C# - Putting Datatable Comma Separated Values In A String?

My datatable consists of a column named "ID". The no. of values in this column varies.Sometimes this Datatable fetches 3 IDs in that ID column, sometimes 2. Now if for example, my datatable has three values as 1,2,3. What I want is to put these three values in a string and separate them by commas as folows:-

string test= "1,2,3";

If Datatable has 2 values, then string should be as follows:-

string test= "1,2";

I did try but in vain.

edit:-

DataTable dt=new DataTable;
dt = obj.GetIDs();
for (int i = 0; i < dt.Rows.Count; i++)
{
string test= "What should be here????";
}
edit 2
foreach(DataRow dr in dt.Rows)
{
string str = str + "," + Convert.ToString(dr("ID"));
}


@Rajeev ::Tried this..it says dr is a variable but used as a method. What's wrong?