This article discusses what memory leaks are and lists some possible causes of memory leaks. Additionally, this article discusses the perceived memory leak and how it can be misinterpreted as a true memory leak.
Memory leaks can cause an application to run out of resources and can cause an application to crash. It is important to identify memory leaks. The problem of memory leaks has plagued developers in C and C++ for years. In Microsoft Visual Studio 2005 or in Microsoft Visual Studio .NET, a comprehensive garbage collection package and managed memory can stop memory leaks, but, under some circumstances, a program may appear to be leaking memory.
Definition of memory leak
A memory leak occurs when memory is allocated in a program and is never returned to the operating system, even though the program does not use the memory any longer. The following are the four basic types of memory leaks:
- In a manually managed memory environment: Memory is dynamically allocated and referenced by a pointer. The pointer is erased before the memory is freed. After the pointer is erased, the memory can no longer be accessed and therefore cannot be freed.
- In a dynamically managed memory environment: Memory is disposed of but never collected, because a reference to the object is still active. Because a reference to the object is still active, the garbage collector never collects that memory. This can occur with a reference that is set by the system or the program.
- In a dynamically managed memory environment: The garbage collector can collect and free the memory but never returns it to the operating system. This occurs when the garbage collector cannot move the objects that are still in use to one portion of the memory and free the rest.
- In any memory environment: Poor memory management can result when many large objects are declared and never permitted to leave scope. As a result, memory is used and never freed.
Because of the garbage collection package that is implemented in the Microsoft .NET Framework, it is not possible to have a memory leak in managed code. This suggests two questions: How then can a memory leak occur? Why does it appear that you have a memory leak?
A memory leak can occur in a .NET Framework application when you use unmanaged code as part of the application. This unmanaged code can leak memory, and the .NET Framework runtime cannot address that problem.
Additionally, a project may only appear to have a memory leak. This condition can occur if many large objects (such as DataTable
objects) are declared and then added to a collection (such as a DataSet
). The resources that these objects own may never be released, and the resources are left alive for the whole run of the program. This appears to be a leak, but actually it is just a symptom of the way that memory is being allocated in the program.
For example, you have a DataSet
. Every time that a new query is run, you add a new DataTable
element to that DataSet
to hold the data that is returned. If there are large amounts of data that you never dispose of, the data stays alive as long as the DataSet
is still in use. If this occurs enough times, it is possible to run out of memory. This is not a memory leak, but instead it is a problem in managing the memory. See the following code example:
Dim DS As DataSet Dim cn As New SqlClient.SqlConnection("data source=localhost;initial catalog=Northwind;integrated security=SSPI") cn.Open() Dim da As New SqlClient.SqlDataAdapter("Select * from Employees", cn) Dim i As Integer DS = New DataSet() For i = 0 To 1000 da.Fill(DS, "Table" + i.ToString) Next
This example is just a snippet of code. This example assumes that Microsoft SQL Server is installed on the local computer and that the user who is running this code has access to the Northwind
database that is included with SQL Server.
Although this code is obviously inefficient and not practical, it is meant to demonstrate that if objects are added to a collection (such as adding the tables to the DataSet
collection), the objects are kept active as long as the collection remains alive. If a collection is declared at the global level of the program, and objects are declared throughout the program and added to that collection, this means that even though the objects are no longer in scope, the objects remain alive because they are still being referenced.
Each time that this occurs, the amount of memory that the program is using increases. The memory does not decrease until the end of the program or the release of the objects from the collection. When you watch the program on a performance monitor, this appears to be a memory leak, but it is not. The program still has control over the memory but has chosen not to release it. The fact that the program still has control prevents this from being a memory leak, but the fact that the program keeps increasing the amount of memory used can make it appear to be a memory leak.
Symptoms of memory leak
When the amount of memory that a program is using continues to increase during the execution, this is a symptom of a memory leak. (You can watch this count of memory through a performance monitor.) The amount of memory that the program uses can eventually cause the program to run out of resources and to crash.