Linq Conditional where clause when parameter can be null

Question

I need to filter my records on the basis of "courseId" which is passed as parameter -
Here is my code -

public IEnumerable<string> GetStudentIdsByRoleAndCourse(string role, string courseId = null)
{
  var query = (from item in Data.TableForQuery<MasterTable>()
               where item.RoleId == role
               select item).ToList();

  if (courseId != null)
  {
    return (from item in query
            let student = Students.GetModelById(item.StudentId)
            where student.DisplayInReports && student.CourseId == courseId 
            orderby student.Name
            select student.Id).ToList();
  }

  return (from item in query
          let student = Students.GetModelById(item.StudentId)
          where student.DisplayInReports 
          orderby student.Name
          select student.Id).ToList();
}

Because I was not convinced with writing separate return statements -
I have now modified the code as below - to cater for the nullable parameter courseId

public IEnumerable<string> GetStudentIdsByRoleAndCourse(string role, string courseId = null)
{
  var query = (from item in Data.TableForQuery<MasterTable>()
               where item.RoleId == role
               select item).ToList();
  
    return (from item in query
            let student = Students.GetModelById(item.StudentId)
            where student.DisplayInReports 
            && (courseId == null || student.CourseId == courseId)
            orderby student.Name
            select student.Id).ToList();        
} 

Wondering if there is even better way than above to achieve same.

Thanks a lot!

Answer 1

Query syntax for Linq here makes this harder than it should be. I'd recommend using Linq's method syntax because it can be built as pieces.

public IEnumerable<string> GetStudentIdsByRoleAndCourse(string role, string courseId = null)
{
    var filteredQuery = Data.TableForQuery<MasterTable>()
                            .Where(_ => _.RoleId == role)
                            .Select(_ => Students.GetModelById(_.StudentId))
                            .Where(_ => _.DisplayInReports);
    if(courseId != null)
    {
        filteredQuery = filteredQuery.Where(_ => _.CourseId == courseId);
    }
    return filteredQuery.OrderBy(_ => _.Name)
                        .Select(_ => _.Id)
                        /*ToList is completely optional since your method is only 
                          IEnumerable<string>, calling ToList every where is
                          actually a bad thing*/
                        .ToList();
  
}