if(a!=null)

if(a.HasValue)

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

if(a is not null)

if(a is {b: not null})

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

G>            if (obj.TryGetPropertyValue("entity", out var entityNode) &&
G>                obj.TryGetPropertyValue("dataPath", out var dataPathNode) &&
G>                entityNode is JsonValue entityValue &&
G>                dataPathNode is JsonValue dataPathValue &&
G>                entityValue.TryGetValue<string>(out var entityId) &&
G>                dataPathValue.TryGetValue<string>(out var dataPath))
G>            {
G>            }
G>

            else
            {
              throw new Exception("Что-то пошло не так");
            }

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>

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

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

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;
    }
}

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.

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 { } stages)

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

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

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

G>void ProcessContent(JsonNode node)
G>{
G>    switch (node)
G>    {
G>        case JsonObject obj:
G>

 case JsonObject obj:
    var x = obj.Deserialize<Option?>();