GCHandle.Free Method
Definition
Releases a GCHandle.
public:
void Free();
[System.Security.SecurityCritical]
public void Free ();
member this.Free : unit -> unit
Public Sub Free ()
- Attributes
Exceptions
The handle was freed or never initialized.
Examples
The following example shows an App
class that creates a handle to a managed object using the GCHandle.Alloc
method, which prevents the managed object from being collected. A call to the EnumWindows
method passes a delegate and a managed object (both declared as managed types, but not shown), and casts the handle to an IntPtr. The unmanaged function passes the type back to the caller as a parameter of the callback function.
using System;
using System.IO;
using System.Threading;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Security.Permissions;
public delegate bool CallBack(int handle, IntPtr param);
internal static class NativeMethods
{
// passing managed object as LPARAM
// BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
[DllImport("user32.dll")]
internal static extern bool EnumWindows(CallBack cb, IntPtr param);
}
public class App
{
public static void Main()
{
Run();
}
[SecurityPermission(SecurityAction.Demand, UnmanagedCode = true)]
public static void Run()
{
TextWriter tw = Console.Out;
GCHandle gch = GCHandle.Alloc(tw);
CallBack cewp = new CallBack(CaptureEnumWindowsProc);
// platform invoke will prevent delegate to be garbage collected
// before call ends
NativeMethods.EnumWindows(cewp, GCHandle.ToIntPtr(gch));
gch.Free();
}
private static bool CaptureEnumWindowsProc(int handle, IntPtr param)
{
GCHandle gch = GCHandle.FromIntPtr(param);
TextWriter tw = (TextWriter)gch.Target;
tw.WriteLine(handle);
return true;
}
}
Imports System.IO
Imports System.Threading
Imports System.Windows.Forms
Imports System.Runtime.InteropServices
Imports System.Security.Permissions
Public Delegate Function CallBack(ByVal handle As Integer, ByVal param As IntPtr) As Boolean
Friend Module NativeMethods
' passing managed object as LPARAM
' BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
<DllImport("user32.dll")>
Friend Function EnumWindows(ByVal cb As CallBack, ByVal param As IntPtr) As Boolean
End Function
End Module
Module App
Sub Main()
Run()
End Sub
<SecurityPermission(SecurityAction.Demand, UnmanagedCode:=True)>
Sub Run()
Dim tw As TextWriter = Console.Out
Dim gch As GCHandle = GCHandle.Alloc(tw)
Dim cewp As CallBack
cewp = AddressOf CaptureEnumWindowsProc
' platform invoke will prevent delegate to be garbage collected
' before call ends
NativeMethods.EnumWindows(cewp, GCHandle.ToIntPtr(gch))
gch.Free()
End Sub
Function CaptureEnumWindowsProc(ByVal handle As Integer, ByVal param As IntPtr) As Boolean
Dim gch As GCHandle = GCHandle.FromIntPtr(param)
Dim tw As TextWriter = CType(gch.Target, TextWriter)
tw.WriteLine(handle)
Return True
End Function
End Module
Remarks
The caller must ensure that for a given handle, Free is called only once.
Security
SecurityCriticalAttribute
requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.