C# Signature:

[DllImport("kernel32.dll", CharSet = CharSet.Auto)]
static extern long PackageFullNameFromId(ref PACKAGE_ID packageId, ref UInt32 packageFullNameLength, StringBuilder packageFullName);

User-Defined Types:

PACKAGE_ID

Sample Code:

        PACKAGE_ID packageId = new PACKAGE_ID();
        packageId.name = "MyPackage";
        packageId.publisher = "CN=PublisherName";
        StringBuilder packageFullName = new StringBuilder(255);
        UInt32 capacity = (UInt32)packageFullName.Capacity;
        long errorCode = PackageFullNameFromId(ref packageId, ref capacity, packageFullName);