Call associate/disassociate by C# in CRM 2015/2016 and Dynamics 365

Use following function to associate records in n to n relationship

public void Associate()
        {
            string relationshipName = "bcrm_dot_document_dot_party_n_to_n";
            string firstEntityName = "dot_document";
            Guid firstEntityId = new Guid("69410885-8457-E811-80DC-005056BE7607");
            string secondEntityName = "dot_party";
            Guid secondEntityId = new Guid("A3BE0B8D-9C4A-E711-80D5-005056BE4FB8");

            bool areEntitiesAlreadyAssociated = RelationshipExists(relationshipName, firstEntityId, firstEntityName, secondEntityId, secondEntityName);

            if(areEntitiesAlreadyAssociated == false)
            {
                Relationship relationship = new Relationship(relationshipName);
                EntityReferenceCollection relatedEntities = new EntityReferenceCollection();
                EntityReference secondaryEntity = new EntityReference(secondEntityName, secondEntityId);
                relatedEntities.Add(secondaryEntity);

                crmService.Associate(firstEntityName, firstEntityId, relationship, relatedEntities);
            }            
        }

Use following function to disassociate records in n to n relationship

public void Disassociate()
        {
            string relationshipName = "bcrm_dot_document_dot_party_n_to_n";
            string firstEntityName = "dot_document";
            Guid firstEntityId = new Guid("69410885-8457-E811-80DC-005056BE7607");
            string secondEntityName = "dot_party";
            Guid secondEntityId = new Guid("A3BE0B8D-9C4A-E711-80D5-005056BE4FB8");
            Relationship relationship = new Relationship(relationshipName);

            EntityReferenceCollection relatedEntities = new EntityReferenceCollection();
            EntityReference secondaryEntity = new EntityReference(secondEntityName, secondEntityId);
            relatedEntities.Add(secondaryEntity);

            crmService.Disassociate(firstEntityName, firstEntityId, relationship, relatedEntities);
        }

private bool RelationshipExists(string relationshipname, Guid entity1Id, string entity1Name,Guid entity2Id, string entity2Name)
        {
            string relationship1EtityName = string.Format("{0}id", entity1Name);
            string relationship2EntityName = string.Format("{0}id", entity2Name);

            //This check is added for self-referenced relationships
            if (entity1Name.Equals(entity2Name, StringComparison.InvariantCultureIgnoreCase))
            {
                relationship1EtityName = string.Format("{0}idone", entity1Name);
                relationship1EtityName = string.Format("{0}idtwo", entity1Name);
            }

            QueryExpression query = new QueryExpression(entity1Name)
            {
                ColumnSet = new ColumnSet(false)
            };

            LinkEntity link = query.AddLink(relationshipname,
                string.Format("{0}id", entity1Name), relationship1EtityName);
            link.LinkCriteria.AddCondition(relationship1EtityName,
                ConditionOperator.Equal, new object[] { entity1Id });
            link.LinkCriteria.AddCondition(relationship2EntityName,
                ConditionOperator.Equal, new object[] { entity2Id });

            EntityCollection entColl = crmService.RetrieveMultiple(query);
            return entColl.Entities.Count != 0;
        }

References: https://community.dynamics.com/crm/b/hardworkdays/archive/2012/05/17/ms-crm-2011-code-which-rechecks-that-n-n-relation-exists-between-2-records

http://www.mscrmtutorial.com/2014/07/associate-and-disassociate-many-to-many.html

https://community.dynamics.com/crm/b/mylifemicrosoftdynamiccrm/archive/2017/04/09/ms-dynamics-crm-associate-disassociate-message-plugin

https://community.dynamics.com/enterprise/b/crmfortressdynamics365/archive/2017/08/24/dynamics-365-teams-addmembers-removemembers-vs-associate-disassociate