Why box field is required in integer in ILDASM

Question

.method public hidebysig instance void fun(string h) cil managed

{

// Code size 133 (0x85)

.maxstack 2

.locals init (int32 V_0)

IL_0000: nop

IL_0001: ldstr "base called from "

IL_0006: ldarg.0

IL_0007: ldfld int64 ConsoleApp6.Base::x

IL_000c: box [System.Runtime]System.Int64

IL_0011: call string [System.Runtime]System.String::Concat(object,

object)

IL_0016: call void [System.Console]System.Console::Write(string)

IL_001b: nop

IL_001c: nop

IL_001d: ldstr "x value next after declartion is"

IL_0022: ldarg.0

IL_0023: ldfld int64 ConsoleApp6.Base::x

IL_0028: box [System.Runtime]System.Int64

IL_002d: call string [System.Runtime]System.String::Concat(object,

object)