Here is one my example of in-memory DataTables master - detail relationship.
Create table with code like:
DataTable dtMaster = new DataTable("Master");
In this example I will add Columns without specifying type:
dtMaster.Columns.Add("Id");
Create new row:
DataRow drMaster = dtMaster.NewRow();
drMaster["Id"] = 1;
Add row to DataTable:
dtMaster.Rows.Add(drMaster);
Now details table:
DataTable dtDetail = new DataTable("Details");
dtDetail.Columns.Add("Id");
dtDetail.Columns.Add("FkDetailMasterId");

DataRow drDetail = dtDetail.NewRow();
drDetail["Id"] = 1;
drDetail["FkDetailMasterId"] = 1;
dtDetail.Rows.Add(drDetail);
Create DataSet and add relation:
DataSet ds = new DataSet();

ds.Tables.Add("Master");
ds.Tables.Add("Details");

DataRelation relMasterDetail = new DataRelation("MyRelation"
	, ds.Tables["Master"].Columns["Id"]
	, ds.Tables["Details"].Columns["FkDetailMasterId"]
);

ds.Relations.Add(relMasterDetail);
Example download from here.