New Post: How to Query STRING VALUES from Dataframe

This is because the columns of the data frames are vectors that are factors. Even in pure R coercion of factors can be a stumbling block.

I suggest you access the data frame by columns, like the following which gives the expected answer.

         engine.Evaluate("data(mpg, package='ggplot2')");
         var mpg = engine.Evaluate("mpg").AsDataFrame();
         var manufacturer = mpg[0].AsFactor().GetFactors();
         Assert.AreEqual("audi", manufacturer[0]);  // Vorsprung durch technik

I'll open a feature request to see whether R.NET can match the behavior of R.