C# Signature:

[DllImport("msi.dll", SetLastError=true)]
static extern UInt32 MsiApplyPatch(string patchPackage, string installPackage, INSTALLTYPE installType, string commandLine);

VB Signature:

Declare Function MsiApplyPatch Lib "msi.dll" (TODO) As TODO

User-Defined Types:

enum INSTALLTYPE
{
    INSTALLTYPE_DEFAULT = 0, // Searches system for products to patch. installPackage must be null
    INSTALLTYPE_NETWORK_IMAGE = 1, // Administrative installation
    INSTALLTYPE_SINGLE_INSTANCE = 2, // Patch product specified by installPackage
}

Alternative Managed API:

Do you know one? Please contribute it!

Notes:

Parameters

patchPackage
   Full path to the patch package
installPackage
   See User-Defined Types
installType
   Type of installation. See User-Defined Types
commandLine
   Specifies command line property settings

Tips & Tricks:

Please add some!

Sample Code:

UInt32 ret = MsiApplyPatch("TestInstall.msp", "", INSTALLTYPE_DEFAULT, "REINSTALL=ALL REINSTALLMODE=omus REBOOT=Suppress");