VB到C＃的运行顺序有什么区别

Question

So at work i am moving all our old VB code to C# and i encountered a problem.

In the VB Code it is

 Do While (iterator.NextCourse(course_list, prof_relation, prof_list, index))
    course = course_list(index)

    course_title_id = CType(course(CourseListQueries.GetCourseListCols.CourseTitleId), Integer)
    course_id = CType(course(CourseListQueries.GetCourseListCols.CourseId), Integer)

    prof_name_list = String.Empty

    For Each prof As DataRowView In prof_list
       If (Not IsDBNull(prof(CourseListQueries.GetCourseProfsCols.ProfId))) Then
          last_name = CType(prof(CourseListQueries.GetCourseProfsCols.LastName), String)
          first_name = CType(prof(CourseListQueries.GetCourseProfsCols.FirstName), String)
              If (IsDBNull(prof(CourseListQueries.GetCourseProfsCols.PreferredName))) Then
                 preferred_name = first_name
              Else
                 preferred_name = CType(prof(CourseListQueries.GetCourseProfsCols.PreferredName), String)
              End If

           prof_name = preferred_name.Substring(0, 1) & ". " & last_name

              If (prof_name_list <> String.Empty) And (prof_name <> String.Empty) Then
                  prof_name_list &= " / "
              End If

           prof_name_list &= prof_name
        End If
  Next

which works fine it works while the method returns bool, but that method is passed by ref. so it links the relation of 2 DataTables.

in C# that code is like

do
{
    code...
    foreach (DataRowView  prof In prof_list) /* crashes here with null object reference*/
      {
          code...
      }
}while(iterator.NextCourse(course_list, prof_relation,ref prof_list, ref index));

so i just want to know, Because in VB the do-while is physically on one line does it run in a different order than the c# method? How would i go about mending this situation because c# runs top to bottom and only once it's done an iteration does it enter the while method and set those references.

Answer 1

The difference between Do ... Loop While and Do While ... Loop is when the condition is checked.

In the first syntax (equivalent to do { ... } while in C#) the block is executed once, THEN the condition is checked to see if the block should be executed again.

In a Do While ... Loop block (equivalent to while { ... } in C#) the condition is checked first , then the code is executed if the condition is true.

So in your case moving the while to the top of the statement block would make the two fragments equivalent:

while(iterator.NextCourse(course_list, prof_relation,ref prof_list, ref index))
{
    code...
    foreach (DataRowView  prof In prof_list) /* crashes here with null object reference*/
      {
          code...
      }
}

Answer 2

In C# while can also go at the top of the loop , so in your example this would be:

while (iterator.NextCourse(course_list, prof_relation,ref prof_list, ref index))
{
    // code...
    foreach (DataRowView  prof In prof_list) /* crashes here with null object reference*/
    {
        // code...
    }
}

Answer 3

Yes, the VB code runs in a different order than the C# code in your question. The test is happening at the beginning of the loop in your VB code and at the end of the loop in the C# version.

If you change your C# version to a while loop instead of a do while (as in @Richard Ev's answer), they will do the same thing.

Answer 4

you could also try an infinite for loop with a if else break

for(;;)
{
    if (iterator.NextCourse(course_list, prof_relation,ref prof_list, ref index))
    {
        code....
    }
    else 
        break;
}