How do you set a null to a string in PowerShell?

SetNullString.ps1

Add-Type "
public class MyClass {
    public string MyString {get;set;}
    public object GetNull { get { return null; } }
    public void SetNull() { MyString = null; }
}"

$myObject = New-Object -TypeName MyClass
$myObject.MyString = "Hey!" 
$myObject.MyString = $null
$myObject.Mystring -eq [String]::Empty
$myObject.MyString -eq $null
$myObject.MyString = $myObject.GetNull
$myObject.Mystring -eq [String]::Empty
$myObject.MyString -eq $null
$myObject.SetNull()
$myObject.Mystring -eq [String]::Empty
$myObject.MyString -eq $null

<# Results in 

True
False
True
False
False
True

#>

$myObject.MyString = [NullString]::Value
$myObject.MyString -eq $null # returns true