Cannot assign LINQ query to variable - vb.net

Question

I want to declare a variable to hold the query result from LINQ to SQL like to the following:
Dim query As IQueryable(Of Student)
If isNoUserName Then
query = From st In db.Students _
Order By st.AssignedId Ascending _
Where (st.studentId = studentId _
Select st
Else
'' error here
query = From st In db.Students _
Order By st.firstName Ascending _
Join user In db.MSUsers On user.UserId Equals st.UserId _
Where st.studentId = studentId _
Select st.firstName, st.lastName, user.userName
End If
Return query
Error : conversions from 'System.Linq.IQueryable(Of )' to 'System.Linq.IQueryable(Of Student)'.
How do I declare variable "query" to hold both data from "Student" and "User" tables?
Thank you.

Answer 1

query is declared as being IQueryable<Student>, but it you look at the second query, you aren't actually returning students - you are returning a compiler-generated type composed of firstName, lastName and userName. Try changing the second query to end with:
Select st;
Edit:
Re needing data other than st, then you might try a trick to give query a type; I'll use C# for my illustration, as my VB is read-only:
var query = Enumerable.Repeat(new {firstName="",lastName="",userName=""}, 0);
...
if(isNoUserName) {
query = ...
select new {st.firstName, st.lastName, userName = ""};
} else {
query = ...
select new {st.firstName, st.lastName, user.userName };
}