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SafeHandle.ReleaseHandle Method

When overridden in a derived class, executes the code required to free the handle.

Namespace:  System.Runtime.InteropServices
Assembly:  mscorlib (in mscorlib.dll)

protected abstract bool ReleaseHandle()

Return Value

Type: System.Boolean
true if the handle is released successfully; otherwise, in the event of a catastrophic failure, false. In this case, it generates a releaseHandleFailed MDA Managed Debugging Assistant.

The ReleaseHandle method is guaranteed to be called only once and only if the handle is valid as defined by the IsInvalid property. Implement this method in your SafeHandle derived classes to execute any code that is required to free the handle. Because one of the functions of SafeHandle is to guarantee prevention of resource leaks, the code in your implementation of ReleaseHandle must never fail. The garbage collector calls ReleaseHandle after normal finalizers have been run for objects that were garbage collected at the same time. The garbage collector guarantees the resources to invoke this method and that the method will not be interrupted while it is in progress. This method will be prepared as a constrained execution region (CER) at instance construction time (along with all the methods in its statically determinable call graph). Although this prevents thread abort interrupts, you must still be careful not to introduce any fault paths in your overridden ReleaseHandle method. In particular, apply the ReliabilityContractAttribute attribute to any methods you call from ReleaseHandle. In most cases this code should be:

ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)

Additionally, for simple cleanup (for example, calling the Win32 API CloseHandle on a file handle) you can check the return value for the single platform invoke call. For complex cleanup, you may have a lot of program logic and many method calls, some of which might fail. You must ensure that your program logic has fallback code for each of those cases.

If ReleaseHandle returns false for any reason, it generates a releaseHandleFailed MDA Managed Debugging Assistant. This helps you detect cases where your attempt to release resources fails.

The following code example releases the handle and is part of a larger example provided for the SafeHandle class.

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
override protected bool ReleaseHandle()
{
    // Here, we must obey all rules for constrained execution regions. 
    return NativeMethods.CloseHandle(handle);
    // If ReleaseHandle failed, it can be reported via the 
    // "releaseHandleFailed" managed debugging assistant (MDA).  This
    // MDA is disabled by default, but can be enabled in a debugger 
    // or during testing to diagnose handle corruption problems. 
    // We do not throw an exception because most code could not recover 
    // from the problem.
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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