【正经的知识增加了】接口默认方法的底层实现(翻译)

interface INormal {
    void Normal();
}

interface IDefaultMethod {
    void Default() => WriteLine("IDefaultMethod.Default");
}

class CNormal : INormal {
    public void Normal() => WriteLine("CNormal.Normal");
}

class CDefault : IDefaultMethod {
    // Nothing to do here!
}

class CDefaultOwnImpl : IDefaultMethod {
    void IDefaultMethod.Default() => WriteLine("CDefaultOwnImpl.IDefaultMethod.Default");
}

// Test out the Normal/DefaultMethod Interfaces
INormal iNormal = new CNormal();
iNormal.Normal(); // prints "CNormal.Normal"

IDefaultMethod iDefault = new CDefault();
iDefault.Default(); // prints "IDefaultMethod.Default"

IDefaultMethod iDefaultOwnImpl = new CDefaultOwnImpl();
iDefaultOwnImpl.Default(); // prints "CDefaultOwnImpl.IDefaultMethod.Default"

private static void ShowInterfaceMapping(Type @implemetation, Type @interface) {
    InterfaceMapping map = @implemetation.GetInterfaceMap(@interface);
    Console.WriteLine($"{map.TargetType}: GetInterfaceMap({map.InterfaceType})");
    for (int counter = 0; counter < map.InterfaceMethods.Length; counter++) {
        MethodInfo im = map.InterfaceMethods[counter];
        MethodInfo tm = map.TargetMethods[counter];
        Console.WriteLine($"   {im.DeclaringType}::{im.Name} --> {tm.DeclaringType}::{tm.Name} ({(im == tm ? "same" : "different")})");
        Console.WriteLine("       MethodHandle 0x{0:X} --> MethodHandle 0x{1:X}",
            im.MethodHandle.Value.ToInt64(), tm.MethodHandle.Value.ToInt64());
        Console.WriteLine("       FunctionPtr  0x{0:X} --> FunctionPtr  0x{1:X}",
            im.MethodHandle.GetFunctionPointer().ToInt64(), tm.MethodHandle.GetFunctionPointer().ToInt64());
    }
    Console.WriteLine();
}

//ShowInterfaceMapping(typeof(CNormal), @interface: typeof(INormal));
//ShowInterfaceMapping(typeof(CDefault), @interface: typeof(IDefaultMethod));
//ShowInterfaceMapping(typeof(CDefaultOwnImpl), @interface: typeof(IDefaultMethod));

TestApp.CNormal: GetInterfaceMap(TestApp.INormal)
   TestApp.INormal::Normal --> TestApp.CNormal::Normal (different)
       MethodHandle 0x7FF993916A80 --> MethodHandle 0x7FF993916B10
       FunctionPtr  0x7FF99385FC50 --> FunctionPtr  0x7FF993861880

TestApp.CDefault: GetInterfaceMap(TestApp.IDefaultMethod)
   TestApp.IDefaultMethod::Default --> TestApp.IDefaultMethod::Default (same)
       MethodHandle 0x7FF993916BD8 --> MethodHandle 0x7FF993916BD8
       FunctionPtr  0x7FF99385FC78 --> FunctionPtr  0x7FF99385FC78

TestApp.CDefaultOwnImpl: GetInterfaceMap(TestApp.IDefaultMethod)
   TestApp.IDefaultMethod::Default --> TestApp.CDefaultOwnImpl::TestApp.IDefaultMethod.Default (different)
       MethodHandle 0x7FF993916BD8 --> MethodHandle 0x7FF993916D10
       FunctionPtr  0x7FF99385FC78 --> FunctionPtr  0x7FF9938663A0

> dumpmt -md 00007ff8bcc31dd8
EEClass:         00007FF8BCC2C420
Module:          00007FF8BCC0F788
Name:            TestApp.INormal
mdToken:         0000000002000002
File:            C:\DefaultInterfaceMethods\TestApp\bin\Debug\netcoreapp3.0\TestApp.dll
BaseSize:        0x0
ComponentSize:   0x0
Slots in VTable: 1
Number of IFaces in IFaceMap: 0
--------------------------------------
MethodDesc Table
           Entry       MethodDesc    JIT Name
00007FF8BCB70580 00007FF8BCC31DC8   NONE TestApp.INormal.Normal()

> dumpmd 00007FF8BCC31DC8                                    
Method Name:          TestApp.INormal.Normal()               
Class:                00007ff8bcc2c420                       
MethodTable:          00007ff8bcc31dd8                       
mdToken:              0000000006000001                       
Module:               00007ff8bcc0f788                       
IsJitted:             no                                     
Current CodeAddr:     ffffffffffffffff                       
Version History:                                             
  ILCodeVersion:      0000000000000000                       
  ReJIT ID:           0                                      
  IL Addr:            0000000000000000                       
     CodeAddr:           0000000000000000  (MinOptJitted)    
     NativeCodeVersion:  0000000000000000 

> dumpmt -md 00007ff8bcc31e68
EEClass:         00007FF8BCC2C498
Module:          00007FF8BCC0F788
Name:            TestApp.IDefaultMethod
mdToken:         0000000002000003
File:            C:\DefaultInterfaceMethods\TestApp\bin\Debug\netcoreapp3.0\TestApp.dll
BaseSize:        0x0
ComponentSize:   0x0
Slots in VTable: 1
Number of IFaces in IFaceMap: 0
--------------------------------------
MethodDesc Table
           Entry       MethodDesc    JIT Name
00007FF8BCB70590 00007FF8BCC31E58    JIT TestApp.IDefaultMethod.Default()


> dumpmd 00007FF8BCC31E58
Method Name:          TestApp.IDefaultMethod.Default()
Class:                00007ff8bcc2c498
MethodTable:          00007ff8bcc31e68
mdToken:              0000000006000002
Module:               00007ff8bcc0f788
IsJitted:             yes
Current CodeAddr:     00007ff8bcb765c0
Version History:
  ILCodeVersion:      0000000000000000
  ReJIT ID:           0
  IL Addr:            0000000000000000
     CodeAddr:           00007ff8bcb765c0  (MinOptJitted)
     NativeCodeVersion:  0000000000000000

- coreclr.dll!MethodTable::FindDefaultInterfaceImplementation(MethodDesc *pInterfaceMD, MethodTable *pInterfaceMT, MethodDesc **ppDefaultMethod, int allowVariance, int throwOnConflict) Line 6985	C++
- coreclr.dll!MethodTable::FindDispatchImpl(unsigned int typeID, unsigned int slotNumber, DispatchSlot *pImplSlot, int throwOnConflict) Line 6851	C++
- coreclr.dll!MethodTable::FindDispatchSlot(unsigned int typeID, unsigned int slotNumber, int throwOnConflict) Line 7251	C++
- coreclr.dll!VirtualCallStubManager::Resolver(MethodTable *pMT, DispatchToken token, OBJECTREF *protectedObj, unsigned __int64 *ppTarget, int throwOnConflict) Line 2208	C++
- coreclr.dll!VirtualCallStubManager::ResolveWorker(StubCallSite *pCallSite, OBJECTREF *protectedObj, DispatchToken token, VirtualCallStubManager::StubKind stubKind) Line 1874	C++
- coreclr.dll!VSD_ResolveWorker(TransitionBlock *pTransitionBlock, unsigned __int64 siteAddrForRegisterIndirect, unsigned __int64 token, unsigned __int64 flags) Line 1683	C++
- coreclr.dll!ResolveWorkerAsmStub() Line 42	Unknown

interface IA
{
    void M();
}
interface IB : IA
{
    override void M() { WriteLine("IB"); }
}
class Base : IA
{
    void IA.M() { WriteLine("Base"); }
}
class Derived : Base, IB // allowed?
{
    static void Main()
    {
        Ia a = new Derived();
        a.M();           // what does it do?
    }
}

public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }

    if (source is ICollection<TSource> collectionoft)
    {
        return collectionoft.Count;
    }

    if (source is IIListProvider<TSource> listProv)
    {
        return listProv.GetCount(onlyIfCheap: false);
    }

    if (source is ICollection collection)
    {
        return collection.Count;
    }

    int count = 0;
    using (IEnumerator<TSource> e = source.GetEnumerator())
    {
        checked
        {
            while (e.MoveNext())
            {
                count++;
            }
        }
    }

    return count;
}