Finding String of Substring in VB without using library function

Part of your confusion is that .Net strings are much more than just character buffers. I'm going to assume that you can at least use strings. If you can't, use need to declare character arrays instead. That out of the way, this should get you there as a 1:1 translation:

Private Shared Function search(ByVal a As String, ByVal b As String) As Integer
    Dim i As Integer = 0
    Dim j As Integer = 0
    Dim firstOcc As Integer

    While i < a.Length

        While a.Chars(i)<>b.Chars(0) AndAlso i < a.Length
            i += 1
        End While

        If i >= a.Length Then Return -1 'search can not continue

        firstOcc = i

        While a.Chars(i)=b.Chars(j) AndAlso i < a.Length AndAlso j < b.Length
            i += 1
            j += 1
        End While

        If j = b.Length Then Return firstOcc
        If i = a.Length Then Return -1

        i = firstOcc + 1
        j = 0
    End While
    Return 0
End Function

Shared Sub Main() As Integer
    Dim a As String
    Dim b As String
    Dim loc As Integer

    Console.Write("Enter the main string :")
    a = Console.ReadLine()

    Console.Write("Enter the search string :")
    b = Console.ReadLine()

    loc = search(a, b)

    If loc = -1 Then
      Console.WriteLine("Not found")
    Else
      Console.WriteLine("Found at location {0:D}",loc+1)
    End If 

    Console.ReadKey(True)
End Sub

But please don't ever actually use that. All you really need is this:

Private Shared Function search(ByVal haystack as String, ByVal needle As String) As Integer
     Return haystack.IndexOf(needle)
End Function

VB has a built-in function called InStr, it's part of the language. It returns an integer specifying the start position of the first occurrence of one string within another.

http://msdn.microsoft.com/en-us/library/8460tsh1(v=VS.80).aspx

Pete

Try this one, this will return a List(Of Integer) containing the index to all occurrence's of the find text within the source text, after the specified search starting position.

Option Strict On
Public Class Form1
''' <summary>
''' Returns an array of indexes where the find text occurred in the source text.
''' </summary>
''' <param name="Source">The text you are searching.</param>
''' <param name="Find">The text you are searching for.</param>
''' <param name="StartIndex"></param>
''' <returns>Returns an array of indexes where the find text occurred in the source text.</returns>
''' <remarks></remarks>
Function FindInString(Source As String, Find As String, StartIndex As Integer) As List(Of Integer)
    If StartIndex > Source.Length - Find.Length Then Return New List(Of Integer)
    If StartIndex < 0 Then Return New List(Of Integer)
    If Find.Length > Source.Length Then Return New List(Of Integer)
    Dim Results As New List(Of Integer)
    For I = StartIndex To (Source.Length) - Find.Length
        Dim TestString As String = String.Empty
        For II = I To I + Find.Length - 1
            TestString = TestString & Source(II)
        Next
        If TestString = Find Then Results.Add(I)
    Next
    Return Results
End Function
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Try
        Dim Search As String = "Hello world, this world is an interesting world"
        Dim Find As String = "world"
        Dim Indexes As List(Of Integer) = New List(Of Integer)
        Try
            Indexes = FindInString(Search, Find, 0)
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
        RichTextBox1.Text = "Search:" & vbCrLf
        RichTextBox1.Text = RichTextBox1.Text & Search & vbCrLf & vbCrLf
        RichTextBox1.Text = RichTextBox1.Text & "Find:" & vbCrLf
        RichTextBox1.Text = RichTextBox1.Text & Find & vbCrLf & vbCrLf
        RichTextBox1.Text = RichTextBox1.Text & "-----------" & vbCrLf
        RichTextBox1.Text = RichTextBox1.Text & "Result Indexes:" & vbCrLf & vbCrLf
        For Each i As Integer In Indexes
            RichTextBox1.Text = RichTextBox1.Text & i.ToString & vbCr
        Next
    Catch ex As Exception
        MsgBox(ex.ToString)
    End Try
End Sub
End Class

Here is another way, where there is no use of .Net functions.

Function FindInString(Source As String, Find As String, StartIndex As Integer) As Integer()
    If StartIndex > Len(Source) - Len(Find) Then Return {}
    If StartIndex < 0 Then Return {}
    If Len(Find) > Len(Source) Then Return {}
    Dim Results As Integer() = {}, ResultCount As Integer = -1
    For I = StartIndex To Len(Source) - Len(Find)
        Dim TestString As String = ""
        For II = I To I + Len(Find) - 1
            TestString = TestString & Source(II)
        Next
        If TestString = Find Then
            ResultCount += 1
            ReDim Preserve Results(ResultCount)
            Results(ResultCount) = I
        End If
    Next
    Return Results
End Function