Silverlight RIA (now WCF RIA) services allows developers to create n-tier applications on top of ASP.Net. There are numerous videos and articles on this new technology. This article assumes you know all about RIA Services, DomainDataSources and how FilterDescriptors can be used with ControlParameters to query data from the server side.
I recently needed to save the filtered result set and didn’t find this to be straight forward. I did come up with the solution described here.
Let’s assume you have a simple Contacts database and the user is able to filter by FirstName, LastName etc. Now let’s assume you need to be able to save the filtered result-set (or alternatively bulk modify the filtered result-set). The challenge is to retrieve the Contact id’s with the client side filters applied.
If we were to implement a save function as follows:
IQueryable SaveContacts()
{
var query = from contact in
Context.Contacts select contact.Id
// use the id's in query
}
This would return all the contacts in the database and not the filtered set. Thankfully, the DomainService class implements a virtual function where we can intercept the client side filter expression:
public virtual IEnumerable Query(QueryDescription queryDescription, out int totalCount);
We however have to merge this expression with the Context.Contacts IQueryable. This didn’t prove to be straight forward as you have to know a little bit about Expressions. I overrode the function like so to save the query for later retrieval:
IQueryable savedQuery;
public override System.Collections.IEnumerable Query(QueryDescription queryDescription,
out int totalCount)
{
if (queryDescription.Method.Name == "SaveContacts")
savedQuery = queryDescription.Query as IQueryable;
return base.Query(queryDescription, out totalCount);
}
We now have the query expression which contains the “Where” clause generated by the client side FilterDescriptors. We have to merge this query expression into the Context.Contacts query to get the filtered query. The following implementation of SaveContacts demonstrates how to accomplish this:
// The FindWhereMedhod recursively looks for the "Where" clause in the query
// expression
MethodCallExpression FindWhereMethod(MethodCallExpression expression)
{
if (expression == null)
return null;
if (expression.Method.Name == "Where")
return expression;
foreach(Expression sub in expression.Arguments)
{
MethodCallExpression result = FindWhereMethod(sub as MethodCallExpression);
if (result != null)
return result;
}
return null;
}
public IQueryable SaveContacts()
{
IQueryable q = GetContacts();
MethodCallExpression method = savedQuery.Expression as MethodCallExpression;
MethodCallExpression whereMethod = FindWhereMethod(method);
IQueryable contactIds;
if (whereMethod != null)
{
ParameterExpression pe = Expression.Parameter(typeof(Contact), "contact");
UnaryExpression unaryExpr = whereMethod.Arguments[1] as UnaryExpression;
MethodCallExpression whereCallExpression = Expression.Call( typeof(Queryable),
"Where",
new Type[] { q.ElementType },
q.Expression, unaryExpr.Operand);
IQueryable merged = q.Provider.CreateQuery(whereCallExpression) as IQueryable;
contactIds = from c in merged
select c.Id;
}
else
{
contactIds = from c in q
select c.Id;
}
// You now have a query merged with the client side contact id's. You can now do
// whatever you'd like with the contact id's including save them via SqlCommands
DbCommand command = Context.GetCommand(contactIds);
string connectionStr = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
using (TransactionScope scope = new TransactionScope())
using (SqlConnection connection = new SqlConnection(connectionStr))
{
connection.Open();
try
{
// There are other queries here that necessitate the transaction scope however,
// these have been removed for simplicity
string insertCmd = "insert into ContactResultSet (ContactId) select Id from ({0}) as subQuery";
insertCmd = string.Format(insertCmd, command.CommandText);
SqlCommand cmd = new SqlCommand(insertCmd, connection);
// We have to copy over the parameters
foreach (SqlParameter param in command.Parameters)
cmd.Parameters.AddWithValue(param.ParameterName, param.Value);
cmd.ExecuteNonQuery();
scope.Complete();
}
finally
{
connection.Close();
}
}
}
It had been two years since my last real vacation and five years since I last visited Mumbai. Recent tragic events in Mumbai only added to my desire to make a trip out to visit family and once again experience the sights and sounds of the city where I was born. 
