I've found and fixed this bug before. There are 2 other ways to handle it

Dapper has a static configuration for things like TypeMappers, and you can change the default mapping for string to use varchar with: Dapper.SqlMapper.AddTypeMap(typeof(string),System.Data.DbType.AnsiString). I typically set that in the app startup, because I avoid NVARCHAR almost entirely (to save the extra byte per character, since I rarely need anything outside of ANSI.)

Or, one could use stored procedures. Assuming you take in a parameter that is the correct type for your indexed predicate, the conversion happens once when the SPROC is called, not done by the optimizer in the query.

I still have mixed feelings about overuse of SQL stored procedures, but this is a classic example of where on of their benefits is revealed: they are a defined interface for the database, where DB-specific types can be handled instead of polluting your code with specifics about your DB.

(This is also a problem for other type mismatches like DateTime/Date, numeric types, etc.)

Sprocs are how I handle complex queries rather than embedding them in our server applications. It's definitely saved me from running into problems like this. And it comes with another advantage of giving DBAs more control to manage performance (DBAs do not like hearing that they can't take care of a performance issue that's cropped up because the query is compiled into an application)