How to add file type validation , when trying to add existing item in solution explorer of visual studio experimental instance?

Is there any visual studio event to call at the time of "add existing Item" to validate?

I am afraid that there is not a specific Visual Studio event that fires before items are added during the "Add Existing Item" process.

If you want to validate file type before adding files to projects/solution,here is a workaround:

you can implement your custom validation logic by creating a separate menu like "validate file type".

1.Create a custom menu item in your extension.

https://learn.microsoft.com/en-us/visualstudio/extensibility/creating-an-extension-with-a-menu-command?view=vs-2022

2.Validate the file type based on its file type. If the file type is invalid, show a message box with an appropriate error message. Otherwise, add the file to the project programmatically.

string fileExtension = Path.GetExtension(fileName).ToLower();
if (Array.IndexOF([".pdf", ".doc", ".docx"],fileExtension) == -1 )
{
 // error message
}else{
// add file to project
}

Hope it can help you.