Using the Visual Studio extension

Install the Visual Studio Extension in VS2019+ (even free community edition is fine)
Open your solution in Visual Studio

Converting whole project/solution

Right click the solution or project node to convert in the solution explorer and select "Convert to VB" or "Convert to C#".
The Output window will show progress - it may seem to pause for several minutes at times on large projects.
At the end of the process you'll be given the choice to automatically update any references (from non-converted projects and the solution) to reference the now converted projects. Cancelling the process at this point will leave all original files intact, and converted projects/files on disk to be manually inspected.
When the conversion finishes, the Output window will show a summary of the conversion's success, and one of the converted files will be opened as an example.
Once you're happy with the resulting conversion, you may want to delete all files with (for example) ".vb" in their name recursively from a folder:
- In powershell: Get-ChildItem -Recurse *.vb* | Remove-Item ⚠️ Deletes files recursively!
- Windows command line: DEL /S /Q *.vb* ⚠️ Deletes files recursively!

Converting a single file

Right click the .vb or .cs file to convert in the solution explorer and select "Convert to VB" or "Convert to C#".
The Output window will show progress - it may seem to pause for several minutes since related projects must be compiled.
When the conversion finishes, the Output window will show a summary of the conversion's success.
The converted file will be saved to disk adjacent to the original file (but with the appropriate converted file extension), and opened in a code window. It won't be part of any project or solution.

Converting a snippet

Open a .vb or .cs file containing code to be converted.
Highlight the relevant text.
Right click on the highlighted text and select "Convert to VB" or "Convert to C#".
The Output window will show progress - it may seem to pause for several minutes since related projects must be compiled.
When the conversion finishes, the Output window will show a summary of the conversion's success.
The converted snippet will be saved to disk adjacent to the original file (but with the appropriate converted file extension), and opened in a code window. It won't be part of any project or solution.

Selected text conversion context menu

Alternatively you can use the online snippet converter: https://codeconverter.icsharpcode.net/ However, it is less accurate due to the lack of project context.

Cancelling

Closing Visual Studio, or starting a new conversion cancels the in-progress conversion