See the question and my original answer on StackOverflow

Here is a pure C# console app sample, equivalent of @Drake's C/C++ code. I've written it using code from an open source project called DirectN that defines thousands of c# Windows interop types (DirectX, etc.), including Code Audio API, etc.

class Program
{
    static void Main(string[] args)
    {
        // using DirectN
        var enumerator = (IMMDeviceEnumerator)new MMDeviceEnumerator();

        // or call GetDevice(...) with an id
        enumerator.GetDefaultAudioEndpoint(
            __MIDL___MIDL_itf_mmdeviceapi_0000_0000_0001.eCapture,
            __MIDL___MIDL_itf_mmdeviceapi_0000_0000_0002.eConsole,
            out var device).ThrowOnError();

        const int CLSCTX_ALL = 23;
        device.Activate(typeof(IDeviceTopology).GUID, CLSCTX_ALL, null, out var iface).ThrowOnError();

        var topology = (IDeviceTopology)iface;
        topology.GetConnector(0, out var connector).ThrowOnError();

        var part = (IPart)connector;
        if (part.Activate(CLSCTX_ALL, typeof(IAudioAutoGainControl).GUID, out iface).IsError)
        {
            Console.WriteLine("AGC not supported.");
            return;
        }

        var control = (IAudioAutoGainControl)iface;
        control.SetEnabled(true, IntPtr.Zero);
    }

    [ComImport]
    [Guid("bcde0395-e52f-467c-8e3d-c4579291692e")] // CLSID_MMDeviceEnumerator
    class MMDeviceEnumerator
    {
    }
}

You can use either the DirectN's nuget package, or copy to your project only the needed .cs files (and their dependencies). Here, you would need the following: 

HRESULT.cs                                          
HRESULTS.cs                                         
IAudioAutoGainControl.cs                            
IAudioVolumeLevel.cs                                
IConnector.cs                                       
IControlChangeNotify.cs                             
IControlInterface.cs                                
IDeviceTopology.cs                                  
IMMDevice.cs                                        
IMMDeviceCollection.cs                              
IMMDeviceEnumerator.cs                              
IMMNotificationClient.cs                            
IPart.cs                                            
IPartsList.cs                                       
IPerChannelDbLevel.cs                               
ISubunit.cs                                         
PROPERTYKEY.cs                                      
PropertyType.cs                                     
PropVariant.cs                                      
_tagpropertykey.cs                                  
__MIDL___MIDL_itf_devicetopology_0000_0000_0011.cs  
__MIDL___MIDL_itf_devicetopology_0000_0000_0012.cs  
__MIDL___MIDL_itf_devicetopology_0000_0000_0013.cs  
__MIDL___MIDL_itf_mmdeviceapi_0000_0000_0001.cs     
__MIDL___MIDL_itf_mmdeviceapi_0000_0000_0002.cs