Function

An Osy# app runs in two places — the server, which holds the data, and the browser, which holds the screen — and the runtime moves between them in…

A function is where your app's logic lives — a top-level unit of work, written like a C# method, that runs on the server. It is transactional by…

A C-style cast converts between the numeric types. It truncates toward zero, and it is checked — a value the target type cannot hold fails loudly…

Increment or decrement a numeric variable or property by one. In statement position (i++;) and as a for increment the pre- and post-forms are…

An array literal [a, b, c] is a value: a list you can return, assign, pass as an argument, or supply as a UI prop. Its element type is inferred from…

Sign with a private key, verify with a public one. This is what a signed JWT is made of, and a signed JWT is how almost every service authenticates a…

How to build a duration and read it back. Construct one with new TimeSpan(...) or a TimeSpan.FromX factory (including TimeSpan.FromMilliseconds)…

Update a variable or property in place: x op= y is shorthand for x = x op y. Arithmetic forms need a numeric lvalue; ??= assigns only when the left…

Build a temporal value from its components. DateTime.New takes a date, optionally with a time (defaulting to midnight); DateOnly.New takes a calendar…

Tests whether a string contains, begins with, or ends with another string. The match is case-sensitive and literal — like C#'s String.Contains, the…

Explicit conversion between types — number to text, text to number, a Guid to its text form. Osy# will not convert silently where information could…

Encrypt and decrypt values under your app's key, which the platform mints, protects, and rotates for you. There is no key parameter — you never…

HMAC-SHA-256 authenticates a message under a shared key — proving it came from a key holder and was not altered, which a plain hash cannot do. Always…

MD5 of a string's UTF-8 bytes rendered as 32-character lowercase hex — the canonical C# fingerprint form. Deterministic, so it pushes down into SQL…

SHA-256 of a string's UTF-8 bytes as 64-character lowercase hex — the secure default hash. Deterministic, so it pushes down into SQL. Use it wherever…

Read the current instant. `DateTime.UtcNow` (and the platform-idiomatic `DurableClock.Now`/…) return the current time from the platform's replay-safe…

Move a DateTime forward or back. AddDays/AddHours/AddMinutes/AddSeconds/AddMilliseconds take a fractional amount and are exact to the tick. AddMonths…

Parallel.For, Parallel.ForEach and Parallel.ForEachAsync run one body per item, each as a task beside the others, and wait for all of them; failures…

Read the human-facing words of an enum value in code: its Label (the [Label] text, or the member name when there is none), its Description (the…

A sequence of consecutive integers, count of them, starting at start. It is how you iterate by index — including in a render block, which has foreach…

The C# Guid statics. `Guid.Empty` is the all-zero Guid constant, spelled without parens; `Guid.NewGuid()` mints a fresh unique Guid. Guid.Empty…

A value built out of references to other values — a linked list, a tree, a chain of parents — may nest up to 500 levels. Past that, serializing it…

A function declared `IEnumerable<T>` whose body uses `yield return` is a C# iterator: each `yield return` adds one item to the sequence it hands…

C#'s index from the end (^n) and ranges (a..b) inside an indexer. ^1 is the last element of a string, an array or a list; a range slices a string…

Sort a local List<T> in memory by a key selector, returning a NEW sorted List<T> (the source is untouched). Ascending or descending, stable, exactly…

Order a local List<T> IN PLACE by a comparison you write: a lambda taking two elements and answering an int — negative when the first goes first, 0…

Reorder or shorten a local List<T> IN PLACE, changing the list you are holding rather than answering a new one. Sort() orders by the elements…

Positional get/set on a List<T> by integer index, exactly like C#'s List<T>.this[int]. The index must be an integer; the result is the element type…

A local function is a helper declared inside the body that uses it — `int Sq(int x) => x * x;`. It can be called before or after its declaration, can…

The numeric helpers, each returning the type C# says it returns. Abs, Min, Max and Clamp answer in the WIDEST of their arguments, so a decimal stays…

The platform numeric types are int, long, decimal, and double. Literals follow C# exactly, suffixes (L, m, d) included: a bare decimal-point literal…

A function's NAME, written where a function value is wanted, is that function — `Func<int, int> f = Double;`, `xs.Select(Double)`, `xs.Where(IsDue)`…

A pattern asks a question about a value's SHAPE — is it 7, is it over 0 and under 10, does it have an X of 1, is it a list that starts with 1 — and…

A QR code for a piece of text, as rows of modules a page draws with ordinary boxes — level M, the smallest version that holds it, the mask the…

`Raise(e)` says that the event `e` happened — an instance of a class marked `[Event]`. It is written in the same commit as the work around it, and…

Read a component off a DateTime — its year, month, day, hour, minute, second — or its day of the week (a DayOfWeek, as in C#), its day of the year (1…

Matching, replacing and splitting with regular expressions. A pattern means the same thing wherever your code runs — the platform makes the browser…

The byte-in, byte-out half of the crypto surface. Real request-signing schemes chain their HMACs — each step's raw output becomes the next step's key…

C#'s cancellation, with C#'s meaning. A CancellationTokenSource hands out a Token; cancel the source and every task that was handed the token stops —…

Build a string from literal text and embedded expressions with $"…{expr}…". A hole may carry a .NET format specifier after a colon ({amount:F2})…

Formats a byte count as the words a person reads — "0 B", "1.5 KB", "2.7 MB". Binary units (1024) with the conventional KB/MB/GB labels. An ordinary…

Produce a new string from an old one: upper-case the first letter, replace every occurrence of a substring, repeat it, or take characters from one…

Joins a list of values into one string with NOTHING between the elements. It is Text.Join with no separator, and the same operation as string.Concat…

The C# string.IndexOf: the index of the FIRST ordinal occurrence of a substring, or -1 if absent. The 3-arg overload resumes the forward search at…

Joins a list of values into one string, placing the separator between each pair. It is the inverse of Text.Split, and the same operation as…

The C# string.LastIndexOf: the index of the LAST ordinal occurrence of a substring, or -1 if absent. The 3-arg overload starts the backward search at…

Ask a string a question without changing it: its length, whether it is empty or blank, and whether it contains, starts with, or ends with a piece of…

Matches a string against a wildcard pattern, where % stands for any run of characters and _ for exactly one. It is the only wildcard search in the…

The C# string.Split: breaks a string on a separator and returns the substrings as a List<string> — iterable with foreach and queryable with .Count /…

Capitalises the first letter of each word and lower-cases the rest, leaving a word that is already entirely upper-case untouched so acronyms survive…

Trim whitespace from one end of a string, or pad it out to a width with a fill string. Trimming uses the full Unicode whitespace set. Padding never…

Shorten a string to at most maxLength characters — INCLUDING the ellipsis — cutting back to the last word boundary rather than mid-word. The bound…

Osy# is C#-shaped, and leaves out three things on purpose: unsafe code (pointers, fixed, stackalloc, function pointers, native interop), float, and…

Authenticator-app codes (RFC 6238 — the six-digit, thirty-second codes every authenticator app shows), which is what a second factor is built on…

A function returns several values by declaring a tuple return type and returning a parenthesized list. The caller reads them by name (`t.ok`), or…

Locals can declare an explicit type instead of var; the declared type pins the binding, and every later assignment to the name is checked against it…

Every body accumulates its writes in a unit of work rather than sending them one at a time. UnitOfWork.Commit() makes everything accumulated so far…

Marks form a stack on the unit of work. UnitOfWork.Mark(label) records everything staged so far and answers a UnitOfWorkMark; Undo() rolls back to…

Task.WhenAll waits for every task and answers their results in order; Task.WhenAny answers the first task to finish, and the others keep running, as…

Wait before carrying on — between checks on a job another system is still running, before trying a call again, or for a moment in a component. `await…

A passkey (W3C Web Authentication) has a server half and a browser half. `WebAuthn.*` runs on the server: it makes the options a browser is handed…

Mark a function `[Action]` and the app offers it to be done BY NAME — by a rule, a macro, a tool — with everything a caller needs derived from the…

A function marked `[CommitsAlone]` runs in a unit of work of its own. What it writes is committed when it returns and stays committed whatever its…

An event is a class marked `[Event]`: something that happens to a record, declared once by the code that owns the record. It is raised where it…

`[Interest(nameof(Triggers))]` beside the `[On]` of a handler of EVERY event names a function that answers which event types it cares about now — the…

`[OnStart]` marks a function the platform runs, as the app, whenever the app version is loaded — on every node that serves it, after every compile…

`[On]` on a function makes it a HANDLER of the event its one parameter is. Every handler of an event — in the app and in every package that sees it —…

`[OncePerDelivery]` on a function that reads makes it answer once per delivery: the events one commit raised are delivered together, and a handler…

C#'s async and await, with C#'s meaning. A call to a Task-returning method that you do not await where you make it starts a task that runs beside the…

break leaves the enclosing loop; continue skips the rest of this pass. Inside a switch, break leaves the SWITCH, not the loop around it — the one…

A value fixed at compile time and folded into the places it is used. Declare one at the TOP LEVEL to share it across the whole app, on a component…

The C-style counting loop. Runs init once, then repeats the body while cond holds, running update after each pass. Any clause may be omitted; for…

Walks a collection — a query result, a list, or a parent's children. The normal way to iterate; reach for a for loop only when you need the index.

Formats a number to a string with a .NET format specifier — F2 for two decimal places, N0 for a grouped whole number, C for currency, P for a…

A function is a top-level unit of work, written like a C# method — a return type, a name, typed parameters, a body. It runs transactionally. Called…

`[Authorize(Policy)]` on a function runs it only for a caller who satisfies the named `policy` — checked when it is called from a page, an API or a…

Conditional branching, exactly as in C#. The condition must be a bool — there is no truthiness, so a null or a number is not a condition.

The C# compile-time name fold: validates the symbol and folds to its simple-name string literal — the last identifier of the chain. Invalid symbols…

`Secret.Name` in a function body evaluates to the declared secret's value — the key itself, as a string, read at the moment it is used…

Branch on a value against constant case labels. Only the matched section runs (no fall-through); a default section handles the rest. break exits the…

Choose a VALUE by matching a subject against patterns. Arms are tried in order and the first match wins. An arm is any C# pattern — a constant, a…

Raises a fault. It ends the function immediately, and the rows the function wrote are discarded rather than half-written. An exception is a class:…

Handles a fault. C#'s syntax, including typed catches, catch filters and finally. The rows written inside a try block that throws are discarded — so…

C#'s `using`, exactly: `Dispose()` is called on every way out of the block — normal exit, return, break, continue, or an exception — last declared…

Declares a local whose type is inferred from its initializer, exactly as in C#. The local is still statically typed — var is about not repeating the…

`@"…"` is a string where a backslash is just a backslash. Nothing inside is an escape, `""` writes a single quote, and the text may run across lines…

Repeats while a bool condition holds. Reach for it when the number of iterations is not known up front — otherwise a foreach or a for loop says more.

A lambda is an inline function: a parameter list, `=>`, and a body. Its parameter types come from where it is used, or you write them yourself —…

A `stream<T>` function produces its results one at a time instead of all at once, and a `live var` bound to one renders each item the moment it…