When merging a DataTable of fresh data into my DataSet, the expression
columns in my other table that links to that table (using the
PARENT(RELATION).FIELD syntax) are not updated. The only way I have managed
to get it working is using an ugly workaround whereby I re-set the
DataColumn's expression and re-set the DataGrid's source straight after the
merge.

The following code should illustrate my problem. Create a new Form, drop a
DataGrid and two Buttons on it and paste the following code in the
appropriate places.

private DataSet _Set;
private DataTable _Parent;
private DataTable _Child;

private void button1_Click(object sender,EventArgs e)
{
   _Parent = new DataTable("PARENT");
   _Parent.PrimaryKey = new DataColumn[]
       {_Parent.Columns.Add("ID")};
   _Parent.Columns.Add("TEXT1");
   _Parent.Rows.Add(new object[] {"1","10"});
   _Parent.Rows.Add(new object[] {"2","20"});
   _Parent.Rows.Add(new object[] {"3","30"});

   _Child = new DataTable("CHILD");
   _Child.PrimaryKey = new DataColumn[]
       {_Child.Columns.Add("ID")};
   _Child.Columns.Add("TEXT2");
   _Child.Rows.Add(new object[] {"1","one"});
   _Child.Rows.Add(new object[] {"2","two"});
   _Child.Rows.Add(new object[] {"3","three"});

   _Set = new DataSet();
   _Set.Tables.Add(_Parent);
   _Set.Tables.Add(_Child);
   _Set.Relations.Add("PARENTCHILD",
       _Parent.PrimaryKey[0],_Child.PrimaryKey[0]);
   _Child.Columns.Add("TEXT1",typeof(string),
       "PARENT(PARENTCHILD).TEXT1");

   dataGrid.DataSource = _Child;
}

private void button2_Click(object sender, System.EventArgs e)
{
   DataTable newParent = _Parent.Clone();
   newParent.Rows.Add(new object[] {"1","ten"});
   newParent.Rows.Add(new object[] {"2","twenty"});
   newParent.Rows.Add(new object[] {"3","thirty"});
   _Set.Merge(newParent);

   //the following 4 lines make up my ugly workaround.
   DataColumn col = _Child.Columns["TEXT1"];
   col.Expression = col.Expression;
   dataGrid.DataSource = null;
   dataGrid.DataSource = _Child;
}

Nato