One to One mapping and join two condition, how to set splitOn

[jjh01]

my sql is like this string sql = SELECT * FROM B JOIN A ON B.ColumnD = A.ColumnD AND B.ColumnE = A.ColumnE

class TestA
{
    public string ColumnD { get; set; }
    public string ColumnE { get; set; }
    public string ColumnF { get; set; }
    public string ColumnG { get; set; }
}

class TestB
{
    public string ColumnA { get; set; }
    public string ColumnB { get; set; }
    public string ColumnC { get; set; }
    public string ColumnD { get; set; }
    public string ColumnE { get; set; }
    public TestA Item { get; set; }
}

this is one to one mapping result

#IEnumerable<TestB> GetTestB()
{
    return Connection.Query<TestB, TestA, TestB>(
       sql,
        (b, a) =>
        {
            b.Item = a;
            return b;
        },
        splitOn: "ColumnD,ColumnE");
}

but the result TestA always NULL splitOn setting wrong?

[NickCraver]

The problem you're seeing here is almost certainly related to the duplicate columns of the same name. Can you show what a column list from that query looks like? The order coming back from the database is what matters, Dapper will look for the first ColumnD and the first ColumnE after that.