fix: optional parameter ordering CS1737

bindgen/src/gen_cs/mod.rs

@@ -480,6 +480,17 @@ pub mod filters {
         Ok(textwrap::indent(&wrapped, &" ".repeat(spaces)))
     }
 
+    /// Orders fields in a way that avoids CS1737 errors
+    pub fn order_fields(fields: &[Field]) -> Result<(Vec<Field>, bool), askama::Error> {
+        let original_fields = fields.to_vec();
+        let mut fields = original_fields.clone();
+        // fields with default values must come after fields without default values
+        // https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-messages/cs1737
+        fields.sort_by_key(|field| field.default_value().is_some());
+        let is_reordered = fields != original_fields;
+        Ok((fields, is_reordered))
+    }
+
     /// Panic with message
     pub fn panic(message: &str) -> Result<String, askama::Error> {
         panic!("{}", message)

bindgen/templates/RecordTemplate.cs

@@ -3,10 +3,18 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */#}
 
 {%- let rec = ci.get_record_definition(name).unwrap() %}
+{%- let (ordered_fields, is_reordered) = rec.fields()|order_fields %}
 
 {%- call cs::docstring(rec, 0) %}
+{%- if is_reordered %}
+/// <remarks>
+/// <b>UniFFI Warning:</b> Optional parameters have been reordered because
+/// of a C# syntax limitation. Use named parameters for compatibility with
+/// future ordering changes.
+/// </remarks>
+{%- endif %}
 public record {{ type_name }} (
-    {%- for field in rec.fields() %}
+    {%- for field in ordered_fields %}
     {%- call cs::docstring(field, 4) %}
     {{ field|type_name }} {{ field.name()|var_name -}}
     {%- match field.default_value() %}
@@ -29,7 +37,7 @@ class {{ rec|ffi_converter_name }}: FfiConverterRustBuffer<{{ type_name }}> {
     public override {{ type_name }} Read(BigEndianStream stream) {
         return new {{ type_name }}(
         {%- for field in rec.fields() %}
-            {{ field|read_fn }}(stream){% if !loop.last %},{% endif%}
+            {{ field.name()|var_name }}: {{ field|read_fn }}(stream){% if !loop.last %},{% endif%}
         {%- endfor %}
         );
     }

dotnet-tests/UniffiCS.binding_tests/OptionalParameterTests.cs

@@ -0,0 +1,27 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+using uniffi.uniffi_cs_optional_parameters;
+using static uniffi.uniffi_cs_optional_parameters.UniffiCsOptionalParametersMethods;
+
+namespace UniffiCS.binding_tests;
+
+public class OptionalParameterTests
+{
+    [Fact]
+    public void OptionalParameter_CanBeOmitted()
+    {
+        var person = new Person(isSpecial: false);
+        string message = Hello(person);
+        Assert.Equal("Hello stranger!", message);
+    }
+
+    [Fact]
+    public void OptionalParameter_CanBeSpecified()
+    {
+        var person = new Person(name: "John Connor", isSpecial: false);
+        string message = Hello(person);
+        Assert.Equal("Hello John Connor!", message);
+    }
+}

fixtures/Cargo.toml

@@ -12,6 +12,7 @@ crate-type = ["cdylib", "lib"]
 global-methods-class-name = { path = "global-methods-class-name" }
 uniffi-cs-custom-types-builtin = { path = "custom-types-builtin" }
 uniffi-cs-disposable-fixture = { path = "disposable" }
+uniffi-cs-optional-parameters-fixture = { path = "optional-parameters" }
 uniffi-example-arithmetic = { path = "../3rd-party/uniffi-rs/examples/arithmetic" }
 uniffi-example-callbacks = { path = "../3rd-party/uniffi-rs/examples/callbacks" }
 uniffi-example-custom-types = { path = "../3rd-party/uniffi-rs/examples/custom-types" }

fixtures/optional-parameters/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "uniffi-cs-optional-parameters-fixture"
+version = "1.0.0"
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["lib", "cdylib"]
+name = "uniffi_cs_optional_parameters"
+
+[dependencies]
+uniffi = {path = "../../3rd-party/uniffi-rs/uniffi", features=["build"]}
+uniffi_macros = {path = "../../3rd-party/uniffi-rs/uniffi_macros"}
+
+[build-dependencies]
+uniffi = {path = "../../3rd-party/uniffi-rs/uniffi", features=["bindgen-tests"]}

fixtures/optional-parameters/src/lib.rs

@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+uniffi::setup_scaffolding!();
+
+#[derive(uniffi::Record)]
+pub struct Person {
+    #[uniffi(default = None)]
+    pub name: Option<String>,
+    pub is_special: bool,
+}
+
+#[uniffi::export]
+fn hello(person: Person) -> String {
+    let name = person.name.unwrap_or("stranger".to_string());
+    if person.is_special {
+        format!("Hello {}! You are special!", name)
+    } else {
+        format!("Hello {}!", name)
+    }
+}

fixtures/src/lib.rs

@@ -20,4 +20,5 @@ mod uniffi_fixtures {
     global_methods_class_name::uniffi_reexport_scaffolding!();
     uniffi_cs_custom_types_builtin::uniffi_reexport_scaffolding!();
     uniffi_cs_disposable::uniffi_reexport_scaffolding!();
+    uniffi_cs_optional_parameters::uniffi_reexport_scaffolding!();
 }