Join 2 DataTables on dynamic number of columns

Since you are using LINQ to Objects, there is no need to use expression trees. You can solve your problem with a custom equality comparer.

Create an equality comparer that can compare equality between two DataRow objects based on the values of specific columns. Here is an example:

public class MyEqualityComparer : IEqualityComparer<DataRow>
{
    private readonly string[] columnNames;

    public MyEqualityComparer(string[] columnNames)
    {
        this.columnNames = columnNames;
    }

    public bool Equals(DataRow x, DataRow y)
    {
        return columnNames.All(cn => x[cn].Equals(y[cn]));
    }

    public int GetHashCode(DataRow obj)
    {
        unchecked
        {
            int hash = 19;
            foreach (var value in columnNames.Select(cn => obj[cn]))
            {
                hash = hash * 31 + value.GetHashCode();
            }
            return hash;
        }
    }
}

Then you can use it to make the join like this:

public class TwoRows
{
    public DataRow Row1 { get; set; }
    public DataRow Row2 { get; set; }
}

private static List<TwoRows> LeftOuterJoin(
    List<string> joinColumnNames,
    DataTable leftTable,
    DataTable rightTable)
{
    return leftTable
        .AsEnumerable()
        .GroupJoin(
            rightTable.AsEnumerable(),
            l => l,
            r => r,
            (l, rlist) => new {LeftValue = l, RightValues = rlist},
            new MyEqualityComparer(joinColumnNames.ToArray()))
        .SelectMany(
            x => x.RightValues.DefaultIfEmpty(rightTable.NewRow()),
            (x, y) => new TwoRows {Row1 = x.LeftValue, Row2 = y})
        .ToList();
}

Please note that I am using method syntax because I don't think that you can use a custom equality comparer otherwise.

Please note that the method does a left outer join, not a full outer join. Based on the example you provided, you seem to want a full outer join. To do this you need to do two left outer joins (see this answer). Here is how the full method would look like:

private static DataTable FullOuterJoin(
    List<string> joinColumnNames,
    DataTable pullX,
    DataTable pullY)
{
    var pullYOtherColumns =
        pullY.Columns
            .Cast<DataColumn>()
            .Where(x => !joinColumnNames.Contains(x.ColumnName))
            .ToList();

    var allColumns = 
        pullX.Columns
            .Cast<DataColumn>()
            .Concat(pullYOtherColumns)
            .ToArray();

    var allColumnsClone =
        allColumns
            .Select(x => new DataColumn(x.ColumnName, x.DataType))
            .ToArray();

    DataTable joinedTable = new DataTable();

    joinedTable.Columns.AddRange(allColumnsClone);

    var first =
        LeftOuterJoin(joinColumnNames, pullX, pullY);

    var resultRows = new List<DataRow>();

    foreach (var item in first)
    {
        DataRow newRow = joinedTable.NewRow();
        foreach (DataColumn col in joinedTable.Columns)
        {
            var value = pullX.Columns.Contains(col.ColumnName)
                ? item.Row1[col.ColumnName]
                : item.Row2[col.ColumnName];

            newRow[col.ColumnName] = value;
        }
        resultRows.Add(newRow);
    }

    var second =
        LeftOuterJoin(joinColumnNames, pullY, pullX);

    foreach (var item in second)
    {
        DataRow newRow = joinedTable.NewRow();
        foreach (DataColumn col in joinedTable.Columns)
        {
            var value = pullY.Columns.Contains(col.ColumnName)
                ? item.Row1[col.ColumnName]
                : item.Row2[col.ColumnName];

            newRow[col.ColumnName] = value;
        }
        resultRows.Add(newRow);
    }

    var uniqueRows =
        resultRows
            .Distinct(
                new MyEqualityComparer(
                    joinedTable.Columns
                        .Cast<DataColumn>()
                        .Select(x => x.ColumnName)
                        .ToArray()));

    foreach (var uniqueRow in uniqueRows)
        joinedTable.Rows.Add(uniqueRow);


    return joinedTable;
}

Please note also how I clone the columns. You cannot use the same column object in two tables.