This would be a C# equivalent (you don't need to define IUnknown methods, and this is implicit):

[Guid("F885120E-3789-4FD9-865E-DC9B4A6412D2"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IARPUninstallStringLauncher
{
    [PreserveSig]
    int LaunchUninstallStringAndWait(
            IntPtr hKey,
            [MarshalAs(UnmanagedType.LPWStr)] string Item,
            bool bModify,
            IntPtr hWnd);

    [PreserveSig]
    int RemoveBrokenItemFromInstalledProgramsList(
        IntPtr hKey,
        [MarshalAs(UnmanagedType.LPWStr)] string Item);
}

You can declare the coclass like this (yes, with an empty body)

[Guid("FCC74B77-EC3E-4DD8-A80B-008A702075A9"), ComImport]
public class UninstallStringLauncher
{
}

And to create and use the object, you can try a code like this

var launcher = (IARPUninstallStringLauncher)new UninstallStringLauncher();

And make sure the bitness (x86/x64) of your COM object matches the bitness of your C# app.