.Net Entity two optional parameters

Question

I am new to .Net Entity framework and was looking for a way to write an function that can take in two optional parameters. Is there a better way to do this ?

public IQueryable<Template> GetTemplateAgentKeyDiseaseId(Guid agentKey, Guid diseaseId)
    {
        if (diseaseId != null)
        {
            return from t in UnitOfWork.GetRepository<Template>().Get()
                   join r in UnitOfWork.GetRepository<Regimen>().Get() on t.Id equals r.TemplateId
                   join rp in UnitOfWork.GetRepository<RegimenPart>().Get() on r.Id equals rp.RegimenId
                   join re in UnitOfWork.GetRepository<RegimenEntry>().Get() on rp.Id equals re.RegimenPartId
                   join a in UnitOfWork.GetRepository<Agent>().Get() on re.AgentVersionKey equals a.VersionKey
                   where a.VersionKey == agentKey && t.IsCurrentVersion
                   select t;
        }
        else if (agentKey != null) 
        {

            return from t in UnitOfWork.GetRepository<Template>().Get()
                   join r in UnitOfWork.GetRepository<Regimen>().Get() on t.Id equals r.TemplateId
                   join rp in UnitOfWork.GetRepository<RegimenPart>().Get() on r.Id equals rp.RegimenId
                   join re in UnitOfWork.GetRepository<RegimenEntry>().Get() on rp.Id equals re.RegimenPartId
                   join a in UnitOfWork.GetRepository<Agent>().Get() on re.AgentVersionKey equals a.VersionKey
                   where t.IsCurrentVersion && t.ExternalDiseaseId == diseaseId
                   select t;
        }else
        {
            return from t in UnitOfWork.GetRepository<Template>().Get()
                   join r in UnitOfWork.GetRepository<Regimen>().Get() on t.Id equals r.TemplateId
                   join rp in UnitOfWork.GetRepository<RegimenPart>().Get() on r.Id equals rp.RegimenId
                   join re in UnitOfWork.GetRepository<RegimenEntry>().Get() on rp.Id equals re.RegimenPartId
                   join a in UnitOfWork.GetRepository<Agent>().Get() on re.AgentVersionKey equals a.VersionKey
                   where a.VersionKey == agentKey && t.IsCurrentVersion && t.ExternalDiseaseId == diseaseId
                   select t;
        }

Answer 1

1. You have the ifs if (diseaseId != null) reversed
2. Guid is struct so it will never be null, use Guid?

You can extract the common part:

        public IQueryable<Template> GetTemplateAgentKeyDiseaseId(Guid? agentKey, Guid? diseaseId)
        {
            //Common part
            var query = from t in UnitOfWork.GetRepository<Template>().Get()
                        join r in UnitOfWork.GetRepository<Regimen>().Get() on t.Id equals r.TemplateId
                        join rp in UnitOfWork.GetRepository<RegimenPart>().Get() on r.Id equals rp.RegimenId
                        join re in UnitOfWork.GetRepository<RegimenEntry>().Get() on rp.Id equals re.RegimenPartId
                        join a in UnitOfWork.GetRepository<Agent>().Get() on re.AgentVersionKey equals a.VersionKey
                        where t.IsCurrentVersion
                        select new
                        {
                            T = t,
                            VersionKey = a.VersionKey,
                        };

            //Extra filters
            if (agentKey != null)
            {
                query = query.Where(o => o.VersionKey == agentKey);
            }
            if (diseaseId!= null)
            {
                query = query.Where(o => o.T.ExternalDiseaseId == diseaseId);
            }

            return query.Select(o => o.T);
        }

(Since I don't have the classes I wasn't able to compile this, you may need to adjust this code)