background

Released three years ago C#8.0 There is an important improvement in... Called Interface default implementation , From then on , The method defined in the interface can contain the method body , Default implementation . But for the default implementation of the interface , The implementation class or sub interface cannot override this method base call , Just like subclass overriding methods can be base.Method() like that . for example ：

public interface IService

{

    void Proccess()

    {

        Console.WriteLine("Proccessing");

    }

}

public class Service : IService

{

    public void Proccess()

    {

        Console.WriteLine("Before Proccess");

        base(IService).Proccess(); //  Currently not supported , It is also the part that this article needs to discuss 

        Console.WriteLine("End Proccess");

    }

}

The original C# The team listed this feature as a plan for the next step （ Click here to view details ）, However, three years later, it has not been put on the agenda . The lack of this feature is undoubtedly a great limitation , Sometimes we do need interfaces base Call to implement certain requirements . This article will introduce two ways to implement it .

Method 1： Use reflection to find the interface implementation and call it

The core idea of this method is , Use reflection to find the interface implementation you need to call MethodInfo, And then build DynamicMethod Use OpCodes.Call Just call it .

First, we define the method signature used to represent the interface method base call .

public static void Base<TInterface>(this TInterface instance, Expression<Action<TInterface>> selector);

public static TReturn Base<TInterface, TReturn>(this TInterface instance, Expression<Func<TInterface, TReturn>> selector);

So the example in the previous section can be rewritten as ：

public class Service : IService

{

    public void Proccess()

    {

        Console.WriteLine("Before Proccess");

        this.Base<IService>(m => m.Proccess());

        Console.WriteLine("End Proccess");

    }

}

So next , We need the basis lambda Expression to find its corresponding interface implementation , And then call it .

The first step is based on lambda Expression acquisition MethodInfo And parameters . It should be noted that , We also need to support the calling of attributes , In fact, attributes are also a method , So it can be handled together .

private static (MethodInfo method, IReadOnlyList<Expression> args) GetMethodAndArguments(Expression exp) => exp switch

{

    LambdaExpression lambda => GetMethodAndArguments(lambda.Body),

    UnaryExpression unary => GetMethodAndArguments(unary.Operand),

    MethodCallExpression methodCall => (methodCall.Method!, methodCall.Arguments),

    MemberExpression { Member: PropertyInfo prop } => (prop.GetGetMethod(true) ?? throw new MissingMethodException($"No getter in propery {prop.Name}"), Array.Empty<Expression>()),

    _ => throw new InvalidOperationException("The expression refers to neither a method nor a readable property.")

};

The second step , utilize Type.GetInterfaceMap Get the interface implementation method to be called . The key points to note here are ,instanceType.GetInterfaceMap(interfaceType).InterfaceMethods All methods of the interface will be returned , So you can't just match by method name , Because there may be various overloads 、 The generic parameter 、 also new Keyword to declare a method with the same name , So you can follow the Method name + Declaration type + Method parameter + Method generic parameters The only way to determine （ In the following code block IfMatch The implementation of the ）

internal readonly record struct InterfaceMethodInfo(Type InstanceType, Type InterfaceType, MethodInfo Method);

private static MethodInfo GetInterfaceMethod(InterfaceMethodInfo info)

{

    var (instanceType, interfaceType, method) = info;

    var parameters = method.GetParameters();

    var genericArguments = method.GetGenericArguments();

    var interfaceMethods = instanceType

        .GetInterfaceMap(interfaceType)

        .InterfaceMethods

        .Where(m => IfMatch(method, genericArguments, parameters, m))

        .ToArray();

    var interfaceMethod = interfaceMethods.Length switch

    {

        0 => throw new MissingMethodException($"Can not find method {method.Name} in type {instanceType.Name}"),

        > 1 => throw new AmbiguousMatchException($"Found more than one method {method.Name} in type {instanceType.Name}"),

        1 when interfaceMethods[0].IsAbstract => throw new InvalidOperationException($"The method {interfaceMethods[0].Name} is abstract"),

        _ => interfaceMethods[0]

    };

    if (method.IsGenericMethod)

        interfaceMethod = interfaceMethod.MakeGenericMethod(method.GetGenericArguments());

    return interfaceMethod;

}

The third step , Use the obtained interface method , structure DynamicMethod. The emphasis is on using OpCodes.Call, It means to call a method in a non - virtual way , Even if the method is virtual , And don't look for a rewrite of it , Instead, it calls itself directly .

private static DynamicMethod GetDynamicMethod(Type interfaceType, MethodInfo method, IEnumerable<Type> argumentTypes)

{

    var dynamicMethod = new DynamicMethod(

        name: "__IL_" + method.GetFullName(),

        returnType: method.ReturnType,

        parameterTypes: new[] { interfaceType, typeof(object[]) },

        owner: typeof(object),

        skipVisibility: true);

    var il = dynamicMethod.GetILGenerator();

    il.Emit(OpCodes.Ldarg_0);

    var i = 0;

    foreach (var argumentType in argumentTypes)

    {

        il.Emit(OpCodes.Ldarg_1);

        il.Emit(OpCodes.Ldc_I4, i);

        il.Emit(OpCodes.Ldelem, typeof(object));

        if (argumentType.IsValueType)

        {

            il.Emit(OpCodes.Unbox_Any, argumentType);

        }

        ++i;

    }

    il.Emit(OpCodes.Call, method);

    il.Emit(OpCodes.Ret);

    return dynamicMethod;

}

Last , take DynamicMethod The conversion to strongly typed delegates is complete . Considering the optimization of performance , The final delegate can be cached , You don't have to build it again next time .

The complete code points here

Method 2： Using function pointers

This method and method 1 Be the same in essentials while differing in minor points , The difference is that , In the method 1 The second step , That is, to find the interface method MethodInfo after , Get its function pointer , Then use this pointer to construct a delegate . This method is actually the method I first found , Method 1 Is its improvement . There is no more introduction here

Method 3： utilize Fody Interface methods are modified at compile time IL Of call call

Method 1 It's possible , But the visible performance loss is large , Even with caching . So I used Fody Write a plug-in InterfaceBaseInvoke.Fody.

The core idea is to find the target interface method at compile time , And then use call Just call it with the command . This minimizes performance loss . For the usage of this plug-in, please refer to Project introduction .

Performance testing

Method The average time Memory allocation
Of the parent class base call 0.0000 ns-
Method 1（DynamicMethod）691.3687 ns776 B
Method 2（FunctionPointer）1,391.9345 ns1,168 B
Method 3（InterfaceBaseInvoke.Fody）0.0066 ns-

summary

This paper discusses several methods to realize the interface base Method called , Among them, the performance is InterfaceBaseInvoke.Fody The best , stay C# The official support has previously recommended . Welcome to use , Be careful , Thank you. .

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