CRM 4 how to retrieve custom attribute value in C#

DynamicEntity someEntity;

TargetRetrieveDynamic retrieveTarget = new TargetRetrieveDynamic();

retrieveTarget.EntityId = <id of entity>;

retrieveTarget.EntityName = EntityName.<entity name>.ToString();

RetrieveRequest retrieveRequest = new RetrieveRequest();

retrieveRequest.ColumnSet = new ColumnSet(new string[] { "your_custom_attribute" });

retrieveRequest.ReturnDynamicEntities = true;

retrieveRequest.Target = retrieveTarget;

RetrieveResponse retrieveResponse = (RetrieveResponse)service.Execute(retrieveRequest);

someEntity = retrieveResponse.BusinessEntity as DynamicEntity;

String result = (String)someEntity["your_custom_attribute"];

String value = result.Value.ToString();