Issue: C# Assigning variable to inline delegate and using it in the delegate body causes "Using unassigned variable" error

[pmunin]

Expected Behavior:

Action<Node, Action<Node> > traverseTree = (node, visitNode)=>
{
     visitNode(node);
     node.Children.ForEach(c=>traverseTree(c, visitNode)); //Compiler throws "Using of unassigned variable 'traverseTree'"
}

Actual Behavior:

Action<Node, Action<Node> > traverseTree = null;
traverseTree = (node, visitNode)=>
{
     visitNode(node);
     node.Children.ForEach(c=>traverseTree(c, visitNode));
}

Another good use case which is very common provided by @jnm2:

SomeDelegate oneTimeHandler = null;
obj.SomeEvent += oneTimeHandler = (s, e) =>
{
    obj.SomeEvent -= oneTimeHandler;
    // ...
};

And combined with #15659 it could be something like this:

obj.SomeEvent += var oneTimeHandler = (s, e) =>
{
    obj.SomeEvent -= oneTimeHandler;
    // ...
};

[HaloFour]

That is expected behavior. The variable has not been assigned yet. This applies to any expressions that attempt to reference the variable before it has been assigned, even if that variable will be assigned as a result of the expression:

int x = x + 1; // CS0165: Use of unassigned local variable 'x'

For delegates specifically, local functions alleviate the need for this as their hoisted scope allows them to be self-referential.

[pmunin]

int x = x+1; is different, because variable is used by execution flow before it was assigned.
In case of assigning to a delegate - its body execution is deferred and will happen obviously when the variable assignment is accomplished.

[HaloFour]

The compiler does not track the lambda body as separate from the body of the current method, particularly in the case of a closure. In any case this feature is addressed by local functions which are being added to C#.

[pmunin]

Just found out how this feature is implemented in F#:

let rec fact x =
    if x < 1 then 1
    else x * fact (x - 1)

[CyrusNajmabadi]

Closing this out. We're doing all language design now at dotnet/csharplang. If you're still interested in this idea let us know and we can migrate this over to a discussion in that repo. Thanks!

Note: this is doable with local-functions, and that woudl be the recommended approach here.