CODEWITHSUNDEEP
c#multithreadingwaithandle
user107586
user107586

Reputation: 81

How does disposing a waithandle not cause an infinite loop?

From what I can see in the source code for WaitHandle in both src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs src/runtime/src/libraries/System.Private.CoreLib/src/System/Threading/WaitHandle.cs

namely:

    public virtual void Close() => Dispose();

    protected virtual void Dispose(bool explicitDisposing)
    {
        _waitHandle?.Close();
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

It seems calling Close() will call Dispose() which will call Dispose(bool) which will call Close()...
second verse, same as the first for calling Dispose().

So how does this not result in an infinite loop?
Especially since none of these methods do anything which would make the _waitHandle be null, as far as I can understand.

Upvotes: 0

Views: 50

Answers (1)

jaros
jaros

Reputation: 164

This is not causing infinite recursion, because similarly named method Close () is called on different classes. The code you are quoting is from WaitHandle class. The private field in it has a different type:

private SafeWaitHandle? _waitHandle;

If you keep digging, SafeHandle (which is the parent of SafeWaitHandle) has the field and an abstract method:

// https://github.com/dotnet/runtime/blob/1d1bf92fcf43aa6981804dc53c5174445069c9e4/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/SafeHandle.cs
protected IntPtr handle;
protected abstract bool ReleaseHandle();

ReleaseHandle() is overridden in a partial .cs file and resolves to WinAPI native call CloseHandle (handle):

// https://github.com/microsoft/referencesource/blob/master/mscorlib/microsoft/win32/safehandles/safewaithandle.cs
override protected bool ReleaseHandle()
{
    return Win32Native.CloseHandle(handle);
}

The main takeaway is that System.Threading classes are thin wrappers around underylying WinAPI system objects. Everything will have to eventually resolve to a WinAPI call.

See also: Dispose pattern by Microsoft.

Upvotes: 3

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