A quick overview of the different ways to call unmanaged APIs from managed code, with .Net and also with Mono.

The inspiration for this post came after reading a couple of articles. The first relating to SharpDX:

A new managed .NET/C# Direct3D 11 API generated from DirectX SDK headers

The second:

Techniques of calling unmanaged code from .NET and their speed

that in substance is on the same topic of this post but doesn't provide enough sample code, in particular for the final benchmark.

Native library

In the following examples we will use a function exported from a phantomatic library called Native.dll (Native.so for Mono on Unix/Linux), written in C and compiled with Cdecl calling convention.

//
// Native.h
//

void DoWithIntPointer(int a, int b, int* r);

//
// Native.c
//

#include "Native.h";

void DoWithIntPointer(int a, int b, int* r) {
    *r = a + b;
}

DoWithIntPointer simply calculates the sum of two integer but having parameters passed by value and by reference it will allow us to see some peculiarities.

Explicit P/Invoke

Let's start with a classic P/Invoke example:

//
// TestPInvoke.cs
//

using System.Runtime.InteropServices;

class TestPInvoke {    
    [DllImport(
        "Native.dll", 
        CallingConvention = CallingConvention.Cdecl
    )]
    private static extern void DoWithIntPointer(
        int a, 
        int b, 
        out int r
    );
    
    public static void Main() {
        int result = 0;
        DoWithIntPointer(1, 2, out result);
    }
}

The extern keyword tells the compiler that DoWithIntPointer is defined elsewhere while the DllImport attribute provides directions to trace it.

Implicit P/Invoke - C++/Cli

With C++/Cli we can write wrappers for native libraries with relative ease but it cannot be used with Mono. Here we have the C++/Cli wrapper for our Native library:

//
// NativeCppCliWrapper.cpp
//

#include "Native.h";

namespace NativeCppCliWrapper
{

using namespace System;
using namespace System::Runtime::InteropServices;

public ref class Wrapper {
public:
    static void CallDoWithIntPointer(
        Int32 a, 
        Int32 b, 
        [Out] Int32% r
    ) {
        int tmp;
        DoWithIntPointer(a, b, &tmp);
        r = tmp;
    }
};

}

After compiling, the wrapper can be used as any other assembly:

//
// TestCppCli.cs
//

using NativeCppCliWrapper;

class TestCppCli {
    public static void Main() {
        int result = 0;
        Wrapper.CallDoWithIntPointer(1, 2, out result);
    }
}

If we dig through the IL code generated by C++/Cli we can see that CallDoWithIntPointer invokes:

 IL_0004: call void modopt(
      [mscorlib]System.Runtime.CompilerServices.CallConvCdecl
   )  '<module>'::DoWithIntPointer(int32, int32, int32*)

And DoWithIntPointer is described by the following metadata:

.method assembly static pinvokeimpl("" lasterr cdecl)
 void modopt(
    [mscorlib]System.Runtime.CompilerServices.CallConvCdecl
    ) DoWithIntPointer (
      int32 '',
      int32 '',
      int32* ''
    ) native unmanaged preservesig 
{
 .custom instance void
 [mscorlib]System.Security.SuppressUnmanagedCodeSecurityAttribute::.ctor()
   = ( 01 00 00 00 )
}

Converted in C# (with IlSpy):

[SuppressUnmanagedCodeSecurity]
[DllImport("", 
    CallingConvention = CallingConvention.Cdecl, 
    SetLastError = true
)]
[MethodImpl(MethodImplOptions.Unmanaged)]
internal unsafe static extern void DoWithIntPointer(
    int, 
    int, 
    int*
);

Does this remind us of anything? Yes, it is very similar to the extern declaration that we have seen previously but among the differences we can note an attribute called SuppressUnmanagedCodeSecurity. MSDN tells us that:

This attribute is primarily used to increase performance; however, the performance gains come with significant security risks.

Security risks apart it can be used with explicit P/Invoke, it is not an exclusive of C++/Cli.

In other situations the native code is called in a more sophisticated way, for example if we poke inside IL code of SlimDX we can find things like this:

.method public hidebysig 
 instance valuetype SlimDX.Result Optimize () cil managed 
{
 // Method begins at RVA 0xd0824
 // Code size 25 (0x19)
 .maxstack 3

 IL_0000: ldarg.0
 IL_0001: call instance valuetype 
  IUnknown* SlimDX.ComObject::get_UnknownPointer()
 IL_0006: dup
 IL_0007: ldind.i4
 IL_0008: ldc.i4.s 68
 IL_000a: add
 IL_000b: ldind.i4
 IL_000c: calli System.Int32 modopt(
   System.Runtime.CompilerServices.IsLong
  ) modopt(
   System.Runtime.CompilerServices.CallConvStdcall
  )(System.IntPtr)
 IL_0011: ldnull
 IL_0012: ldnull
 IL_0013: call valuetype SlimDX.Result 
  SlimDX.Result::Record
  <class SlimDX.Direct3D11.Direct3D11Exception>(
   int32, object, object
  )
 IL_0018: ret
}

The calli instruction is used to invoke a native method given the address of the method itself. We will see how to take advantage of calli without C++/Cli in the last example.

Dynamic P/Invoke

In order to employ the previos techniques tha native library must be know at compile time, while it must be located in a certain path at runtime. With dynamic P/Invoke we can obtain a greater degree of flexibility.

In the next example we will benefit by an assembly called CSLoadLibrary but slightly modified, in particular to run on Unix/Linux via Mono (see the download link at the end of this post for the modified version). CSLoadLibrary contains an UnmanagedLibrary class that provides access to native libraries through standard Windows APIs (LoadLibrary, GetProcAddress, FreeLibrary) or Unix/Linux counterparts (dlopen, dlsym, dlclose).

//
// TestDelegate.cs
//

using System.Runtime.InteropServices;
using CSLoadLibrary;

class TestDelegate {
    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
    delegate void DelegateWithIntPointer(
        int a, 
        int b, 
        out int r
    );
    
    public static void Main() {
        UnmanagedLibrary nativeLib = 
            new UnmanagedLibrary(
                "Native"
            );
        
        DelegateWithIntPointer doWithIntPointer = 
            nativeLib.GetUnmanagedFunction
                <DelegateWithIntPointer>(
                "DoWithIntPointer"
            );

        int result = 0;
        doWithIntPointer(1, 2, out result);
    }
}

In practice, given the name of the native library to load, an UnmanagedLibrary object is instantiated. Then, with GetUnmanagedFunction we obtain a delegate pointing to our native function, DoWithIntPointer. Naturally the signature of the delegate must match the signature of the native function.

Dynamic P/Invoke – Explicit P/Invoke

This time, instead of using CSLoadLibrary, the delegate is created via Reflection, replicating the extern declaration shown in the Explicit P/Invoke example.

//
// TestDynamicS.cs
//

using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.InteropServices;
using System.Security;

class TestDynamicS {
    delegate void DelegateWithIntPointer(
        int a, 
        int b, 
        out int r
    );
    
    public static void Main() {
        DelegateWithIntPointer doWithIntPointer = 
            GetDynamicSDelegate
                <DelegateWithIntPointer>(
                "Native", 
                "DoWithIntPointer", 
                CallingConvention.Cdecl
            );
            
        int result = 0;
        doWithIntPointer(1, 2, out result);
    }

    private static TDelegate GetDynamicSDelegate
        <TDelegate>(
        string libraryName, 
        string entryPoint, 
        CallingConvention callingConvention
    ) where TDelegate : class 
    {
        Type delegateType = typeof(TDelegate);
        MethodInfo invokeInfo = delegateType.GetMethod("Invoke");
        // Gets the return type for the P/Invoke method.
        Type invokeReturnType = invokeInfo.ReturnType;
        // Gets the parameter types for the P/Invoke method.
        ParameterInfo[] invokeParameters = 
            invokeInfo.GetParameters();
        Type[] invokeParameterTypes = 
            new Type[
                invokeParameters.Length
            ];
        for (int i = 0; i < invokeParameters.Length; i++) {
            invokeParameterTypes[i] = 
                invokeParameters[i].ParameterType;
        }

        // Defines an assembly with a module and a type.
        AssemblyName assemblyName = 
            new AssemblyName(
                "TestAssembly"
            );
        AssemblyBuilder assemblyBuilder = 
            AppDomain.CurrentDomain.DefineDynamicAssembly(
                assemblyName, 
                AssemblyBuilderAccess.Run
            );
        ModuleBuilder moduleBuilder = 
            assemblyBuilder.DefineDynamicModule(
                "TestModule"
            );
        TypeBuilder typeBuilder = 
            moduleBuilder.DefineType(
                "TestDynamicS"
            );
            
        //Defines a P/Invoke method called Invoke.
        MethodBuilder methodBuilder = 
            typeBuilder.DefinePInvokeMethod(
                "Invoke", 
                libraryName + ".dll", 
                entryPoint, 
                MethodAttributes.Public | 
                    MethodAttributes.Static | 
                    MethodAttributes.PinvokeImpl,
                CallingConventions.Standard, 
                invokeReturnType, 
                invokeParameterTypes, 
                callingConvention, 
                CharSet.Ansi
            );
        methodBuilder.SetImplementationFlags(
            methodBuilder.GetMethodImplementationFlags() | 
            MethodImplAttributes.PreserveSig
        );
        
        // Adds SuppressUnmanagedCodeSecurityAttribute to 
        // the method.
        Type attributeType = 
            typeof(
                SuppressUnmanagedCodeSecurityAttribute
            );
        ConstructorInfo attributeConstructorInfo = 
            attributeType.GetConstructor(
                new Type[] {}
            );
        CustomAttributeBuilder attributeBuilder = 
            new CustomAttributeBuilder(
                attributeConstructorInfo, 
                new object[] {}
            );
        methodBuilder.SetCustomAttribute(attributeBuilder);

        // Finishes the type.
        Type newType = typeBuilder.CreateType();
        
        object tmp = 
            (object)Delegate.CreateDelegate(
                delegateType, 
                newType.GetMethod("Invoke")
            );
        return (TDelegate)tmp;
    }
}

Though we are adding the SuppressUnmanagedCodeSecurity attribute, it is not essential.

Dynamic P/Invoke - Emit Calli

The last example is more complicated. Here again, we make use of UnmanagedLibrary to get the native function's address. Then, through Reflection, we create a dynamic method which internally passes the address of the native function to calli, the instruction seen previously.

//
// TestCalli.cs
//

using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.InteropServices;
using CSLoadLibrary;

class TestCalli {
    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
    delegate void DelegateWithIntPointer(
        int a, 
        int b, 
        out int r
    );
    
    public static void Main() {        
        UnmanagedLibrary nativeLib = 
            new UnmanagedLibrary(
                "Native"
            );
        IntPtr nativeMethodAddress = 
            nativeLib.GetUnmanagedFunctionAddress(
                "DoWithIntPointer"
            );
        DelegateWithIntPointer doWithIntPointer = 
            GetCalliDelegate
                <DelegateWithIntPointer>(
                nativeMethodAddress
            );
        int result = 0;
        doWithIntPointer(1, 2, out result);
    }
    
    private static TDelegate GetCalliDelegate
        <TDelegate>(
        IntPtr methodAddress
    ) where TDelegate : class
    {
        Type delegateType = typeof(TDelegate);
        MethodInfo invokeInfo = delegateType.GetMethod("Invoke");
        // Gets the return type for the dynamic method and calli.
        // Note: for calli, a type such as System.Int32& must be
        // converted to System.Int32* otherwise the execution 
        // will be slower.
        Type invokeReturnType = invokeInfo.ReturnType;
        Type calliReturnType = 
            GetPointerTypeIfReference(
                invokeInfo.ReturnType
            );
        // Gets the parameter types for the dynamic method 
        // and calli.
        ParameterInfo[] invokeParameters = 
            invokeInfo.GetParameters();
        Type[] invokeParameterTypes = 
            new Type[
                invokeParameters.Length
            ];
        Type[] calliParameterTypes = 
            new Type[
                invokeParameters.Length
            ];
        for (int i = 0; i < invokeParameters.Length; i++) {
            invokeParameterTypes[i] = 
                invokeParameters[i].ParameterType;
            calliParameterTypes[i] = 
                GetPointerTypeIfReference(
                    invokeParameters[i].ParameterType
                );
        }

        // Defines the dynamic method.
        DynamicMethod calliMethod = 
            new DynamicMethod(
                "CalliInvoke", 
                invokeReturnType, 
                invokeParameterTypes, 
                typeof(TestCalli), 
                true
            );
            
        // Gets an ILGenerator.
        ILGenerator generator = calliMethod.GetILGenerator();   
        // Emits instructions for loading the parameters into 
        // the stack.
        for (int i = 0; i < calliParameterTypes.Length; i++) {
            if (i == 0) {
                generator.Emit(OpCodes.Ldarg_0);
            } else if (i == 1) {
                generator.Emit(OpCodes.Ldarg_1);
            } else if (i == 2) {
                generator.Emit(OpCodes.Ldarg_2);
            } else if (i == 3) {
                generator.Emit(OpCodes.Ldarg_3);
            } else {
                generator.Emit(OpCodes.Ldarg, i);
            }
        }
        // Emits instruction for loading the address of the
        //native function into the stack.
        switch (IntPtr.Size) {
            case 4:
                generator.Emit(
                    OpCodes.Ldc_I4, 
                    methodAddress.ToInt32()
                );
                break;
            case 8:
                generator.Emit(
                    OpCodes.Ldc_I8, 
                    methodAddress.ToInt64()
                );
                break;
            default:
                throw new PlatformNotSupportedException();
        }
        // Emits calli opcode.
        generator.EmitCalli(
            OpCodes.Calli, 
            CallingConvention.Cdecl,
            calliReturnType, 
            calliParameterTypes
        );
        // Emits instruction for returning a value.
        generator.Emit(OpCodes.Ret);

        object tmp = 
            (object)calliMethod.CreateDelegate(
                delegateType
            );
        return (TDelegate)tmp;
    }
    
    private static Type GetPointerTypeIfReference(Type type) {
        if (type.IsByRef) {
            return Type.GetType(type.FullName.Replace("&", "*"));
        }
        return type;
    }
}

The method GetPointerTypeIfReference converts the type of a parameter like Int32& to Int32*, otherwise calli executes correctly but results slower.

Benchmark

Hardware: CPU Intel Core i3-2310M 2.1 GHz, RAM 4 GB.
Software: VMware Player 3.1.4. on Windows 7 x64.

[ms] x 100,000,000 iterations

SUC means that the test has been executed with the SuppressUnmanagedCodeSecurity attribute.

Conclusion

The above results seem a bit weird and the only certain thing appears to be that Dynamic P/Invoke is always faster if we resort to calli. Anyway, we have seen different ways to invoke unmanaged code from managed code, each with its own pros and cons, and if necessary we can use them or conduct further tests.

Download

Benchmark source code.

Other resources

About interoperability:

• Interop with Native Libraries
• Invoke Native C++ DLL from .NET Code

About calli:

• Calli is not Verifiable
• C#/.NET: Why is Calli Faster Than a Delegate Call?
• OpenGL Import für Delphi .Net