Hence to retrieve the list members for a given entity we need to identify the type of entity being supported by the list i.e Leads/Accounts/Contacts and then using the link entity functionality link it with the listmember entity to get the list.
Following is the snippet of getting collection of accounts from a particular marketing list id.
//initialize QueryExpression for adding link entities
QueryExpression qe = new QueryExpression();
qe.EntityName = EntityName.account.ToString();
//Initialize columnset
ColumnSet col = new ColumnSet();
//add columns to columnset for the acc to retrieve each acc from the acc list
col.AddColumns(new string[] { “accountid”, “name”, “address1_line1”, “address1_line2”, “address1_city”, “address1_stateorprovince”, “address1_postalcode”});
qe.ColumnSet = col;
// link from account to listmember
LinkEntity le = new LinkEntity();
le.LinkFromEntityName =
le.LinkFromAttributeName = “accountid”;
le.LinkToEntityName = EntityName.listmember.ToString();
le.LinkToAttributeName = “entityid”;
//link from listmember to list
LinkEntity le2 = new LinkEntity();
le2.LinkFromEntityName = EntityName.listmember.ToString();
le2.LinkFromAttributeName = “listid”;
le2.LinkToEntityName = EntityName.list.ToString();
le2.LinkToAttributeName = “listid”;
// add condition for listid
ConditionExpression ce = new ConditionExpression();
ce.AttributeName = “listid”;
ce.Operator = ConditionOperator.Equal;
ce.Values = new object[] { strList };//here “strList” is the marketing list id provided to this function.
//add condition to linkentity
le2.LinkCriteria = new FilterExpression();
le2.LinkCriteria.Conditions.Add(ce);
//add linkentity2 to linkentity
le.LinkEntities.Add(le2);
//add linkentities to Query Expression which is passed for getting collection of accounts
qe.LinkEntities.Add(le);
//above query expression is passed to retrieve multiple for getting BusinessEntityCollection of accounts.
BusinessEntityCollection bec = service.RetrieveMultiple(qe);
Similarly you could link with Contacts/Leads to get the list of members from a Contact/Lead Marketing list.