Eliminate the remaining classic DllImport entries and dead DllImport comments

Six entry points still took a delegate directly as a P/Invoke parameter,
which [LibraryImport]'s source generator does not support, so they stayed
on classic [DllImport]. Each is routed through a static
[UnmanagedCallersOnly] trampoline instead, following the pattern already
established for highgui's setMouseCallback/createTrackbar:

- face_FacemarkTrain_setFaceDetector / face_FacemarkKazemi_setFaceDetector
  and geometry_SACSegmentation_setCustomModelConstraints support multiple
  concurrent instances, each with its own callback, but neither native
  entry point threaded a userData pointer back to the raw callback - the
  previous per-delegate marshaled thunk was the only thing disambiguating
  instances. Native (face_Facemark.h/geometry.h) now carries a void*
  userData alongside the callback pointer through to the point of
  invocation, mirroring what videoio's ManagedStreamReader already did;
  the managed side boxes the real delegate into a GCHandle-rooted context
  and passes its IntPtr through as that userData.
- videoio_VideoCapture_new6/open3's ManagedStreamReader::ReadCallback/
  SeekCallback already carried userData through to the point of
  invocation, so StreamReaderBridge just needed to become a GCHandle-based
  context holder instead of an instance whose delegates were marshaled
  directly.
- tracking_TrackerKCF_setFeatureExtractor's cv::TrackerKCF::
  FeatureExtractorCallbackFN has no per-instance user-data slot at all
  (already documented as process-wide-singleton-only), so no GCHandle
  context is needed there - only the field the fixed trampoline reads is
  swapped.

Also deleted the dead, commented-out //[DllImport(...)] lines left over
from the original Phase 2 bulk conversion to LibraryImport, scattered
across NativeMethods_highgui.cs, NativeMethods_videoio.cs, and the core/
imgproc NativeMethods files - they had no live counterpart to convert and
were pure noise.

Testing:
- dotnet build src/OpenCvSharp/OpenCvSharp.csproj -c Release: 0 warnings, 0 errors.
- cmake --build (OpenCvSharpExtern, Release) for the face_Facemark.h/geometry.h changes.
- dotnet test test/OpenCvSharp.Tests -f net10.0: 1730 passed, 0 failed, 23 skipped
  (pre-existing, platform/model-file-gated skips) - including FacemarkKazemiTest.
  CustomFaceDetectorIsInvoked, SACSegmentationTest.CustomModelConstraintsRejecting
  AllModelsYieldsZeroModels, TrackerKCFTest.SetFeatureExtractor, and
  VideoCaptureTest.OpenFromCustomStreamReader, which exercise the new
  trampolines end-to-end.

Fixes #2077

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: shimat

src/OpenCvSharp/Internal/PInvoke/NativeMethods/NativeMethods_highgui.cs

@@ -104,8 +104,4 @@ public static partial ExceptionStatus highgui_getTrackbarPos(
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus highgui_setTrackbarMin([MarshalAs(UnmanagedType.LPStr)] string trackbarName, [MarshalAs(UnmanagedType.LPStr)] string winName, int minVal);
-
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, BestFitMapping = false, ThrowOnUnmappableChar = true, ExactSpelling = true)]
-    //public static extern ExceptionStatus highgui_createButton(
-    //    [MarshalAs(UnmanagedType.LPStr)] string barName, IntPtr onChange, IntPtr userData, int type, int initialButtonState, out int returnValue);
 }

src/OpenCvSharp/Internal/PInvoke/NativeMethods/NativeMethods_videoio.cs

@@ -71,10 +71,12 @@ public static partial ExceptionStatus videoio_VideoCapture_new4(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus videoio_VideoCapture_new5(int device, int apiPreference, [In] int[] @params, int paramsLength, out IntPtr returnValue);
 
-    [DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    public static extern ExceptionStatus videoio_VideoCapture_new6(
-        [MarshalAs(UnmanagedType.FunctionPtr)] StreamReaderReadCallback readCallback,
-        [MarshalAs(UnmanagedType.FunctionPtr)] StreamReaderSeekCallback seekCallback,
+    // readCallback/seekCallback are function pointers to static, [UnmanagedCallersOnly] trampolines
+    // (see StreamReaderBridge); userData is a GCHandle to the context rooting the real IStreamReader,
+    // not a caller-supplied value.
+    [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    public static partial ExceptionStatus videoio_VideoCapture_new6(
+        IntPtr readCallback, IntPtr seekCallback,
         IntPtr userData, int apiPreference, [In] int[] @params, int paramsLength, out IntPtr returnValue);
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
@@ -87,11 +89,13 @@ public static partial ExceptionStatus videoio_VideoCapture_open1(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus videoio_VideoCapture_open2(OpenCvSafeHandle obj, int device, int apiPreference, out int returnValue);
 
-    [DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    public static extern ExceptionStatus videoio_VideoCapture_open3(
+    // readCallback/seekCallback are function pointers to static, [UnmanagedCallersOnly] trampolines
+    // (see StreamReaderBridge); userData is a GCHandle to the context rooting the real IStreamReader,
+    // not a caller-supplied value.
+    [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    public static partial ExceptionStatus videoio_VideoCapture_open3(
         OpenCvSafeHandle obj,
-        [MarshalAs(UnmanagedType.FunctionPtr)] StreamReaderReadCallback readCallback,
-        [MarshalAs(UnmanagedType.FunctionPtr)] StreamReaderSeekCallback seekCallback,
+        IntPtr readCallback, IntPtr seekCallback,
         IntPtr userData, int apiPreference, [In] int[] @params, int paramsLength, out int returnValue);
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
@@ -116,9 +120,6 @@ public static partial ExceptionStatus videoio_VideoCapture_open4(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus videoio_VideoCapture_operatorRightShift_Mat(OpenCvSafeHandle obj, IntPtr image);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus videoio_VideoCapture_operatorRightShift_UMat(IntPtr obj, IntPtr image);
-
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     internal static partial ExceptionStatus videoio_VideoCapture_read_OutputArray(OpenCvSafeHandle obj, in OutputArrayProxy image, out int returnValue);
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
@@ -194,9 +195,6 @@ public static partial ExceptionStatus videoio_VideoWriter_open4(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus videoio_VideoWriter_release(OpenCvSafeHandle obj);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus videoio_VideoWriter_OperatorLeftShift(IntPtr obj, IntPtr image);
-
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     internal static partial ExceptionStatus videoio_VideoWriter_write(OpenCvSafeHandle obj, in InputArrayProxy image, out int returnValue);
 

src/OpenCvSharp/Internal/PInvoke/NativeMethods/core/NativeMethods_core_FileStorage.cs

@@ -31,9 +31,6 @@ public static partial ExceptionStatus core_FileStorage_open(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_FileStorage_isOpened(OpenCvSafeHandle obj, out int returnValue);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_FileStorage_release(IntPtr obj);
-
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_FileStorage_releaseAndGetString(
         OpenCvSafeHandle obj, IntPtr outString);

src/OpenCvSharp/Internal/PInvoke/NativeMethods/core/NativeMethods_core_InputArray.cs

@@ -40,16 +40,12 @@ static partial class NativeMethods
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_InputArray_getMat(OpenCvSafeHandle ia, int idx, out IntPtr returnValue);
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_InputArray_getMat_(IntPtr ia, int idx, out IntPtr returnValue);
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_InputArray_getUMat(OpenCvSafeHandle ia, int idx, out IntPtr returnValue);
-        
+
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_InputArray_getMatVector(OpenCvSafeHandle ia, IntPtr mv);
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern void core_InputArray_getUMatVector(IntPtr ia, IntPtr umv);
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_InputArray_getFlags(OpenCvSafeHandle ia, out int returnValue);

src/OpenCvSharp/Internal/PInvoke/NativeMethods/core/NativeMethods_core_Mat.cs

@@ -64,8 +64,6 @@ public static partial ExceptionStatus core_Mat_ones_MatShape(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_Mat_new12(IntPtr mat, out IntPtr returnValue);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_Mat_release(IntPtr mat);
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_Mat_delete(IntPtr mat);
 
@@ -259,12 +257,6 @@ public static partial ExceptionStatus core_Mat_checkVector(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_Mat_stepAt(OpenCvSafeHandle self, int i, out IntPtr returnValue);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_Mat_assignment_FromMat(IntPtr self, IntPtr newMat);
-
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_Mat_assignment_FromScalar(IntPtr self, Scalar scalar);
-        
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_abs_Mat(IntPtr e, out IntPtr returnValue);
 

src/OpenCvSharp/Internal/PInvoke/NativeMethods/core/NativeMethods_core_OutputArray.cs

@@ -12,8 +12,6 @@ static partial class NativeMethods
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_OutputArray_new_byMat(IntPtr mat, out IntPtr returnValue);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_OutputArray_new_byGpuMat(IntPtr mat, out IntPtr returnValue);
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_OutputArray_new_byUMat(IntPtr mat, out IntPtr returnValue);
 

src/OpenCvSharp/Internal/PInvoke/NativeMethods/core/NativeMethods_core_UMat.cs

@@ -39,10 +39,6 @@ static partial class NativeMethods
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_UMat_new9(IntPtr umat, Range[] ranges, out IntPtr returnValue);
 
-
-
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus core_UMat_release(IntPtr mat);
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus core_UMat_delete(IntPtr umat);
 

src/OpenCvSharp/Internal/PInvoke/NativeMethods/face/NativeMethods_face_Facemark.cs

@@ -70,9 +70,12 @@ internal static partial ExceptionStatus face_FacemarkTrain_addTrainingSample(
     internal static partial ExceptionStatus face_FacemarkTrain_getFaces(
         OpenCvSafeHandle obj, in InputArrayProxy image, IntPtr faces, out int returnValue);
 
-    [DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    internal static extern ExceptionStatus face_FacemarkTrain_setFaceDetector(
-        OpenCvSafeHandle obj, FacemarkFaceDetectorNativeCallback callback,
+    // callback is a function pointer to a static, [UnmanagedCallersOnly] trampoline (see
+    // FacemarkFaceDetectorBridge); userData is a GCHandle to the context rooting the real managed
+    // detector, round-tripped opaquely through callbackData/face_Facemark_faceDetectorThunk.
+    [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    internal static partial ExceptionStatus face_FacemarkTrain_setFaceDetector(
+        OpenCvSafeHandle obj, IntPtr callback, IntPtr userData,
         out IntPtr callbackData, out int returnValue);
 
     #endregion
@@ -218,9 +221,12 @@ internal static partial ExceptionStatus face_FacemarkKazemi_training(
     internal static partial ExceptionStatus face_FacemarkKazemi_getFaces(
         OpenCvSafeHandle obj, in InputArrayProxy image, IntPtr faces, out int returnValue);
 
-    [DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    internal static extern ExceptionStatus face_FacemarkKazemi_setFaceDetector(
-        OpenCvSafeHandle obj, FacemarkFaceDetectorNativeCallback callback,
+    // callback is a function pointer to a static, [UnmanagedCallersOnly] trampoline (see
+    // FacemarkFaceDetectorBridge); userData is a GCHandle to the context rooting the real managed
+    // detector, round-tripped opaquely through callbackData/face_Facemark_faceDetectorThunk.
+    [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    internal static partial ExceptionStatus face_FacemarkKazemi_setFaceDetector(
+        OpenCvSafeHandle obj, IntPtr callback, IntPtr userData,
         out IntPtr callbackData, out int returnValue);
 
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]

src/OpenCvSharp/Internal/PInvoke/NativeMethods/geometry/NativeMethods_geometry.cs

@@ -688,10 +688,12 @@ internal static partial ExceptionStatus geometry_SACSegmentation_segment(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     public static partial ExceptionStatus geometry_SACSegmentation_getRandomGeneratorState(OpenCvSafeHandle obj, out ulong returnValue);
 
-    // LibraryImport does not support marshaling delegate parameters, so this one uses classic DllImport.
-    [DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    public static extern ExceptionStatus geometry_SACSegmentation_setCustomModelConstraints(
-        OpenCvSafeHandle obj, SacModelConstraintNativeCallback? callback);
+    // callback is a function pointer to a static, [UnmanagedCallersOnly] trampoline (see
+    // SACSegmentation.SetCustomModelConstraints), or IntPtr.Zero to clear the constraint; userData
+    // is a GCHandle to the context rooting the real managed delegate, not a caller-supplied value.
+    [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
+    public static partial ExceptionStatus geometry_SACSegmentation_setCustomModelConstraints(
+        OpenCvSafeHandle obj, IntPtr callback, IntPtr userData);
 
     // RegionGrowing3D
 

src/OpenCvSharp/Internal/PInvoke/NativeMethods/imgproc/NativeMethods_imgproc.cs

@@ -301,9 +301,6 @@ internal static partial ExceptionStatus imgproc_blendLinear(
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     internal static partial ExceptionStatus imgproc_demosaicing(in InputArrayProxy src, in OutputArrayProxy dst, int code, int dstCn);
 
-    //[DllImport(DllExtern, CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
-    //public static extern ExceptionStatus imgproc_HuMoments(ref Moments.NativeStruct moments, [MarshalAs(UnmanagedType.LPArray)] double[] hu);
-
     [LibraryImport(DllExtern), UnmanagedCallConv(CallConvs = [typeof(CallConvCdecl)])]
     internal static partial ExceptionStatus imgproc_matchTemplate(
         in InputArrayProxy image, in InputArrayProxy templ, in OutputArrayProxy result, int method, in InputArrayProxy mask);