CA2000: Dispose objects before losing scope

Cause

A local object of a IDisposable type is created but the object is not disposed before all references to the object are out of scope.

Rule Description

If a disposable object is not explicitly disposed before all references to it are out of scope, the object will be disposed at some indeterminate time when the garbage collector runs the finalizer of the object. Because an exceptional event might occur that will prevent the finalizer of the object from running, the object should be explicitly disposed instead.

How to Fix Violations

To fix a violation of this rule, call Dispose on the object before all references to it are out of scope.

Note that you can use the using statement (Using in Visual Basic) to wrap objects that implement IDisposable. Objects that are wrapped in this manner will automatically be disposed at the close of the using block.

The following are some situations where the using statement is not enough to protect IDisposable objects and can cause CA2000 to occur.

• Returning a disposable object requires that the object is constructed in a try/finally block outside a using block.

• Initializing members of a disposable object should not be done in the constructor of a using statement.

• Nesting constructors that are protected only by one exception handler. For example,

  using (StreamReader sr = new StreamReader(new FileStream("C:\myfile.txt", FileMode.Create)))
  { ... }
  

  causes CA2000 to occur because a failure in the construction of the StreamReader object can result in the FileStream object never being closed.

• Dynamic objects should use a shadow object to implement the Dispose pattern of IDisposable objects.

When to Suppress Warnings

Do not suppress a warning from this rule unless you have called a method on your object that calls Dispose, such as Close.

Related Rules

CA2213: Disposable fields should be disposed

CA2202: Do not dispose objects multiple times

Example

If you are implementing a method that returns a disposable object, use a try/finally block without a catch block to make sure that the object is disposed. By using a try/finally block, you allow exceptions to be raised at the fault point and make sure that object is disposed.

In the OpenPort1 method, the call to open the ISerializable object SerialPort or the call to SomeMethod can fail. A CA2000 warning is raised on this implementation.

In the OpenPort2 method, two SerialPort objects are declared and set to null:

• tempPort, which is used to test that the method operations succeed.

• port, which is used for the return value of the method.

The tempPort is constructed and opened in a try block, and any other required work is performed in the same try block. At the end of the try block, the opened port is assigned to the port object that will be returned and the tempPort object is set to null.

The finally block checks the value of tempPort. If it is not null, an operation in the method has failed, and tempPort is closed to make sure that any resources are released. The returned port object will contain the opened SerialPort object if the operations of the method succeeded, or it will be null if an operation failed.

Public Function OpenPort1(ByVal PortName As String) As SerialPort

   Dim port As New SerialPort(PortName)
   port.Open()    'CA2000 fires because this might throw
   SomeMethod()   'Other method operations can fail
   Return port

End Function


Public Function OpenPort2(ByVal PortName As String) As SerialPort

   Dim tempPort As SerialPort = Nothing
   Dim port As SerialPort = Nothing

   Try
      tempPort = New SerialPort(PortName)
      tempPort.Open()
      SomeMethod()
      'Add any other methods above this line
      port = tempPort
      tempPort = Nothing

   Finally
      If Not tempPort Is Nothing Then
         tempPort.Close()
      End If


   End Try

   Return port

End Function

Public Function CreateReader1(ByVal x As Integer) As StreamReader
   Dim local As New StreamReader("C:\Temp.txt")
   x += 1
   Return local
End Function


Public Function CreateReader2(ByVal x As Integer) As StreamReader
   Dim local As StreamReader = Nothing
   Dim localTemp As StreamReader = Nothing
   Try
      localTemp = New StreamReader("C:\Temp.txt")
      x += 1
      local = localTemp
      localTemp = Nothing
      Finally
         If (Not (localTemp Is Nothing)) Then
            localTemp.Dispose()
         End If
   End Try
   Return local
End Function

public SerialPort OpenPort1(string portName)
{
   SerialPort port = new SerialPort(portName);
   port.Open();  //CA2000 fires because this might throw
   SomeMethod(); //Other method operations can fail
   return port;
}

public SerialPort OpenPort2(string portName)
{
   SerialPort tempPort = null;
   SerialPort port = null;
   try
   {
      tempPort = new SerialPort(portName);
      tempPort.Open();
      SomeMethod();
      //Add any other methods above this line
      port = tempPort;
      tempPort = null;

   }
   finally
   {
      if (tempPort != null)
      {
         tempPort.Close();
      }
   }
   return port;
}

By default, the Visual Basic compiler has all arithmetic operators check for overflow. Therefore, any Visual Basic arithmetic operation might throw an OverflowException. This could lead to unexpected violations in rules such as CA2000. For example, the following CreateReader1 function will produce a CA2000 violation because the Visual Basic compiler is emitting an overflow checking instruction for the addition that could throw an exception that would cause the StreamReader not to be disposed.

To fix this, you can disable the emitting of overflow checks by the Visual Basic compiler in your project or you can modify your code as in the following CreateReader2 function.

To disable the emitting of overflow checks, right-click the project name in Solution Explorer and then click Properties. Click Compile, click Advanced Compile Options, and then check Remove integer overflow checks.

See Also

Reference

IDisposable

Implementing Finalize and Dispose to Clean Up Unmanaged Resources