System.OutOfMemoryException being thrown - asp.net

Question

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)
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.

Answer 1

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;
}
}

Answer 2

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.