diff --git a/extras/fileformats/extras/application/medical.py b/extras/fileformats/extras/application/medical.py
index 07f4c14..4020014 100644
--- a/extras/fileformats/extras/application/medical.py
+++ b/extras/fileformats/extras/application/medical.py
@@ -1,5 +1,4 @@
 import typing as ty
-from random import Random
 from pathlib import Path
 import pydicom
 from fileformats.core import FileSet
@@ -8,8 +7,10 @@
 
 
 @FileSet.read_metadata.register
-def dicom_read_metadata(dicom: Dicom, specific_tags=None) -> ty.Mapping[str, ty.Any]:
-    dcm = pydicom.dcmread(dicom.fspath, specific_tags=specific_tags)
+def dicom_read_metadata(
+    dicom: Dicom, selected_keys: ty.Optional[ty.Sequence[str]] = None
+) -> ty.Mapping[str, ty.Any]:
+    dcm = pydicom.dcmread(dicom.fspath, specific_tags=selected_keys)
     [getattr(dcm, a, None) for a in dir(dcm)]  # Ensure all keywords are set
     metadata = {
         e.keyword: e.value  # type: ignore[union-attr]
@@ -21,8 +22,8 @@ def dicom_read_metadata(dicom: Dicom, specific_tags=None) -> ty.Mapping[str, ty.
 
 @FileSet.generate_sample_data.register
 def dicom_generate_sample_data(
-    dicom: Dicom, dest_dir: Path, seed: ty.Union[int, Random], stem: ty.Optional[str]
-):
+    dicom: Dicom, dest_dir: Path, seed: int, stem: ty.Optional[str]
+) -> ty.Iterable[Path]:
     return next(
         medimages4tests.dummy.dicom.mri.t1w.siemens.skyra.syngo_d13c.get_image().iterdir()
     )
diff --git a/extras/fileformats/extras/application/serialization.py b/extras/fileformats/extras/application/serialization.py
index cd2a000..8456197 100644
--- a/extras/fileformats/extras/application/serialization.py
+++ b/extras/fileformats/extras/application/serialization.py
@@ -25,13 +25,13 @@ def convert_data_serialization(
 
 
 @DataSerialization.load.register
-def yaml_load(yml: Yaml):
+def yaml_load(yml: Yaml) -> dict:
     with open(yml.fspath) as f:
         data = yaml.load(f, Loader=yaml.Loader)
     return data
 
 
 @DataSerialization.save.register
-def yaml_save(yml: Yaml, data):
+def yaml_save(yml: Yaml, data: dict):
     with open(yml.fspath, "w") as f:
         yaml.dump(data, f)
diff --git a/fileformats/application/serialization.py b/fileformats/application/serialization.py
index fdedc9e..5c25ce0 100644
--- a/fileformats/application/serialization.py
+++ b/fileformats/application/serialization.py
@@ -2,7 +2,7 @@
 import typing as ty
 from random import Random
 from pathlib import Path
-from ..core import hook, DataType
+from ..core import hook, DataType, FileSet
 from ..core.mixin import WithClassifiers
 from ..generic import File
 from ..core.exceptions import FormatMismatchError
@@ -42,12 +42,12 @@ class DataSerialization(WithClassifiers, File):
     iana_mime = None
 
     @hook.extra
-    def load(self):
+    def load(self) -> dict:
         """Load the contents of the file into a dictionary"""
         raise NotImplementedError
 
     @hook.extra
-    def save(data):
+    def save(self, data: dict):
         """Serialise a dictionary to a new file"""
         raise NotImplementedError
 
@@ -93,10 +93,10 @@ class Toml(DataSerialization):
     ext = ".toml"
 
 
-@File.generate_sample_data.register
+@FileSet.generate_sample_data.register
 def generate_json_sample_data(
     js: Json, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     js_file = dest_dir / gen_filename(seed, file_type=js, stem=stem)
     rng = Random(seed + 1)
     with open(js_file, "w") as f:
@@ -107,10 +107,10 @@ def generate_json_sample_data(
     return [js_file]
 
 
-@File.generate_sample_data.register
+@FileSet.generate_sample_data.register
 def generate_yaml_sample_data(
     yml: Yaml, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     yml_file = dest_dir / gen_filename(seed, file_type=yml, stem=stem)
     rng = Random(seed + 1)
     with open(yml_file, "w") as f:
diff --git a/fileformats/core/__init__.py b/fileformats/core/__init__.py
index 9e64352..c3cf12a 100644
--- a/fileformats/core/__init__.py
+++ b/fileformats/core/__init__.py
@@ -1,7 +1,7 @@
 from ._version import __version__
 from .classifier import Classifier, ClassifierCategory
 from .datatype import DataType
-from .fileset import FileSet
+from .fileset import FileSet, MockMixin
 from .field import Field
 from .utils import (
     to_mime,
diff --git a/fileformats/core/fileset.py b/fileformats/core/fileset.py
index 61246b2..08051c3 100644
--- a/fileformats/core/fileset.py
+++ b/fileformats/core/fileset.py
@@ -1,4 +1,3 @@
-from __future__ import annotations
 import os
 from copy import copy
 import struct
@@ -189,18 +188,31 @@ def metadata(self) -> ty.Dict[str, ty.Any]:
         return metadata
 
     @hook.extra
-    def read_metadata(self) -> ty.Dict[str, ty.Any]:
-        """Reads any metadata associated with the fileset and returns it as a dict"""
+    def read_metadata(
+        self, selected_keys: ty.Optional[ty.Sequence[str]] = None
+    ) -> ty.Mapping[str, ty.Any]:
+        """Reads any metadata associated with the fileset and returns it as a dict
+
+        Parameters
+        ----------
+        selected_keys : Sequence[str], optional
+            selected keys to load instead of loading the complete metadata set
+
+        Returns
+        -------
+        metadata : Mapping[str, Any]
+            a mapping from names of the metadata fields to their values
+        """
         raise NotImplementedError
 
     @classmethod
-    def required_properties(cls):
+    def required_properties(cls) -> ty.Generator[str, None, None]:
         """Find all properties required to treat file-set as being in the format specified
         by the class
 
         Returns
         -------
-        iter(str)
+        Generator[str, None, None]
             an iterator over all properties names marked as "required"
         """
         fileset_props = dir(FileSet)
@@ -235,7 +247,7 @@ def required_paths(self) -> ty.Set[Path]:
                     required.add(path)
         return required
 
-    def nested_filesets(self) -> ty.List[FileSet]:
+    def nested_filesets(self) -> ty.List["FileSet"]:
         """Returns all nested filesets that are required for the format
 
         Returns
@@ -609,7 +621,7 @@ def byte_chunks(
         relative_to: ty.Optional[os.PathLike] = None,
         ignore_hidden_files: bool = False,
         ignore_hidden_dirs: bool = False,
-    ) -> ty.Generator[str, ty.Generator[bytes]]:
+    ) -> ty.Generator[ty.Tuple[str, bytes], None, None]:
         """Yields relative paths for all files within the file-set along with iterators
         over their byte-contents. To be used when generating hashes for the file set.
 
@@ -859,7 +871,7 @@ def sample(
 
     @hook.extra
     def generate_sample_data(
-        self, dest_dir: Path, seed: int = 0, stem: str = None
+        self, dest_dir: Path, seed: int = 0, stem: ty.Optional[str] = None
     ) -> ty.Iterable[Path]:
         """Generate test data at the fspaths of the file-set
 
diff --git a/fileformats/core/hook.py b/fileformats/core/hook.py
index 317b535..1bf529a 100644
--- a/fileformats/core/hook.py
+++ b/fileformats/core/hook.py
@@ -1,5 +1,7 @@
 import importlib
 import typing as ty
+import inspect
+from itertools import zip_longest
 import functools
 import attrs
 import urllib.error
@@ -158,5 +160,57 @@ def decorated(obj, *args, **kwargs):
                         )
             raise FileFormatsExtrasError(msg)
 
-    decorated.register = dispatch_method.register
+    decorated.register = ExtraRegisterer(dispatch_method)
     return decorated
+
+
+class ExtraRegisterer:
+    def __init__(self, dispatch):
+        self.dispatch = dispatch
+
+    def __call__(self, function: ty.Callable):
+        method = self.dispatch.__wrapped__
+        msig = inspect.signature(method)
+        fsig = inspect.signature(function)
+
+        def type_match(a, b) -> bool:
+            return isinstance(a, str) or isinstance(b, str) or a == b
+
+        differences = []
+        for i, (mparam, fparam) in enumerate(
+            zip_longest(
+                list(msig.parameters.values())[1:], list(fsig.parameters.values())[1:]
+            )
+        ):
+            if mparam is None:
+                differences.append(
+                    f"found additional argument, '{fparam.name}', at position {i}"
+                )
+                continue
+            if fparam is None:
+                if mparam.default is mparam.empty:
+                    differences.append(
+                        f"override missing required argument '{mparam.name}'"
+                    )
+                continue
+            mname = mparam.name
+            fname = fparam.name
+            mtype = mparam.annotation
+            ftype = fparam.annotation
+            if mname != fname:
+                differences.append(
+                    f"name of parameter at position {i}: {mname} vs {fname}"
+                )
+            elif not type_match(mtype, ftype):
+                differences.append(f"Type of '{mname}' arg: {mtype} vs {ftype}")
+        if not type_match(msig.return_annotation, fsig.return_annotation):
+            differences.append(
+                f"return type: {msig.return_annotation} vs {fsig.return_annotation}"
+            )
+        if differences:
+            raise TypeError(
+                f"Arguments differ between the signature of the "
+                f"decorated method {method} and the registered override {function}:\n"
+                + "\n".join(differences)
+            )
+        self.dispatch.register(function)
diff --git a/fileformats/core/tests/test_hooks.py b/fileformats/core/tests/test_hooks.py
new file mode 100644
index 0000000..3b18559
--- /dev/null
+++ b/fileformats/core/tests/test_hooks.py
@@ -0,0 +1,73 @@
+from pathlib import Path
+import platform
+import typing as ty
+import pytest
+from fileformats.core import hook, MockMixin, FileSet
+from fileformats.testing import Foo
+
+
+def test_sample():
+    test_inst = Foo.sample()
+    assert test_inst.fspath.exists()
+    assert test_inst.fspath.suffix == ".foo"
+
+
+def test_mock():
+    mock = Foo.mock()
+    if platform.system() == "Windows":
+        expected = Path(f"{Path().cwd().drive}\\mock\\foo.foo")
+    else:
+        expected = Path("/mock/foo.foo")
+    assert mock.fspath == expected
+    assert not mock.fspath.exists()
+    assert isinstance(mock, MockMixin)
+
+
+class Woo(FileSet):
+    @hook.extra
+    def test_hook(self, a: int, b: float, c: ty.Optional[str] = None) -> float:
+        raise NotImplementedError
+
+
+def test_hook_signature_no_default():
+    @Woo.test_hook.register
+    def woo_test_hook(woo: Woo, a: int, b: float) -> float:
+        pass
+
+
+def test_hook_signature1():
+
+    with pytest.raises(TypeError, match="missing required argument"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int) -> float:
+            pass
+
+
+def test_hook_signature2():
+
+    with pytest.raises(TypeError, match="name of parameter"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int, d: str) -> float:
+            pass
+
+
+def test_hook_signature3():
+
+    with pytest.raises(TypeError, match="found additional argument"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(
+            woo: Woo, a: int, b: float, c: ty.Optional[str], d: int
+        ) -> float:
+            pass
+
+
+def test_hook_signature4():
+
+    with pytest.raises(TypeError, match="return type"):
+
+        @Woo.test_hook.register
+        def woo_test_hook(woo: Woo, a: int, b: str) -> int:
+            pass
diff --git a/fileformats/core/tests/test_test_extras.py b/fileformats/core/tests/test_test_extras.py
deleted file mode 100644
index f087d94..0000000
--- a/fileformats/core/tests/test_test_extras.py
+++ /dev/null
@@ -1,21 +0,0 @@
-from pathlib import Path
-import platform
-from fileformats.core.fileset import MockMixin
-from fileformats.testing import Foo
-
-
-def test_sample():
-    test_inst = Foo.sample()
-    assert test_inst.fspath.exists()
-    assert test_inst.fspath.suffix == ".foo"
-
-
-def test_mock():
-    mock = Foo.mock()
-    if platform.system() == "Windows":
-        expected = Path(f"{Path().cwd().drive}\\mock\\foo.foo")
-    else:
-        expected = Path("/mock/foo.foo")
-    assert mock.fspath == expected
-    assert not mock.fspath.exists()
-    assert isinstance(mock, MockMixin)
diff --git a/fileformats/generic/__init__.py b/fileformats/generic/__init__.py
index 98ea046..54427fb 100644
--- a/fileformats/generic/__init__.py
+++ b/fileformats/generic/__init__.py
@@ -279,7 +279,7 @@ class SetOf(WithClassifiers, TypedSet):
 @FileSet.generate_sample_data.register
 def fsobject_generate_sample_data(
     fsobject: FsObject, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     a_file = dest_dir / gen_filename(seed, file_type=fsobject, stem=stem)
     a_file.write_text("".join(random.choices(string.printable, k=FILE_LENGTH)))
     return [a_file]
@@ -288,7 +288,7 @@ def fsobject_generate_sample_data(
 @FileSet.generate_sample_data.register
 def file_generate_sample_data(
     file: File, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     fname = gen_filename(seed, file_type=file, stem=stem)
     stem = fname[: -len(file.strext)]
     a_file = dest_dir / fname
@@ -322,7 +322,7 @@ def file_generate_sample_data(
 @FileSet.generate_sample_data.register
 def directory_generate_sample_data(
     directory: Directory, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     rng = Random(str(seed) + directory.mime_like)
     a_dir = dest_dir / gen_filename(rng, stem=stem)
     a_dir.mkdir()
@@ -333,7 +333,7 @@ def directory_generate_sample_data(
 @FileSet.generate_sample_data.register
 def directory_containing_generate_sample_data(
     directory: DirectoryContaining, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     rng = Random(str(seed) + directory.mime_like)
     a_dir = dest_dir / gen_filename(rng, stem=stem)
     a_dir.mkdir()
@@ -345,7 +345,7 @@ def directory_containing_generate_sample_data(
 @FileSet.generate_sample_data.register
 def set_of_sample_data(
     set_of: SetOf, dest_dir: Path, seed: int, stem: ty.Optional[str]
-) -> ty.List[Path]:
+) -> ty.Iterable[Path]:
     rng = Random(str(seed) + set_of.mime_like)
     return list(
         itertools.chain(