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 = 10
let y = 20
let z = x + y

printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

let x = 15
let y = 20
let z = x + y

printfn "x: %i" x
printfn "y: %i" y
printfn "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 = 10
x <- 15

以下示例将让这个概念更加清晰

例

let mutable x = 10
let y = 20
let mutable z = x + y

printfn "Original Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

printfn "Let us change the value of x"
printfn "Value of z will change too."

x <- 15
z <- x + y

printfn "New Values:"
printfn "x: %i" x
printfn "y: %i" y
printfn "z: %i" z

当你编译和执行程序，它产生以下输出 

Original Values:
x: 10
y: 20
z: 30
Let us change the value of x
Value of z will change too.
New Values:
x: 15
y: 20
z: 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 System

type 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) |> ignore

main()

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";}