public void TestConcatenate()
{
var df1 = new DataFrame();
df1.AddColumn("x", new int[] { 1, 2 });
df1.AddColumn("y", new string[] { "a", "b" });
var df2 = new DataFrame();
df2.AddColumn("x", new int[] { 3, 4 });
df2.AddColumn("z", new string[] { "c", "d" });
var result = DataFrame.Concatenate(df1, df2);
CollectionAssert.AreEqual(new string[] { "x", "y", "z" }, result.ColumnNames);
Assert.AreEqual(4, result[0].Count);
Assert.AreEqual(4, result[1].Count);
Assert.AreEqual(4, result[2].Count);
CollectionAssert.AreEqual(new object[] { 1, 2, 3, 4 }, result[0].ToObjectArray());
CollectionAssert.AreEqual(new object[] { "a", "b", null, null }, result[1].ToObjectArray());
CollectionAssert.AreEqual(new object[] { null, null, "c", "d" }, result[2].ToObjectArray());
}