C# Signature:

[DllImport("advapi32.dll", SetLastError = true)]
static extern bool GetUserName(System.Text.StringBuilder sb, ref Int32 length);

VB.NET Signature:

<DllImport("advapi32.dll", EntryPoint:="GetUserNameW", SetLastError:=True)> _
Public Function GetUserName(<MarshalAs(UnmanagedType.LPTSTR)>sb As System.Text.StringBuilder, _
         ByRef length As Integer) As <MarshalAs(UnmanagedType.Bool)>Boolean
End Function

VB Signature:

Declare Function GetUserName Lib "advapi32.dll" Alias "GetUserNameA" (ByVal lpBuffer As String, ByRef nMax As Integer) As Boolean

User-Defined Types:

None.

Alternative Managed API:

Environment.UserName (System.Environment)

Notes:

None.

Tips & Tricks:

Please add some!

Sample Code:

using System;
using System.Text;
using System.Runtime.InteropServices;
namespace GetUserNameExample
{
    class Class1
    {
        [DllImport("Advapi32.dll")]
        static extern bool GetUserName(StringBuilder lpBuffer, ref int nSize);    
        [STAThread]
        static void Main(string[] args)
        {
            StringBuilder Buffer = new StringBuilder(64);
            int nSize=64;
            GetUserName(Buffer, ref nSize);
            Console.WriteLine(Buffer.ToString());
        }
    }
}

Sample Code (VB.NET):

Imports System
Imports System.Text
Imports System.Runtime.InteropServices
Namespace GetUserNameExample

    Class Class1

        <DllImport("advapi32.dll", EntryPoint:="GetUserNameA", SetLastError:=True)> _
        Public Function GetUserName(<MarshalAs(UnamagedType.LPTSTR)>System.Text.StringBuilder sb, _
            ByRef length As Integer) As <MarshalAs(UnmanagedType.Bool)>Boolean
        End Function

        <STAThread> _
        Public Sub Main()

            Dim Buffer = new StringBuilder(64)
            Dim nSize As Integer =64
            GetUserName(Buffer, nSize)
            Console.WriteLine(Buffer.ToString())
        End Sub
    End Class
End Namespace

Sample Code (VB):

Dim xstr as String = Space(255)
Dim max as Integer = 255
Dim rc as Integer

rc = GetUserName(xstr,max)
' max will now contain the total number of
' characters written to the buffer (here: xstr)
MessageBox.Show(Mid(xstr,1,max))

Sample Code (PowerShell):

$sig = @'
[DllImport("advapi32.dll", SetLastError = true)]
public static extern bool GetUserName(System.Text.StringBuilder sb, ref Int32 length);
'@

Add-Type -MemberDefinition $sig -Namespace Advapi32 -Name Util

$size = 64
$str = New-Object System.Text.StringBuilder -ArgumentList $size

[Advapi32.util]::GetUserName($str,[ref]$size) |Out-Null
$str.ToString()