter Huang
Microsoft Online Partner Support

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