This is a C# Roslyn code analyzer that will help you replace var with the actual type name by using the Ctrl-. shortcut, or clicking the light bulb. Just put the text cursor inside of var in C# code to get started.
To control if var is underlined as an error, warning, etc. you can add a custom .ruleset file to your solution. Here is how to create one:
Here is how to use a custom .ruleset: