if (stage() is { } stages)

BE>if (stage() is { } stages)
BE>

if (stage() is { Count: > 0 } stages)

BE>if (stage() is { } stages)
BE>

To check if the string s is non-null, you can write any of the following forms

if (s is object o) ... // o is of type object
if (s is string x) ... // x is of type string
if (s is {} x) ... // x is of type string
if (s is {}) ...

if (stage() is not null stages)
    stages.SomeMethod();

if(a!=null)

if(a.HasValue)

if(a?.b?.HasValue==true)

if(a is not null)

if(a is {b: not null})

BE>if (stage() is { } stages)
BE>

let xs = [1;2;3]
(* было *)
let x = xs.[0] // => 1
(* стало *)
let x = xs[0] // => 1
let x = xs [0] // =>  error FS3217: Это значение не является функцией, и применить его невозможно. Вы хотели обратиться к индексатору с помощью "xs[index]"?

> let xs:List<Stage> = null;;
val xs : List<Stage> = null

> let r = match xs with | :? List<Stage> as x -> x.GetType().ToString() | _ -> "noname";;

  let r = match xs with | :? List<Stage> as x -> x.GetType().ToString() | _ -> "noname";;
  ------------------------^^^^^^^^^^^^^^

stdin(15,25): warning FS0067: Данная проверка типа или понижение будет всегда сохраняться

val r : string = "noname"

if (stage() is not null stages)
Q>    stages.SomeMethod();
Q>

if (stage() is not null and var stages)
    stages.SomeMethod();

if (stage() is not null stages)
Q>    stages.SomeMethod();
Q>

if (input is not (float or double))
{
    return;
}

void ProcessContent(JsonNode node)
{
    switch (node)
    {
        case JsonObject obj:
            if (obj.TryGetPropertyValue("entity", out var entityNode) &&
                obj.TryGetPropertyValue("dataPath", out var dataPathNode) &&
                entityNode is JsonValue entityValue &&
                dataPathNode is JsonValue dataPathValue &&
                entityValue.TryGetValue<string>(out var entityId) &&
                dataPathValue.TryGetValue<string>(out var dataPath))
            {
                if (dataPath.Split("/") is
                    ["api", "v1.0", "dynamicEntityTable", var projectName, var moduleName, var name])
                {
                    // ....
                }
                else
                {
                    // ....
                }
            }
            foreach (var pair in obj)
            {
                await ProcessContent(pair.Value);
            }

            break;
        case JsonArray arr:
            foreach (var subNode in arr)
            {
                await ProcessContent(subNode);
            }

            break;
    }
}

S>if(a!=null)
S>

S>if(a.HasValue)
S>

S>if(a?.b?.HasValue==true)
S>

S>if(a is not null)
S>

bool CheckFor42(object a)
{
  return a == 42; // fail
  return a is 42;  // fine
}

  return a is int && (a as int) == 42;

S>if(a is {b: not null}) 
S>

G> entityValue.TryGetValue<string>(out var entityId) &&
G> dataPathValue.TryGetValue<string>(out var dataPath))

entityValue.TryGetValue(out string entityId) &&
dataPathValue.TryGetValue(out string dataPath))

BE>if (stage() is { } stages)

❌ DO NOT return null values from collection properties or from methods returning collections. Return an empty collection or an empty array instead.

The general rule is that null and empty (0 item) collections or arrays should be treated the same.