F＃中的变量是不可变的，这意味着一旦变量绑定到一个值，它就不能被改变。 它们实际上被编译为静态只读属性。
下面的例子说明了这一点。
例
let x = 10let y = 20let z = x + yprintfn "x: %i" xprintfn "y: %i" yprintfn "z: %i" zlet x = 15let y = 20let z = x + yprintfn "x: %i" xprintfn "y: %i" yprintfn "z: %i" z
当你编译和执行程序，它显示了以下错误消息
Duplicate definition of value 'x'Duplicate definition of value 'Y'Duplicate definition of value 'Z'
可变变量
有时您需要更改存储在变量中的值。 要指定在程序的后面部分中声明的和分配的变量的值可能有变化，F＃提供mutable。 您可以使用此关键字声明和分配可变变量，其值将更改。mutable允许在可变变量中声明和赋值。
您可以使用theletkeyword为可变变量分配一些初始值。 但是，要为其分配新的后续值，需要使用<-运算符。
例如，
let mutable x = 10x <- 15
以下示例将让这个概念更加清晰
例
let mutable x = 10let y = 20let mutable z = x + yprintfn "Original Values:"printfn "x: %i" xprintfn "y: %i" yprintfn "z: %i" zprintfn "Let us change the value of x"printfn "Value of z will change too."x <- 15z <- x + yprintfn "New Values:"printfn "x: %i" xprintfn "y: %i" yprintfn "z: %i" z
当你编译和执行程序，它产生以下输出
Original Values:x: 10y: 20z: 30Let us change the value of xValue of z will change too.New Values:x: 15y: 20z: 35
可变数据的使用
可变数据通常需要和在数据处理中使用，特别是与记录数据的结构。下面的例子说明了这一点
open Systemtype studentData = { ID : int; mutable IsRegistered : bool; mutable RegisteredText : string; }let getStudent id = { ID = id; IsRegistered = false; RegisteredText = null; }let registerStudents (students : studentData list) = students |> List.iter(fun st -> st.IsRegistered <- true st.RegisteredText <- sprintf "Registered %s" (DateTime.Now.ToString("hh:mm:ss")) Threading.Thread.Sleep(1000) (* Putting thread to sleep for 1 second to simulate processing overhead. *))let printData (students : studentData list) = students |> List.iter (fun x -> printfn "%A" x)let main() = let students = List.init 3 getStudent printfn "Before Process:" printData students printfn "After process:" registerStudents students printData students Console.ReadKey(true) |> ignoremain()当你编译和执行程序，它产生以下输出
Before Process:{ID = 0;IsRegistered = false;RegisteredText = null;}{ID = 1;IsRegistered = false;RegisteredText = null;}{ID = 2;IsRegistered = false;RegisteredText = null;}After process:{ID = 0;IsRegistered = true;RegisteredText = "Registered 05:39:15";}{ID = 1;IsRegistered = true;RegisteredText = "Registered 05:39:16";}{ID = 2;IsRegistered = true;RegisteredText = "Registered 05:39:17";}