C#/.NET 当我们在写事件 += 和 -= 的时候，方法是如何转换成事件处理器的

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// 这是简化的代码。 public event FileSystemEventHandler Changed;

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public delegate void FileSystemEventHandler(object sender, FileSystemEventArgs e);

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void Subscribe(FileSystemWatcher watcher) { watcher.Changed = new FileSystemEventHandler(OnChanged); } void OnChanged(object sender, FileSystemEventArgs e) { }

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// 方法组。 watcher.Changed = OnChanged;

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// Lambda 表达式。 watcher.Changed = (sender, e) => Console.WriteLine(e.ChangeType);

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// Lambda 表达式。 watcher.Changed = (sender, e) => { // 事件引发时，代码会在这里执行。 };

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// 匿名方法。 watcher.Changed = delegate (object sender, FileSystemEventArgs e) { // 事件引发时，代码会在这里执行。 };

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// 委托类型的局部变量（或者字段）。 FileSystemEventHandler onChanged = (sender, e) => Console.WriteLine(e.ChangeType); watcher.Changed = onChanged;

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// 局部方法（或者局部静态方法）。 watcher.Changed = OnChanged; void OnChanged(object sender, FileSystemEventArgs e) { }

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FileSystemEventHandler onChanged1 = (sender, e) => Console.WriteLine(e.ChangeType); Action<object, FileSystemEventArgs> onChanged2 = (sender, e) => Console.WriteLine(e.ChangeType);

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void Subscribe(FileSystemWatcher watcher) { watcher.Changed = new FileSystemEventHandler(OnChanged); watcher.Changed -= new FileSystemEventHandler(OnChanged); } void OnChanged(object sender, FileSystemEventArgs e) { }

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watcher.Changed = OnChanged; watcher.Changed -= OnChanged;

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public event FileSystemEventHandler Changed { add { onChangedHandler = (FileSystemEventHandler)Delegate.Combine(onChangedHandler, value); } remove { onChangedHandler = (FileSystemEventHandler)Delegate.Remove(onChangedHandler, value); } }

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public override bool Equals(object? obj) { if (obj == null || !InternalEqualTypes(this, obj)) return false; Delegate d = (Delegate)obj; // do an optimistic check first. This is hopefully cheap enough to be worth if (_target == d._target && _methodPtr == d._methodPtr && _methodPtrAux == d._methodPtrAux) return true; // even though the fields were not all equals the delegates may still match // When target carries the delegate itself the 2 targets (delegates) may be different instances // but the delegates are logically the same // It may also happen that the method pointer was not jitted when creating one delegate and jitted in the other // if that's the case the delegates may still be equals but we need to make a more complicated check if (_methodPtrAux == IntPtr.Zero) { if (d._methodPtrAux != IntPtr.Zero) return false; // different delegate kind // they are both closed over the first arg if (_target != d._target) return false; // fall through method handle check } else { if (d._methodPtrAux == IntPtr.Zero) return false; // different delegate kind // Ignore the target as it will be the delegate instance, though it may be a different one /* if (_methodPtr != d._methodPtr) return false; */ if (_methodPtrAux == d._methodPtrAux) return true; // fall through method handle check } // method ptrs don't match, go down long path // if (_methodBase == null || d._methodBase == null || !(_methodBase is MethodInfo) || !(d._methodBase is MethodInfo)) return Delegate.InternalEqualMethodHandles(this, d); else return _methodBase.Equals(d._methodBase); }

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void Subscribe(FileSystemWatcher watcher) { watcher.Changed = new FileSystemEventHandler(OnChanged); watcher.Changed -= new FileSystemEventHandler(OnChanged); } void OnChanged(object sender, FileSystemEventArgs e) { }

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// this 就是对象，OnChanged 就是方法信息。 this.OnChanged