C# Signature:

[DllImport("Advapi32.dll", SetLastError=true, EntryPoint="CredWriteW", CharSet=CharSet.Unicode)]
static extern bool CredWrite([In] Credential userCredential, [In] UInt32 flags);

VB Signature:

Declare Function CredWrite Lib "advapi32.dll" (TODO) As TODO

User-Defined Types:

enum CRED_TYPE : uint
{
     CRED_TYPE_GENERIC = 1,
     CRED_TYPE_DOMAIN_PASSWORD = 2,
     CRED_TYPE_DOMAIN_CERTIFICATE = 3,
     CRED_TYPE_DOMAIN_VISIBLE_PASSWORD = 4
}

enum CRED_PERSIST : uint
{
     CRED_PERSIST_SESSION = 1,
     CRED_PERSIST_LOCAL_MACHINE = 2,
     CRED_PERSIST_ENTERPRISE = 3
}

[StructLayout(LayoutKind.Sequential)]
private struct Credential
{
     public UInt32 flags;
     public UInt32 type;
     public string targetName;
     public string comment;
     //public System.Runtime.InteropServices.FILETIME lastWritten; // .NET 1.1
     public System.Runtime.InteropServices.ComTypes.FILETIME lastWritten; // .NET 2.0
     public UInt32 credentialBlobSize;
     public IntPtr credentialBlob;
     public UInt32 persist;
     public UInt32 attributeCount;
     public IntPtr credAttribute;
     public string targetAlias;
     public string userName;
}

Notes:

If it's legal for PInvoke.net to swipe Microsoft's own code, the stuff here is even better:

    http://blogs.msdn.com/peerchan/pages/487834.aspx

The stuff on this page about needing custom marshaling are just wrong.

Tips & Tricks:

Needs extensive custom marshaling code. See CredRead to for an example on the custom marshaling code.

Sample Code:

C#

internal static void AddDomainUserCredential(string target, string userName, string password)
{
     Credential userCredential = new Credential();

     userCredential.targetName = target;
     userCredential.type = (UInt32)CRED_TYPE.CRED_TYPE_DOMAIN_PASSWORD;
     userCredential.userName = userName;
     userCredential.attributeCount = 0;
     userCredential.persist = (UInt32)CRED_PERSIST.CRED_PERSIST_ENTERPRISE;
     byte[] bpassword = Encoding.Unicode.GetBytes(password);
     userCredential.credentialBlobSize = (UInt32)bpassword.Length;
     userCredential.credentialBlob = Marshal.StringToCoTaskMemUni(password);
     if (!Unmanaged.CredWrite(ref userCredential, 0))
     {
     throw new System.ComponentModel.Win32Exception(Marshal.GetLastWin32Error());
     }
}

PowerShell

$sig = @"
[DllImport("Advapi32.dll", SetLastError=true, EntryPoint="CredWriteW", CharSet=CharSet.Ansi)]
public static extern bool CredWrite([In] Credential userCredential, [In] UInt32 flags);

[StructLayout(LayoutKind.Sequential)]
public struct Credential
{
     public UInt32 flags;
     public UInt32 type;
     public string targetName;
     public string comment;
     public System.Runtime.InteropServices.ComTypes.FILETIME lastWritten;
     public UInt32 credentialBlobSize;
     public IntPtr credentialBlob;
     public UInt32 persist;
     public UInt32 attributeCount;
     public IntPtr Attributes;
     public string targetAlias;
     public string userName;
}
"@
$type = Add-Type -MemberDefinition $sig -Namespace "ADVAPI32" -Name 'Util' -PassThru

$cred = New-Object ADVAPI32.Util+Credential
$cred.targetName = 'server1'
$cred.type = 2
$cred.userName = 'home\user'
$cred.attributeCount = 0
$cred.persist = 2
$password = "password"
$cred.credentialBlobSize = [System.Text.Encoding]::Unicode.GetBytes($password).length
$cred.credentialBlob = [System.Runtime.InteropServices.Marshal]::StringToCoTaskMemUni($password)
[ADVAPI32.Util]::CredWrite($cred,0)

Alternative Managed API:

Do you know one? Please contribute it!