Collapse methods with same name into same COM slot

The IOpenFileDialog declared in the .NET 7 in such way that methods from IFileDialog duplicated which lead to incorrect counting of COM slot location

Author: kant2002

WinFormsComInterop.SourceGenerator.Tests/RuntimeCallableWrapperTest.cs

@@ -2427,6 +2427,138 @@ unsafe partial class C : global::Foo.IStr2
             }
         }
     }
+}";
+            Assert.AreEqual(expectedOutput, output);
+        }
+
+        [TestMethod]
+        public void InheritanceSameMethods()
+        {
+            string source = @"
+namespace Foo
+{
+    using System.Runtime.InteropServices;
+
+    [Guid(""22DD68D1-86FD-4332-8666-9ABEDEA2D24C"")]
+    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    public interface IStr
+    {
+        void Read(byte* pv, uint cb, uint* pcbRead);
+        string Name { get; }
+    }
+
+    [Guid(""22DD68D2-86FD-4332-8666-9ABEDEA2D24C"")]
+    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
+    public interface IStr2: IStr
+    {
+        void Read(byte* pv, uint cb, uint* pcbRead);
+        string Name { get; }
+        void Write(byte* pv, uint cb, uint* pcbRead);
+    }
+
+    [RuntimeCallableWrapper(typeof(IStr2))]
+    partial class C
+    {
+    }
+}";
+            string output = this.GetGeneratedOutput(source, NullableContextOptions.Disable);
+
+            Assert.IsNotNull(output);
+
+            var expectedOutput = @"// <auto-generated>
+// Code generated by COM Proxy Code Generator.
+// Changes may cause incorrect behavior and will be lost if the code is
+// regenerated.
+// </auto-generated>
+#nullable enable
+using Marshal = System.Runtime.InteropServices.Marshal;
+
+namespace Foo
+{
+    [System.Runtime.Versioning.SupportedOSPlatform(""windows"")]
+    unsafe partial class C : global::Foo.IStr2
+    {
+        void global::Foo.IStr2.Read(byte* pv, uint cb, uint* pcbRead)
+        {
+            var targetInterface = new System.Guid(""22DD68D2-86FD-4332-8666-9ABEDEA2D24C"");
+            var result = Marshal.QueryInterface(this.instance, ref targetInterface, out var thisPtr);
+            if (result != 0)
+            {
+                throw new System.InvalidCastException();
+            }
+
+            try
+            {
+                var comDispatch = (System.IntPtr*)thisPtr;
+                var vtbl = (System.IntPtr*)comDispatch[0];
+                result = ((delegate* unmanaged<System.IntPtr, byte*, uint, uint*, int>)vtbl[3])(thisPtr, pv, cb, pcbRead);
+                if (result != 0)
+                {
+                    Marshal.ThrowExceptionForHR(result);
+                }
+
+            }
+            finally
+            {
+                Marshal.Release(thisPtr);
+            }
+        }
+        string global::Foo.IStr2.Name
+        {
+            get
+            {
+                var targetInterface = new System.Guid(""22DD68D2-86FD-4332-8666-9ABEDEA2D24C"");
+                var result = Marshal.QueryInterface(this.instance, ref targetInterface, out var thisPtr);
+                if (result != 0)
+                {
+                    throw new System.InvalidCastException();
+                }
+
+                try
+                {
+                    var comDispatch = (System.IntPtr*)thisPtr;
+                    var vtbl = (System.IntPtr*)comDispatch[0];
+                    System.IntPtr retVal;
+                    result = ((delegate* unmanaged<System.IntPtr, System.IntPtr*, int>)vtbl[4])(thisPtr, &retVal);
+                    if (result != 0)
+                    {
+                        Marshal.ThrowExceptionForHR(result);
+                    }
+
+                    return retVal == System.IntPtr.Zero ? null : (string)Marshal.PtrToStringUni(retVal);
+                }
+                finally
+                {
+                    Marshal.Release(thisPtr);
+                }
+            }
+        }
+        void global::Foo.IStr2.Write(byte* pv, uint cb, uint* pcbRead)
+        {
+            var targetInterface = new System.Guid(""22DD68D2-86FD-4332-8666-9ABEDEA2D24C"");
+            var result = Marshal.QueryInterface(this.instance, ref targetInterface, out var thisPtr);
+            if (result != 0)
+            {
+                throw new System.InvalidCastException();
+            }
+
+            try
+            {
+                var comDispatch = (System.IntPtr*)thisPtr;
+                var vtbl = (System.IntPtr*)comDispatch[0];
+                result = ((delegate* unmanaged<System.IntPtr, byte*, uint, uint*, int>)vtbl[5])(thisPtr, pv, cb, pcbRead);
+                if (result != 0)
+                {
+                    Marshal.ThrowExceptionForHR(result);
+                }
+
+            }
+            finally
+            {
+                Marshal.Release(thisPtr);
+            }
+        }
+    }
 }";
             Assert.AreEqual(expectedOutput, output);
         }

WinFormsComInterop.SourceGenerator/WrapperGenerationContext.cs

@@ -93,6 +93,26 @@ public MethodGenerationContext CreateMethodGenerationContext(
                 return existingContext;
             }
 
+            var originalMethod = method;
+            IMethodSymbol? previousMethod = null;
+            MethodGenerationContext? previousMethodContext = null;
+            if (method.ContainingType.AllInterfaces.Length != 0)
+            {
+                foreach (var parentInterface in method.ContainingType.AllInterfaces)
+                {
+                    var sameMembers = parentInterface.GetMembers(method.Name);
+                    
+                    // I do not distinguish between methods with same name and different parameters.
+                    var previousMember = sameMembers.SingleOrDefault();
+                    if (previousMember != null)
+                    {
+                        previousMethod = (IMethodSymbol)previousMember;
+                        previousMethodContext = CreateMethodGenerationContext(classSymbol, previousMethod, ref comSlotNumber);
+                        break;
+                    }
+                }
+            }
+
             var preserveSigAttribute = method.GetAttributes().FirstOrDefault(ad =>
             {
                 var attributeName = ad.AttributeClass?.ToDisplayString();
@@ -101,13 +121,17 @@ public MethodGenerationContext CreateMethodGenerationContext(
             });
             var preserveSignature = preserveSigAttribute != null;
             preserveSignature |= (method.MethodImplementationFlags & System.Reflection.MethodImplAttributes.PreserveSig) == System.Reflection.MethodImplAttributes.PreserveSig;
-            var methodContext = new MethodGenerationContext(method, classSymbol.Type, this)
+            var methodContext = new MethodGenerationContext(originalMethod, classSymbol.Type, this)
             {
                 PreserveSignature = preserveSignature,
-                ComSlotNumber = comSlotNumber,
+                ComSlotNumber = previousMethodContext?.ComSlotNumber ?? comSlotNumber,
             };
             contextCache.Add(key, methodContext);
-            comSlotNumber++;
+            if (previousMethodContext is null)
+            {
+                comSlotNumber++;
+            }
+
             return methodContext;
         }