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feat: add Span<T> params to C# struct Create methods for zero-allocation stackalloc support #8970

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feat: add Span<T> params to C# struct Create methods for zero-allocation stackalloc support #8970

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6e77942
feat: add Span<T> params to C# struct Create methods for zero-allocat…
statxc Mar 11, 2026
fc63740
fix: update default Span to ReadOnlySpan
statxc Mar 11, 2026
f673e3d
Merge branch 'master' into csharp/span-params-create-struct
statxc Mar 13, 2026
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215 changes: 157 additions & 58 deletions src/idl_gen_csharp.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,20 @@ class CSharpGenerator : public BaseGenerator {
int length;
};

// Check if a struct (or any of its nested structs) contains array fields.
// Used to decide whether Create/Pack methods need Span overloads.
bool StructHasArrayFields(const StructDef& struct_def) const {
for (auto it = struct_def.fields.vec.begin();
it != struct_def.fields.vec.end(); ++it) {
auto& field = **it;
if (IsArray(field.value.type)) return true;
if (IsStruct(field.value.type) &&
StructHasArrayFields(*field.value.type.struct_def))
return true;
}
return false;
}

public:
CSharpGenerator(const Parser& parser, const std::string& path,
const std::string& file_name)
Expand Down Expand Up @@ -536,9 +550,11 @@ class CSharpGenerator : public BaseGenerator {
}

// Recursively generate arguments for a constructor, to deal with nested
// structs.
// structs. When use_span is true, array parameters use Span<T> instead of
// T[] or T[,] for zero-allocation struct creation.
void GenStructArgs(const StructDef& struct_def, std::string* code_ptr,
const char* nameprefix, size_t array_count = 0) const {
const char* nameprefix, size_t array_count = 0,
bool use_span = false) const {
std::string& code = *code_ptr;
for (auto it = struct_def.fields.vec.begin();
it != struct_def.fields.vec.end(); ++it) {
Expand All @@ -553,17 +569,21 @@ class CSharpGenerator : public BaseGenerator {
// a nested struct, prefix the name with the field name.
GenStructArgs(*field_type.struct_def, code_ptr,
(nameprefix + (EscapeKeyword(field.name) + "_")).c_str(),
array_cnt);
array_cnt, use_span);
} else {
code += ", ";
code += GenTypeBasic(type);
if (field.IsScalarOptional()) {
code += "?";
}
if (array_cnt > 0) {
code += "[";
for (size_t i = 1; i < array_cnt; i++) code += ",";
code += "]";
if (use_span && array_cnt > 0) {
code += "ReadOnlySpan<" + GenTypeBasic(type) + ">";
} else {
code += GenTypeBasic(type);
if (field.IsScalarOptional()) {
code += "?";
}
if (array_cnt > 0) {
code += "[";
for (size_t i = 1; i < array_cnt; i++) code += ",";
code += "]";
}
}
code += " ";
code += nameprefix;
Expand All @@ -574,10 +594,12 @@ class CSharpGenerator : public BaseGenerator {

// Recusively generate struct construction statements of the form:
// builder.putType(name);
// and insert manual padding.
// and insert manual padding. When use_span is true, multi-dimensional array
// accesses use flat indexing (e.g. [i*N+j]) instead of [i,j].
void GenStructBody(const StructDef& struct_def, std::string* code_ptr,
const char* nameprefix, size_t index = 0,
bool in_array = false) const {
bool in_array = false, bool use_span = false,
std::vector<int> array_dims = {}) const {
std::string& code = *code_ptr;
std::string indent((index + 1) * 2, ' ');
code += indent + " builder.Prep(";
Expand All @@ -594,21 +616,23 @@ class CSharpGenerator : public BaseGenerator {
if (IsStruct(field_type)) {
GenStructBody(*field_type.struct_def, code_ptr,
(nameprefix + (field.name + "_")).c_str(), index,
in_array);
in_array, use_span, array_dims);
} else {
const auto& type =
IsArray(field_type) ? field_type.VectorType() : field_type;
const auto index_var = "_idx" + NumToString(index);
auto current_dims = array_dims;
if (IsArray(field_type)) {
code += indent + " for (int " + index_var + " = ";
code += NumToString(field_type.fixed_length);
code += "; " + index_var + " > 0; " + index_var + "--) {\n";
in_array = true;
current_dims.push_back(field_type.fixed_length);
}
if (IsStruct(type)) {
GenStructBody(*field_type.struct_def, code_ptr,
(nameprefix + (field.name + "_")).c_str(), index + 1,
in_array);
in_array, use_span, current_dims);
} else {
code += IsArray(field_type) ? " " : "";
code += indent + " builder.Put";
Expand All @@ -619,9 +643,24 @@ class CSharpGenerator : public BaseGenerator {
size_t array_cnt = index + (IsArray(field_type) ? 1 : 0);
if (array_cnt > 0) {
code += "[";
for (size_t i = 0; in_array && i < array_cnt; i++) {
code += "_idx" + NumToString(i) + "-1";
if (i != (array_cnt - 1)) code += ",";
if (use_span && in_array && array_cnt > 1) {
// Flat indexing for Span<T>: [(_idx0-1)*stride + (_idx1-1)]
for (size_t i = 0; i < array_cnt; i++) {
if (i > 0) code += " + ";
code += "(_idx" + NumToString(i) + "-1)";
int stride = 1;
for (size_t j = i + 1; j < current_dims.size(); j++) {
stride *= current_dims[j];
}
if (stride > 1) {
code += "*" + NumToString(stride);
}
}
} else {
for (size_t i = 0; in_array && i < array_cnt; i++) {
code += "_idx" + NumToString(i) + "-1";
if (i != (array_cnt - 1)) code += ",";
}
}
code += "]";
}
Expand Down Expand Up @@ -1374,7 +1413,22 @@ class CSharpGenerator : public BaseGenerator {
flatbuffers::FieldDef* key_field = nullptr;
if (struct_def.fixed) {
struct_has_create = true;
// create a struct constructor function
const bool has_arrays = StructHasArrayFields(struct_def);
if (has_arrays) {
// Span version: array params become Span<T>, multi-dim uses flat index
code += "#if ENABLE_SPAN_T\n";
code += " public static " + GenOffsetType(struct_def) + " ";
code += "Create";
code += struct_def.name + "(FlatBufferBuilder builder";
GenStructArgs(struct_def, code_ptr, "", 0, true);
code += ") {\n";
GenStructBody(struct_def, code_ptr, "", 0, false, true);
code += " return ";
code += GenOffsetConstruct(struct_def, "builder.Offset");
code += ";\n }\n";
code += "#else\n";
}
// Original version (also the only version when no arrays)
code += " public static " + GenOffsetType(struct_def) + " ";
code += "Create";
code += struct_def.name + "(FlatBufferBuilder builder";
Expand All @@ -1384,6 +1438,9 @@ class CSharpGenerator : public BaseGenerator {
code += " return ";
code += GenOffsetConstruct(struct_def, "builder.Offset");
code += ";\n }\n";
if (has_arrays) {
code += "#endif\n";
}
} else {
// Generate a method that creates a table in one go. This is only possible
// when the table has no struct fields, since those have to be created
Expand Down Expand Up @@ -2365,6 +2422,73 @@ class CSharpGenerator : public BaseGenerator {
code += " }\n";
}

// Generate one pack declaration: either flat (for Span) or multi-dim.
void GenPackDeclArray(std::string& code, const std::string& name,
const Type& field_type,
const std::vector<FieldArrayLength>& array_lengths,
const std::vector<FieldArrayLength>& array_only_lengths,
bool flat) const {
code += " var " + name + " = ";
code += "new " + GenTypeBasic(field_type) + "[";
if (flat) {
int total = 1;
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
total *= array_only_lengths[i].length;
}
code += NumToString(total);
} else {
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
if (i != 0) code += ",";
code += NumToString(array_only_lengths[i].length);
}
}
code += "];\n";
code += " ";
// initialize array
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
auto idx = "idx" + NumToString(i);
code += "for (var " + idx + " = 0; " + idx + " < " +
NumToString(array_only_lengths[i].length) + "; ++" + idx +
") {";
}
if (flat) {
// flat indexing: name[idx0*d1 + idx1]
code += name + "[";
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
if (i > 0) code += " + ";
code += "idx" + NumToString(i);
int stride = 1;
for (size_t j = i + 1; j < array_only_lengths.size(); ++j) {
stride *= array_only_lengths[j].length;
}
if (stride > 1) code += "*" + NumToString(stride);
}
code += "]";
} else {
// multi-dim indexing: name[idx0,idx1]
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
auto idx = "idx" + NumToString(i);
if (i == 0) {
code += name + "[" + idx;
} else {
code += "," + idx;
}
}
code += "]";
}
code += " = _o";
for (size_t i = 0, j = 0; i < array_lengths.size(); ++i) {
code += "." + ConvertCase(array_lengths[i].name, Case::kUpperCamel);
if (array_lengths[i].length <= 0) continue;
code += "[idx" + NumToString(j++) + "]";
}
code += ";";
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
code += "}";
}
code += "\n";
}

void GenStructPackDecl_ObjectAPI(
const StructDef& struct_def, std::string* code_ptr,
std::vector<FieldArrayLength>& array_lengths) const {
Expand Down Expand Up @@ -2394,50 +2518,25 @@ class CSharpGenerator : public BaseGenerator {
for (size_t i = 0; i < array_lengths.size(); ++i) {
name += "_" + array_lengths[i].name;
}
code += " var " + name + " = ";
if (array_only_lengths.size() > 0) {
code += "new " + GenTypeBasic(field_type) + "[";
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
if (i != 0) {
code += ",";
}
code += NumToString(array_only_lengths[i].length);
}
code += "];\n";
code += " ";
// initialize array
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
auto idx = "idx" + NumToString(i);
code += "for (var " + idx + " = 0; " + idx + " < " +
NumToString(array_only_lengths[i].length) + "; ++" + idx +
") {";
}
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
auto idx = "idx" + NumToString(i);
if (i == 0) {
code += name + "[" + idx;
} else {
code += "," + idx;
}
}
code += "] = _o";
for (size_t i = 0, j = 0; i < array_lengths.size(); ++i) {
code += "." + ConvertCase(array_lengths[i].name, Case::kUpperCamel);
if (array_lengths[i].length <= 0) continue;
code += "[idx" + NumToString(j++) + "]";
}
code += ";";
for (size_t i = 0; i < array_only_lengths.size(); ++i) {
code += "}";
}
if (array_only_lengths.size() > 1) {
// Multi-dim case: needs flat arrays for Span, multi-dim for legacy
code += "#if ENABLE_SPAN_T\n";
GenPackDeclArray(code, name, field_type, array_lengths,
array_only_lengths, true);
code += "#else\n";
GenPackDeclArray(code, name, field_type, array_lengths,
array_only_lengths, false);
code += "#endif\n";
} else if (array_only_lengths.size() == 1) {
GenPackDeclArray(code, name, field_type, array_lengths,
array_only_lengths, false);
} else {
code += "_o";
code += " var " + name + " = _o";
for (size_t i = 0; i < array_lengths.size(); ++i) {
code += "." + ConvertCase(array_lengths[i].name, Case::kUpperCamel);
}
code += ";";
code += ";\n";
}
code += "\n";
}
array_lengths.pop_back();
}
Expand Down
24 changes: 16 additions & 8 deletions tests/FlatBuffers.Test/FlatBuffersExampleTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -398,22 +398,32 @@ public void TestFixedLenghtArrays()
float a;
int[] b = new int[15];
sbyte c;
int[,] d_a = new int[2, 2];
TestEnum[] d_b = new TestEnum[2];
TestEnum[,] d_c = new TestEnum[2, 2];
long[,] d_d = new long[2, 2];
int e;
long[] f = new long[2];

a = 0.5f;
for (int i = 0; i < 15; i++) b[i] = i;
c = 1;
d_b[0] = TestEnum.B;
d_b[1] = TestEnum.C;
e = 2;
f[0] = -1;
f[1] = 1;

#if ENABLE_SPAN_T
// Flat arrays for Span<T> parameters
int[] d_a = new int[] { 1, 2, 3, 4 };
TestEnum[] d_c = new TestEnum[] { TestEnum.A, TestEnum.B, TestEnum.C, TestEnum.B };
long[] d_d = new long[] { -1, 1, -2, 2 };
#else
int[,] d_a = new int[2, 2];
TestEnum[,] d_c = new TestEnum[2, 2];
long[,] d_d = new long[2, 2];
d_a[0, 0] = 1;
d_a[0, 1] = 2;
d_a[1, 0] = 3;
d_a[1, 1] = 4;
d_b[0] = TestEnum.B;
d_b[1] = TestEnum.C;
d_c[0, 0] = TestEnum.A;
d_c[0, 1] = TestEnum.B;
d_c[1, 0] = TestEnum.C;
Expand All @@ -422,9 +432,7 @@ public void TestFixedLenghtArrays()
d_d[0, 1] = 1;
d_d[1, 0] = -2;
d_d[1, 1] = 2;
e = 2;
f[0] = -1;
f[1] = 1;
#endif

Offset<ArrayStruct> arrayOffset = ArrayStruct.CreateArrayStruct(
builder, a, b, c, d_a, d_b, d_c, d_d, e, f);
Expand Down
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