update case description = all tasks completed

Entity target = (Entity)context.InputParameters["target"];
//Inheritance OBJECT

EntityReference regardingCaseRef = target["regardingobjectid"] as EntityReference;
Entity regardingCase = service.Retrieve(regardingCaseRef.LogicalName, regardingCaseRef.Id, new ColumnSet(true));
QueryExpression qeRelatedTasks = new QueryExpression("incident");

qeRelatedTasks.Criteria.AddCondition(new ConditionExpression("regardingobjectid", ConditionOperator.Equal, regardingCaseRef));

EntityCollection allRelatedTasks = service.RetrieveMultiple(qeRelatedTasks);

// update case description = all tasks completed
var entityRef = new Entity(retrieved.Entities.LogicalName, retrieved.Entities.Id);
entityRef["Description"] = "all tasks completed";
service.Update(entityRef);