C#: Merge Datarows in the datatable

You can group by multiple properties by using an anonymous type:

var result = list1.GroupBy(x=> new {x.ID, x.VERSION}).Select(
    item => new Example
    {
        ID = item.Key.ID, 
        VERSION = item.Key.VERSION,
        ENTITY = string.Join("/", item.Select(c=>c.ENTITY))
    });

Afterwards select the appropriate properties and feed them into a new object of the type you need.

Output:

EDIT:

In a DataTable you need to access the columns via the [ ] operator but the principle of the grouping remains the same:

Examplary DataTable:

DataTable table = new DataTable();
table.Columns.Add("ID", typeof(int));
table.Columns.Add("VERSION", typeof(string));
table.Columns.Add("ENTITY", typeof(string));

table.Rows.Add(1, "01", "A01");
table.Rows.Add(1, "01", "A02");
table.Rows.Add(2, "01", "A01");
table.Rows.Add(2, "01", "A02");

The grouping:

var result = table.AsEnumerable().GroupBy(x => new { ID = x["ID"], VERSION = x["VERSION"]}).Select(
    item => new Example
    {
        ID = (int)item.Key.ID,
        VERSION = (string)item.Key.VERSION,
        ENTITY = string.Join("/", item.Select(c => c["ENTITY"]))
    });