Issue using Framework 4.0 in Powershell with [system.string]::join-Collection of common programming errors

This is due to the new overloads to string.Join added in version 4 of the .NET framework:

PS> [Environment]::Version.ToString()
2.0.50727.5456
PS> [string]::Join.OverloadDefinitions
static string Join(string separator, string[] value)
static string Join(string separator, string[] value, int startIndex, int count)

PS> [Environment]::Version.ToString()
4.0.30319.269
PS> [string]::Join.OverloadDefinitions
static string Join(string separator, Params string[] value)
static string Join(string separator, Params System.Object[] values)
static string Join[T](string separator, System.Collections.Generic.IEnumerable[T] values)
static string Join(string separator, System.Collections.Generic.IEnumerable[string] values)
static string Join(string separator, string[] value, int startIndex, int count)

In .NET 2.0, the only applicable method overload takes a string[] as the second argument, so each character from $word.ToCharArray() is coerced to a string.

PS> Trace-Command *Member* { [string]::Join( "-", $word.ToCharArray() ) } -PSHost
...
MemberResolution: Method argument conversion.
MemberResolution:     Converting parameter "-" to "System.String".
MemberResolution: Method argument conversion.
MemberResolution:     Converting parameter "System.Char[]" to "System.String[]".
MemberResolution: Calling Method: static string Join(string separator, string[] value)

In .NET 4, the object[] overload is preferred. Rather than boxing each character, the entire array is coerced to an object, sending a single-element object array to string.Join, which then has ToString called to produce the "System.Char[]" output.

Method argument conversion.
    Converting parameter "-" to "System.String".
Method argument conversion.
    Converting parameter "System.Char[]" to "System.Object".
Method argument conversion.
    Converting parameter "System.Object[]" to "System.Object[]".
Calling Method: static string Join(string separator, Params System.Object[] values)

To get the old behavior, you can insert a [string[]] cast to influence how the method is resolved (or in this case just use PowerShell’s -join operator, which works correctly everywhere :).

PS> $word = 'test'
PS> [string]::Join( '-', $word.ToCharArray() )
System.Char[]
PS> [string]::Join( '-', [string[]]$word.ToCharArray() )
t-e-s-t
PS> [char[]]$word -join '-'
t-e-s-t