How can i use generic class to write useful codes via linq?

Answer 1

Something like this:

public string GetList(IEnumerable<TaskRelation> relations,
                      int taskId, int relTypeId,
                      Func<TaskRelation, string> projection)
{
    var query = relations.Where(r => r.TaskId == taskId && 
                                     r.RelTypeID == relTypeId)
                         .Select(projection));
                         .ToArray()
    return string.Join("; ", query.ToArray());
}

Then use it like this:

string zones = GetList(reslations, taskID, 14, r => r.RefZone.shortdesc);

Note that I've assumed LINQ to Objects - if this is LINQ to SQL (or something similar) then you should specify IQueryable<TaskRelation> and Expression<Func<TaskRelation, string>> so that the filtering and projecting can be done at the data source.