Buffer.BlockCopy vs unions

BlockCopy .fs

//Results on my machine
//U1 Elapsed Time: 2285 Consumed memory: 81313268
//U2 Elapsed Time: 5729 Consumed memory: 60775624
//U1 Elapsed Time: 2141 Consumed memory: 80048624
//U2 Elapsed Time: 6011 Consumed memory: 57525248
//U1 Elapsed Time: 2208 Consumed memory: 81011948
//U2 Elapsed Time: 5653 Consumed memory: 58437492
open System
type U1 = 
        | F of float32
        | C of char
        | I of int

type U2Tag =
        | F = 0uy
        | C = 1uy
        | I = 2uy

type U2 =
   struct 
      val tag: U2Tag
      val b0 : byte
      val b1 : byte 
      val b2 : byte
      val b3 : byte

      new(x: float32) = 
        let v = [|0uy; 0uy; 0uy; 0uy |]
        Buffer.BlockCopy([|x|], 0 , v, 0, 4)
        { 
            tag = U2Tag.F;  
            b0 = v.[0];
            b1 = v.[1];
            b2 = v.[2];
            b3 = v.[3];
        }

      new(x: int) = 
        let v = [|0uy; 0uy; 0uy; 0uy |]
        Buffer.BlockCopy([|x|], 0 , v, 0, 4)
        { 
            tag = U2Tag.I;  
            b0 = v.[0];
            b1 = v.[1];
            b2 = v.[2];
            b3 = v.[3];
        }
     new(x: char) = 
        let v = [|0uy; 0uy; |]
        Buffer.BlockCopy([|x|], 0 , v, 0, 2)
        { 
            tag = U2Tag.I;  
            b0 = v.[0];
            b1 = v.[1];
            b2 = 0uy;
            b3 = 0uy;
        }
     member this.char : char =
        if this.tag = U2Tag.C
            then let res = Array.zeroCreate 1
                 Buffer.BlockCopy([|this.b0; this.b1|], 0 , res, 0, 2)
                 res.[0]
            else failwith "it is not a char"
     member this.int : int =
        if this.tag = U2Tag.I
            then let res = Array.zeroCreate 1
                 Buffer.BlockCopy([|this.b0; this.b1;this.b2; this.b3|], 0 , res, 0, 4)
                 res.[0]
            else failwith "it is not an int"
     member this.float32 : float32=
        if this.tag = U2Tag.F
            then let res = Array.zeroCreate 1
                 Buffer.BlockCopy([|this.b0; this.b1;this.b2; this.b3|], 0 , res, 0, 4)
                 res.[0]
            else failwith "it is not a float"
   end 
let duration str f = 
    let timer = new System.Diagnostics.Stopwatch()
    let memory = GC.GetTotalMemory(true)
    timer.Start()
    let returnValue = f()
    timer.Stop()
    let memoryUsage = GC.GetTotalMemory(false) - memory
    printfn "%s Elapsed Time: %i Consumed memory: %d" str timer.ElapsedMilliseconds memoryUsage
    returnValue
let max = 1000000
let test data =
    let arr = Array.zeroCreate (max + 1)
    for idx in 0..max do
        let c,i,f = data(idx)
        arr.[idx] <- [|c,i,f|]
    arr
[<EntryPoint>]
let main argv = 
    let u1data  i = (C('a'), I(i), F(float32 i))
    let testu1() = test u1data
    let u2data (i:int) = (new U2('a'), new U2(i), new U2(float32 i))
    let testu2() = test u2data
    for i in 0..10 do
        duration "U1" testu1 |> ignore
        duration "U2" testu2 |> ignore
    
    0 // return an integer exit code