如果源对象被取消引用，Func 和 Action 委托会变为 null 吗？

Question

Define these variables in Class A:

public int CubeAnInt32(int n)
{
    return n * n * n;
}

And these variables in Class B:

public void CubeAnInt32WithDelegate(int k, Func<int, int> delg)
{
    return delg(k);
}

And an overall scope:

/// Note that this is outside the scope of SomeArbitraryCallback()
Func<int, int> cube = default;

public void SomeArbitraryCallback()
{
    var refWithinScope = new ClassA();
    cube = refWithinScope.CubeAnInt32;
    return;
}

public void AnotherCallbackPerformedAfter()
{
    var cubeDependent = new ClassB();

    /// Does this throw a NullReferenceException because of 'cube'?
    /// Has the delegate assigned to 'cube' been dereferenced?
    var result = cubeDependent.CubeAnInt32WithDelegate(0, cube);
    return result;
}

Will a delegate whose object has been "de-scoped" (ie cube) be dereferenced? Or will this delegate reference the assigned object and prevent it from being GC'd?

Answer 1

As per https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/delegates/ - "A delegate is a type that represents references to methods" (emphasis is mine).

So as long as your delegate ( cube ) remains in scope, a reference is held and refWithinScope (of type ClassA ) will not be collected.

Whether you want to have this sort of design is a different discussion: looks like a good candidate for introducing subtle, unintended bugs in future re-factors.

Answer 2

A delegate - concerning referencing null - is in no way different from any other type. So if the variable is null and you try to invoke it, you surely get a NullReferenceException .

public event EventHandler<EventArgs>  MyEvent;

private void RaiseEvent()
{
    MyEvent.Invoke(this, myEventArgs);
}

So of course you need to assign somethig to your delegate, otherwise it's just null .

The same applies to a property. Unless you don't assignt any value to it, it will allways have its default-value, which is null for reference-types:

public string MyString { get; set; }
private void DoSomething()
{
    Console.WriteLine(MyString.Length); // crash here
}

Coming back to your code, Func<int, int> cube = default; will just assign null to cube . So when you call CubeAnInt32WithDelegate before having assigned something to the delegate, eg by calling SomeArbitraryCallback , you will get an NRE.