Dapper2.cs

// Whatever shared state you need, the root of the universe
public class DapperContext
{
   private QueryCache _cache = new QueryCache();
   
   public static DapperContext Instance = new DapperContext();
   
   public Task<IEnumerable<dynamic>> QueryAsync(IDbConnection cnn, string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
   {
       // Real logic happens here
   }
}

// Extension methods act as sugar for accessing the "static" default query instance.
public static partial class SqlMapper
{
   public static Task<IEnumerable<dynamic>> QueryAsync(this IDbConnection cnn, string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null)
   {
      return DapperContext.Instance.QueryAsync(conn, sql, param, transaction, commandTimeout, commandType);
   }
}

// You now write unit tests against the DapperContext directly