Windows Defender Antivirus scan from C# [AccessViolation exception]

Question

We are writing a code to do on-demand scan of a file from C# using Windows Defender APIs.

        [DllImport(@\"C:\\Program Files\\Windows Defender\\MpClient         


        

Answer 1

I couldn't identify the problem here. So I ended up with Antimalware Scan Interface (AMSI) available starting from Windows 10.

I have written a sample C# code here.
One thing I found is AMSI requires Windows defender/any antivirus to be turned on to verify the file passed to API. But triggering a scan through MpClient.dllwill trigger a defender scan even if defender is turned off.

Also ensure your project targets x64 platform.

public enum AMSI_RESULT
    {
        AMSI_RESULT_CLEAN = 0,
        AMSI_RESULT_NOT_DETECTED = 1,
        AMSI_RESULT_DETECTED = 32768
    }

[DllImport("Amsi.dll", EntryPoint = "AmsiInitialize", CallingConvention = CallingConvention.StdCall)]
public static extern int AmsiInitialize([MarshalAs(UnmanagedType.LPWStr)]string appName, out IntPtr amsiContext);

[DllImport("Amsi.dll", EntryPoint = "AmsiUninitialize", CallingConvention = CallingConvention.StdCall)]
public static extern void AmsiUninitialize(IntPtr amsiContext);

[DllImport("Amsi.dll", EntryPoint = "AmsiOpenSession", CallingConvention = CallingConvention.StdCall)]
public static extern int AmsiOpenSession(IntPtr amsiContext, out IntPtr session);

[DllImport("Amsi.dll", EntryPoint = "AmsiCloseSession", CallingConvention = CallingConvention.StdCall)]
public static extern void AmsiCloseSession(IntPtr amsiContext, IntPtr session);

[DllImport("Amsi.dll", EntryPoint = "AmsiScanString", CallingConvention = CallingConvention.StdCall)]
public static extern int AmsiScanString(IntPtr amsiContext, [InAttribute()] [MarshalAsAttribute(UnmanagedType.LPWStr)]string @string, [InAttribute()] [MarshalAsAttribute(UnmanagedType.LPWStr)]string contentName, IntPtr session, out AMSI_RESULT result);
[DllImport("Amsi.dll", EntryPoint = "AmsiScanBuffer", CallingConvention = CallingConvention.StdCall)]
public static extern int AmsiScanBuffer(IntPtr amsiContext, [In] [MarshalAs(UnmanagedType.LPArray)] byte[] buffer, uint length, [In()] [MarshalAs(UnmanagedType.LPWStr)] string contentName, IntPtr session, out AMSI_RESULT result);

//This method apparently exists on MSDN but not in AMSI.dll (version 4.9.10586.0)
[DllImport("Amsi.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
public static extern bool AmsiResultIsMalware(AMSI_RESULT result);

private void CallAntimalwareScanInterface()
{
    IntPtr amsiContext;
    IntPtr session;
    AMSI_RESULT result = 0;
    int returnValue;

    returnValue = AmsiInitialize("VirusScanAPI", out amsiContext); //appName is the name of the application consuming the Amsi.dll. Here my project name is VirusScanAPI.   
    returnValue = AmsiOpenSession(amsiContext, out session);
    returnValue = AmsiScanString(amsiContext, @"X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*", "EICAR", session, out result); //I've used EICAR test string.   
    AmsiCloseSession(amsiContext, session);
    AmsiUninitialize(amsiContext);
}