$ dotnet fsi

Microsoft (R) F# Interactive version 12.0.5.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> System.Security.Cryptography.MD5.Create();;

  System.Security.Cryptography.MD5.Create();;
  -----------------------------^^^

stdin(12,30): error FS0039: The value, constructor, namespace or type 'MD5' is not defined.

 > type ComputeHash = delegate of byte[] -> byte[]
let computeHash  =
    let asm = System.AppDomain.CurrentDomain.GetAssemblies() |> Seq.find (fun x -> x.FullName.StartsWith("System.Security.Cryptography,"))
    let md5 = asm.GetTypes() |> Seq.find (fun x -> x.Name = "MD5")
    let computeHashMeth = md5.GetMethod("ComputeHash", [| typeof<byte[]> |])
    let md5obj = md5.GetMethod("Create",[||]).Invoke(null,[||])
    let d : ComputeHash = computeHashMeth.CreateDelegate(md5obj)
    fun x -> d.Invoke x
;;
type ComputeHash =
  delegate of byte[] -> byte[]
val computeHash: (byte[] -> byte[])

> computeHash [|0uy|];;
val it: byte[] =
  [|147uy; 184uy; 133uy; 173uy; 254uy; 13uy; 160uy; 137uy; 205uy; 246uy; 52uy;
    144uy; 79uy; 213uy; 159uy; 113uy|]