Visual Basic Confusion - vb.net

Question

I have been required to create a program that asks me to find the maximum value of one particular array. I am using multiple forms in this project and have used a user-defined data type and created multiple array under it. There is a first form that is related to this, which defines my defined data type is gStudentRecord and the arrays that define it are last name, Id, and GPA. This second form is where I write all of the code to display what I want. My question is how to get the Max GPA out of that array. I'm sorry if this isn't in very good format, this is the first time I've used Stackoverflow
Public Class frmSecond
Private Sub frmSecond_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim Ctr As Integer
Dim Line As String
lstDisplay.Items.Clear()
lstDisplay.Items.Add("Name".PadRight(25) & "ID".PadRight(16) & "GPA".PadRight(20) & "Out of state".PadRight(10))
For Ctr = 0 To gIndex Step 1
Line = gCourseRoster(Ctr).LastName.PadRight(20) & gCourseRoster(Ctr).ID.PadRight(15) & gCourseRoster(Ctr).GPA.ToString.PadRight(15) & gCourseRoster(Ctr).OutOfState.ToString().PadLeft(5)
lstDisplay.Items.Add(Line)
Next
End Sub
Private Sub btnStats_Click(sender As Object, e As EventArgs) Handles btnStats.Click
Dim Ctr As Integer = 0
Dim Average As Double
Dim Sum As Double
Dim Found As Boolean = False
Dim Pass As Integer
Dim Index As Integer
lstDisplay.Items.Clear()
**For Ctr = 0 To gIndex Step 1
If gCourseRoster(Ctr).GPA > gCourseRoster(Ctr).GPA Then
lstDisplay.Items.Add(gCourseRoster(Ctr).GPA)
End If
Next**
Average = gComputeAverage(Sum)
lstDisplay.Items.Add("Number of Students: " & gNumberOfStudents)
lstDisplay.Items.Add("Average: " & Average)
End Sub
Private Function gComputeAverage(Sum As Double) As Double
Dim Ctr As Integer
Dim Average As Double
For Ctr = 0 To gIndex Step 1
Sum = Sum + gCourseRoster(Ctr).GPA
Next
Average = Sum / gNumberOfStudents
Return Average
End Function
End Class

Answer 1

You can use a Lambda expression to tease it out. The Cast part is converting from the gCourseRoster to a collection of Double by supplying the GPA to the Select statement.
Dim gList As New List(Of gCourseRoster)
gList.Add(New gCourseRoster With {.id = 1, .name = "Bob", .GPA = 3.9})
gList.Add(New gCourseRoster With {.id = 2, .name = "Sarah", .GPA = 3.2})
gList.Add(New gCourseRoster With {.id = 3, .name = "Frank", .GPA = 3.1})
Dim maxGPA = gList.Cast(Of gCourseRoster).Select(Function(c) c.GPA).ToList.Max
MessageBox.Show(maxGPA.ToString)
Output: 3.9