diff --git a/src/mccode_antlr/common/textwrap.py b/src/mccode_antlr/common/textwrap.py
index e905fc15..204fdd55 100644
--- a/src/mccode_antlr/common/textwrap.py
+++ b/src/mccode_antlr/common/textwrap.py
@@ -110,6 +110,20 @@ def hide(content: str) -> str:
         raise NotImplementedError
 
 
+def _own_lines(content: str) -> str:
+    """Put block content between its `%{` and `%}` lines.
+
+    Reading a block keeps everything between the delimiters, so content that was read
+    keeps its newlines as they were, and writing must add only what is missing: adding
+    more grows every block each time it is written and read again.
+    """
+    if not content.startswith('\n'):
+        content = '\n' + content
+    if not content.endswith('\n'):
+        content += '\n'
+    return content
+
+
 class TextWrapper(BaseWrapper):
 
     def wrap(self, text: str, pad: str = None) -> list[str]:
@@ -132,7 +146,7 @@ def end_block_comment() -> str:
         return '*/'
 
     def block(self, name: str, content: str) -> str:
-        return '\n'.join([name + '\n%{\n', content + '\n%}\n'])
+        return f'{name}\n%{{{_own_lines(content)}%}}\n'
 
     def line(self, name: str, items: list[str], sep: str = ' ') -> str:
         items = [item or 'None' for item in items]
@@ -147,7 +161,7 @@ def lines(self, items: list[str]) -> str:
 
     @staticmethod
     def metadata_group(name: str, mimetype: str, item: str, value: str) -> str:
-        return f'{name} "{mimetype}" {item} \n%{{\n{value}\n%}}\n'
+        return f'{name} "{mimetype}" {item}\n%{{{_own_lines(value)}%}}\n'
 
     @staticmethod
     def datatype(data_type: str) -> str:
diff --git a/src/mccode_antlr/comp/comp.py b/src/mccode_antlr/comp/comp.py
index 11d9aaa1..2291dd4a 100644
--- a/src/mccode_antlr/comp/comp.py
+++ b/src/mccode_antlr/comp/comp.py
@@ -165,33 +165,34 @@ def to_file(self, output=None, wrapper=None):
         def _join(raw_tuple):
             return '\n'.join(str(rc) for rc in raw_tuple)
 
-        # Build the parameter type prefix for define/setting/output parameters
-        def _param_type(p):
-            from mccode_antlr.common.expression import DataType, ShapeType
+        def _param_str(p):
+            # The type and default exactly as a parameter list spells them, so that
+            # reading the text back gives the same parameter
+            from mccode_antlr.common.expression import DataType, ObjectType
+            from mccode_antlr.common.expression.sympy_classes import UNSET_SYMPY
             v = p.value
             if v.is_str:
-                return 'string '
-            if v.is_vector and getattr(v, 'data_type', None) == DataType.float and \
-                    getattr(v, '_shape', None) is not None:
-                return 'vector '
-            if hasattr(v, 'data_type') and v.data_type == DataType.int:
-                return 'int '
-            return ''
+                # an archaic `symbol` is read as a string with an identifier default
+                default = str(v)
+                s = 'string ' if default in ('None', 'NULL') or default.startswith('"') else 'symbol '
+            elif v.is_vector:
+                s = 'int *' if v.data_type == DataType.int else 'vector '
+            elif v.data_type == DataType.int:
+                s = 'int '
+            else:
+                s = ''
+            s += p.name
+            if not (v.is_singular and v._exprs[0] is UNSET_SYMPY):
+                value = str(v)
+                if v.object_type == ObjectType.initializer_list:
+                    value = '{' + value + '}'
+                s += '=' + value
+            return s
 
         def _params_str(params):
-            parts = []
-            for p in params:
-                s = _param_type(p) + p.name
-                if p.value.has_value:
-                    s += '=' + str(p.value)
-                parts.append(s)
-            return ', '.join(parts)
-
-        name_line = f'DEFINE COMPONENT {self.name}'
-        if self.dependency:
-            name_line += f'\nDEPENDENCY "{self.dependency}"'
-        print(name_line, file=output)
+            return ', '.join(_param_str(p) for p in params)
 
+        print(f'DEFINE COMPONENT {self.name}', file=output)
         if self.define:
             print(f'DEFINITION PARAMETERS ({_params_str(self.define)})', file=output)
         if self.setting:
@@ -199,25 +200,20 @@ def _params_str(params):
         if self.output:
             print(f'OUTPUT PARAMETERS ({_params_str(self.output)})', file=output)
 
+        # The grammar fixes the order: metadata, then dependency, then NOACC
         for metadata in self.metadata:
             metadata.to_file(output=output, wrapper=wrapper)
-
-        if self.share:
-            print(wrapper.block('SHARE', _join(self.share)), file=output)
-        if self.user:
-            print(wrapper.block('USERVARS', _join(self.user)), file=output)
-        if self.declare:
-            print(wrapper.block('DECLARE', _join(self.declare)), file=output)
-        if self.initialize:
-            print(wrapper.block('INITIALIZE', _join(self.initialize)), file=output)
-        if self.trace:
-            print(wrapper.block('TRACE', _join(self.trace)), file=output)
-        if self.save:
-            print(wrapper.block('SAVE', _join(self.save)), file=output)
-        if self.final:
-            print(wrapper.block('FINALLY', _join(self.final)), file=output)
-        if self.display:
-            print(wrapper.block('MCDISPLAY', _join(self.display)), file=output)
+        if self.dependency:
+            print(f'DEPENDENCY "{self.dependency}"', file=output)
+        if not self.acc:
+            print('NOACC', file=output)
+
+        for name, raw in (('SHARE', self.share), ('USERVARS', self.user),
+                          ('DECLARE', self.declare), ('INITIALIZE', self.initialize),
+                          ('TRACE', self.trace), ('SAVE', self.save),
+                          ('FINALLY', self.final), ('MCDISPLAY', self.display)):
+            if raw:
+                print(wrapper.block(name, _join(raw)), file=output)
 
         print('END', file=output)
 
diff --git a/tests/test_comp_to_file.py b/tests/test_comp_to_file.py
new file mode 100644
index 00000000..0d76a25e
--- /dev/null
+++ b/tests/test_comp_to_file.py
@@ -0,0 +1,75 @@
+"""Comp.to_string writes a definition that reads back as the same component (#391)."""
+from mccode_antlr.comp import Comp
+from mccode_antlr.common import DataType
+
+SOURCE = '''DEFINE COMPONENT Everything
+DEFINITION PARAMETERS (int n=3)
+SETTING PARAMETERS (a, b=1e-4, int c=2, string s="x", string none=0, char *cs=NULL,
+  vector v=NULL, vector w={1, 2, 3}, vector u=nm, double *d, int *i={1, 2}, int *j,
+  symbol big=FLT_MAX)
+OUTPUT PARAMETERS (o)
+METADATA "text/plain" note %{
+a note
+%}
+DEPENDENCY "-lm"
+NOACC
+SHARE %{
+double helper(double x) { return x; }
+%}
+USERVARS %{ int seen; %}
+DECLARE %{
+double total;
+%}
+INITIALIZE %{
+total = 0;
+%}
+TRACE %{
+total += helper(a);
+%}
+SAVE %{
+%}
+FINALLY %{
+total = 0;
+%}
+MCDISPLAY %{
+line(0,0,0, 1,0,0);
+%}
+END
+'''
+
+
+def _read(source):
+    import warnings
+    with warnings.catch_warnings():
+        warnings.simplefilter('ignore')  # `symbol` is archaic, and says so
+        return Comp.from_source(None, None, source)
+
+
+def _parameters(comp):
+    return [(p.name, str(p.value), p.value.data_type, p.value.shape_type, p.value.object_type)
+            for p in comp.define + comp.setting + comp.output]
+
+
+def test_a_written_component_reads_back_the_same():
+    original = _read(SOURCE)
+    written = original.to_string()
+    back = _read(written)
+    assert _parameters(back) == _parameters(original)
+    assert back.dependency == original.dependency == '-lm'
+    assert back.acc is original.acc is False
+    assert [(m.name, m.mimetype, m.value) for m in back.metadata] == \
+        [(m.name, m.mimetype, m.value) for m in original.metadata]
+    for block in ('share', 'user', 'declare', 'initialize', 'trace', 'save', 'final', 'display'):
+        # an inline `%{ ... %}` gains its own lines once, and keeps them thereafter
+        assert [str(r).strip() for r in getattr(back, block)] == \
+            [str(r).strip() for r in getattr(original, block)]
+    assert back.to_string() == written
+
+
+def test_vector_and_array_parameters_keep_their_type_and_default():
+    written = _read(SOURCE).to_string()
+    for text in ('vector v=NULL', 'vector w={1,2,3}', 'vector u=nm', 'vector d,',
+                 'int *i={1,2}', 'int *j,', 'symbol big=FLT_MAX'):
+        assert text in written
+    i = next(p for p in _read(written).setting if p.name == 'i')
+    assert i.value.data_type == DataType.int and i.value.is_vector
diff --git a/tests/test_instr_to_file.py b/tests/test_instr_to_file.py
index 8d17aca0..a02fbd11 100644
--- a/tests/test_instr_to_file.py
+++ b/tests/test_instr_to_file.py
@@ -22,3 +22,26 @@ def test_to_file_with_a_file_name_or_path(tmp_path):
     instr.to_file(tmp_path / 'b.instr')
     assert (tmp_path / 'a.instr').read_text(encoding='utf-8') == expected
     assert (tmp_path / 'b.instr').read_text(encoding='utf-8') == expected
+
+
+def test_written_blocks_do_not_grow_when_read_back():
+    """Writing added a blank line inside every block, so each read and write grew it."""
+    instr = parse_mcstas_instr(dedent("""DEFINE INSTRUMENT blocks(a=1)
+    METADATA "text/plain" note %{
+    instrument note
+    %}
+    DECLARE %{
+    double x;
+    %}
+    TRACE
+    COMPONENT origin = Arm() AT (0, 0, 0) ABSOLUTE
+    EXTEND %{
+    x += a;
+    %}
+    METADATA "text/plain" inote %{
+    instance note
+    %}
+    END
+    """))
+    once = instr.to_string(None)
+    assert parse_mcstas_instr(once).to_string(None) == once