System.OutOfMemoryException being thrown

I am getting a system.outofmemory exception in my code:

While r1.Read()
                    menu1id = r1("id")
                    db.AddInParameter(command2, "@menu1id", DbType.Int32, menu1id)
                    r2 = db.ExecuteReader(command2)
                    command2.Parameters.Clear()
                    menu1heading = r1("Heading")
                    If r1("url") IsNot Nothing And r1("moduleid") = 0 Then
                        url = r1("url").ToString

                        If InStr(url, "file:///") > 0 Then
                            Dim builder As New StringBuilder
                            builder.Append("<a href=")
                            builder.Append(r1("url"))
                            builder.Append(" target=")
                            builder.Append(r1("urltarget"))
                            builder.Append(">")
                            builder.Append(menu1heading)
                            builder.Append("</a>")
                            level1.Add(builder.ToString)
                        Else
                            Dim builder As New StringBuilder
                            builder.Append("<a href=http://")
                            builder.Append(r1("url"))
                            builder.Append(" target=")
                            builder.Append(r1("urltarget"))
                            builder.Append(">")
                            builder.Append(menu1heading)
                            builder.Append("</a>")
                            level1.Add(builder.ToString)
                        End If
                    Else
                        Dim builder As New StringBuilder
                        builder.Append("<a href=~/Default.aspx?id=")
                        builder.Append(r1("id"))
                        builder.Append(">")
                        builder.Append(menu1heading)
                        builder.Append("</a>")
                        level1.Add(builder.ToString)
                    End If


                    While r2.Read
                        menu2id = r2("id")
                        db.AddInParameter(command3, "@menu2id", DbType.Int32, menu2id)
                        r3 = db.ExecuteReader(command3)
                        command3.Parameters.Clear()
                        menu2heading = r2("Heading")

                        If r2("url") IsNot Nothing And r2("moduleid") = 0 Then
                            Dim builder As New StringBuilder
                            builder.Append("<a href=http://")
                            builder.Append(r2("url"))
                            builder.Append(" target=")
                            builder.Append(r2("urltarget"))
                            builder.Append(menu2heading)
                            builder.Append("</a>")
                            level2.Add(builder.ToString)
                        Else
                            Dim builder As New StringBuilder
                            builder.Append("<a href=~/Default.aspx?id=")
                            builder.Append(r2("id"))
                            builder.Append(">")
                            builder.Append(menu2heading)
                            builder.Append("</a>")
                            level2.Add(builder.ToString)
  开发者_开发知识库                      End If


                        While r3.Read
                            menu3heading = r3("Heading")
                            menu3id = r3("id")
                            If r3("url") IsNot Nothing And r3("moduleid") = 0 Then
                                Dim builder As New StringBuilder
                                builder.Append("<a href=http://")
                                builder.Append(r3("url"))
                                builder.Append(" target=")
                                builder.Append(r3("urltarget"))
                                builder.Append(">")
                                builder.Append(menu3heading)
                                builder.Append("</a>")
                                level3.Add(builder.ToString)
                            Else
                                Dim builder As New StringBuilder
                                builder.Append("<a href=~/Default.aspx?id=")
                                builder.Append(r3("id"))
                                builder.Append(">")
                                builder.Append(menu3heading)
                                builder.Append("</a>")
                                level3.Add(builder.ToString)
                            End If

                        End While

                        r3.Close()
                    End While

                    r2.Close()
                End While

                r1.Close()
            End While
            r0.Close()

Please can you tell me how I go about diagnosing and fixing this exception? thanks a lot.

You would be a lot better off changing your logic to a single sql query that returns all of your menu items in one go and then iterating this dataset to build your menu.

You could just try

SELECT id, DepartmentID, GroupingID, Heading, OrderID, 
Publish, moduleid, url, urltarget
FROM Grouping 
WHERE (DepartmentID = 0 AND Publish <> 0)
ORDER BY OrderID

This returns all of the data that the above queries return, including the GroupingID which determines the tree structure. You should be able to load up the results into a collection of objects and then query them using LINQ to build your menu.

Copy your data into the following class annd then use LINQ on a list of them:

public class DataClass
{
public string Id { get; set; }
public string DepartmentID { get; set; }
public string GroupingID { get; set; }
public string Heading { get; set; }
public string OrderID { get; set; }
public string Publish { get; set; }
public string Moduleid { get; set; }
public string Url { get; set; }
public string Urltarget { get; set; }

public List<DataClass> Children { get; set; }

public DataClass(string id, string departmentID, string groupingID, string heading, string orderID, string publish, string moduleid, string url, string urltarget)
{
    Id = id;
    DepartmentID = departmentID; 
    GroupingID = groupingID;
    Heading = heading;
    OrderID = orderID; 
    Publish = publish;
    Moduleid = moduleid;
    Url = url;
    Urltarget = urltarget;
}
}

The method uses a lot of string concatenations, each of them creating lots of new temporary string objects which use up your heap space. This can be avoided by using a StringBuilder to create the concatenated string. The StringBuilder works not only more memory-efficient when concatenating lots of strings, it will also be much faster:

Dim builder As New StringBuilder()
builder.Append("<a href=")
builder.Append("r0("url"))
builder.Append("target=")
builder.Append(r0("urltarget"))
builder.Append(menu0heading)
builder.Append("</a>")

Dim str as String
str = builder.ToString()

Note: As pointed out by David Neale in the comments, in this context it would be better to use a TagBuilder (or an XmlWriter) to create the HTML/XML document tree.