[Rgen] Add support to the export method to provide custom marshalling.

Add a new flag that can be used to let the generator know that a exposed
method or property can have a custom marshalling call.

Later the analyzer will make sure that only when the appropiate flag is
set, that the extra name parameters are allowed.

src/ObjCBindings/ExportAttribute.cs

@@ -27,6 +27,30 @@ public class ExportAttribute<T> : Attribute where T : Enum {
 		///  </summary >
 		public ArgumentSemantic ArgumentSemantic { get; set; } = ArgumentSemantic.None;
 
+		/// <summary>
+		/// Get/Set the native prefix to be used in the custom marshal directive.
+		///
+		/// The generator will only respect this value if the CustomMarshalDirective flag is set.
+		/// If the flag is not set the analyzer will raise an compiling error.
+		/// </summary >
+		public string? NativePrefix { get; set; } = null;
+
+		/// <summary>
+		/// Get/Set the native suffix to be used in the custom marshal directive.
+		///
+		/// The generator will only respect this value if the CustomMarshalDirective flag is set.
+		/// If the flag is not set the analyzer will raise an compiling error.
+		/// </summary >
+		public string? NativeSuffix { get; set; } = null;
+
+		/// <summary>
+		/// Get/Set the library to be used in the custom marshal directive.
+		///
+		/// The generator will only respect this value if the CustomMarshalDirective flag is set.
+		/// If the flag is not set the analyzer will raise an compiling error.
+		/// </summary >
+		public string? Library { get; set; } = null;
+
 		protected ExportAttribute () { }
 
 		/// <summary>

src/ObjCBindings/ExportTag.cs

@@ -49,6 +49,16 @@ public enum Method : Int64 {
 		/// </summary>
 		MarshalNativeExceptions = 1 << 4,
 
+
+		/// <summary>
+		/// Instruct the generator to use a custom marshal directive for the method. When this flag is applied the 
+		/// following name parameters must be provided:
+		/// - NativePrefix: The prefix to be used in the native method name.
+		/// - NativeSuffix: The suffix to be used in the native method name.
+		/// - Library: The library to be used in the custom marshal directive.
+		/// </summary>
+		CustomMarshalDirective = 1 << 5,
+
 	}
 
 	/// <summary>
@@ -78,6 +88,15 @@ public enum Property : Int64 {
 		/// </summary>
 		MarshalNativeExceptions = 1 << 4,
 
+		/// <summary>
+		/// Instruct the generator to use a custom marshal directive for the method. When this flag is applied the 
+		/// following name parameters must be provided:
+		/// - NativePrefix: The prefix to be used in the native method name.
+		/// - NativeSuffix: The suffix to be used in the native method name.
+		/// - Library: The library to be used in the custom marshal directive.
+		/// </summary>
+		CustomMarshalDirective = 1 << 5,
+
 	}
 }
 

src/rgen/Microsoft.Macios.Generator/Attributes/ExportData.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 using System;
 using System.Diagnostics.CodeAnalysis;
+using System.Text;
 using Microsoft.CodeAnalysis;
 using ObjCRuntime;
 
@@ -27,6 +28,27 @@ namespace Microsoft.Macios.Generator.Attributes;
 	/// Argument semantics to use with the selector.
 	/// </summary>
 	public ArgumentSemantic ArgumentSemantic { get; } = ArgumentSemantic.None;
+
+	/// <summary>
+	/// Get the native prefix to be used in the custom marshal directive.
+	///
+	/// Should only be present with the CustomeMarshalDirective flag.
+	/// </summary >
+	public string? NativePrefix { get; init; }
+
+	/// <summary>
+	/// Get the native sufix to be used in the custom marshal directive.
+	///
+	/// Should only be present with the CustomeMarshalDirective flag.
+	/// </summary >
+	public string? NativeSuffix { get; init; }
+
+	/// <summary>
+	/// Get the library to be used in the custom marshal directive.
+	///
+	/// Should only be present with the CustomeMarshalDirective flag.
+	/// </summary >
+	public string? Library { get; init; }
 
 	public ExportData () { }
 
@@ -62,6 +84,12 @@ public static bool TryParse (AttributeData attributeData,
 		string? selector = null;
 		ArgumentSemantic argumentSemantic = ArgumentSemantic.None;
 		T? flags = default;
+
+		// custom marshal directive values
+		string? nativePrefix = null;
+		string? nativeSuffix = null;
+		string? library = null;
+
 		switch (count) {
 		case 1:
 			selector = (string?) attributeData.ConstructorArguments [0].Value!;
@@ -106,14 +134,35 @@ public static bool TryParse (AttributeData attributeData,
 			case "Flags":
 				flags = (T) value.Value!;
 				break;
+			case "NativePrefix":
+				nativePrefix = (string?) value.Value!;
+				break;
+			case "NativeSuffix":
+				nativeSuffix = (string?) value.Value!;
+				break;
+			case "Library":
+				library = (string?) value.Value!;
+				break;
 			default:
 				data = null;
 				return false;
 			}
 		}
 
-		data = flags is not null ?
-			new (selector, argumentSemantic, flags) : new (selector, argumentSemantic);
+		if (flags is not null) {
+			data = new (selector, argumentSemantic, flags) {
+				NativePrefix = nativePrefix,
+				NativeSuffix = nativeSuffix,
+				Library = library
+			};
+			return true;
+		}
+
+		data = new(selector, argumentSemantic) {
+			NativePrefix = nativePrefix, 
+			NativeSuffix = nativeSuffix, 
+			Library = library
+		};
 		return true;
 	}
 
@@ -124,10 +173,18 @@ public bool Equals (ExportData<T> other)
 			return false;
 		if (ArgumentSemantic != other.ArgumentSemantic)
 			return false;
-		if (Flags is not null && other.Flags is not null) {
-			return Flags.Equals (other.Flags);
-		}
-		return false;
+		if (NativePrefix != other.NativePrefix)
+			return false;
+		if (NativeSuffix != other.NativeSuffix)
+			return false;
+		if (Library != other.Library)
+			return false;
+		return (Flags, other.Flags) switch {
+			(null, null) => true,
+			(null, _) => false,
+			(_, null) => false,
+			(_, _) => Flags!.Equals (other.Flags)
+		};
 	}
 
 	/// <inheritdoc />
@@ -155,6 +212,21 @@ public override int GetHashCode ()
 	/// <inheritdoc />
 	public override string ToString ()
 	{
-		return $"{{ Type: '{typeof (T).FullName}', Selector: '{Selector ?? "null"}', ArgumentSemantic: '{ArgumentSemantic}', Flags: '{Flags}' }}";
+		var sb = new StringBuilder ("{ Type: '");
+		sb.Append (typeof (T).FullName);
+		sb.Append ("', Selector: '");
+		sb.Append (Selector ?? "null");
+		sb.Append ("', ArgumentSemantic: '");
+		sb.Append (ArgumentSemantic);
+		sb.Append ("', Flags: '");
+		sb.Append (Flags);
+		sb.Append ("', NativePrefix: '");
+		sb.Append (NativePrefix ?? "null");
+		sb.Append ("', NativeSuffix: '");
+		sb.Append (NativeSuffix ?? "null");
+		sb.Append ("', Library: '");
+		sb.Append (Library ?? "null");
+		sb.Append ("' }");
+		return sb.ToString ();
 	}
 }

tests/rgen/Microsoft.Macios.Generator.Tests/Attributes/ExportDataTests.cs

@@ -54,22 +54,31 @@ public IEnumerator<object []> GetEnumerator ()
 			yield return [
 				Method.Default,
 				new ExportData<Method> ("symbol", ArgumentSemantic.None, Method.Default),
-				"{ Type: 'ObjCBindings.Method', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default' }"
+				"{ Type: 'ObjCBindings.Method', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default', NativePrefix: 'null', NativeSuffix: 'null', Library: 'null' }"
 			];
 			yield return [
 				Method.Default,
 				new ExportData<Method> ("symbol"),
-				"{ Type: 'ObjCBindings.Method', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default' }"
+				"{ Type: 'ObjCBindings.Method', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default', NativePrefix: 'null', NativeSuffix: 'null', Library: 'null' }"
 			];
 			yield return [
 				Property.Default,
 				new ExportData<Property> ("symbol", ArgumentSemantic.Retain, Property.Default),
-				"{ Type: 'ObjCBindings.Property', Selector: 'symbol', ArgumentSemantic: 'Retain', Flags: 'Default' }"
+				"{ Type: 'ObjCBindings.Property', Selector: 'symbol', ArgumentSemantic: 'Retain', Flags: 'Default', NativePrefix: 'null', NativeSuffix: 'null', Library: 'null' }"
 			];
 			yield return [
 				Property.Default,
 				new ExportData<Property> ("symbol"),
-				"{ Type: 'ObjCBindings.Property', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default' }"
+				"{ Type: 'ObjCBindings.Property', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'Default', NativePrefix: 'null', NativeSuffix: 'null', Library: 'null' }"
+			];
+			yield return [
+				Method.Default,
+				new ExportData<Method> ("symbol", ArgumentSemantic.None,
+				Method.Default | Method.CustomMarshalDirective) {
+					NativePrefix = "xamarin_",
+					Library = "__Internal"
+				},
+				"{ Type: 'ObjCBindings.Method', Selector: 'symbol', ArgumentSemantic: 'None', Flags: 'CustomMarshalDirective', NativePrefix: 'xamarin_', NativeSuffix: 'null', Library: '__Internal' }"
 			];
 		}
 
@@ -81,6 +90,7 @@ IEnumerator IEnumerable.GetEnumerator ()
 	[ClassData (typeof (TestDataToString))]
 	void TestFieldDataToString<T> (T @enum, ExportData<T> x, string expected) where T : Enum
 	{
+		var str = x.ToString ();
 		Assert.NotNull (@enum);
 		Assert.Equal (expected, x.ToString ());
 	}

tests/rgen/Microsoft.Macios.Generator.Tests/DataModel/ClassCodeChangesTests.cs

@@ -734,6 +734,78 @@ public partial class MyClass {
 					]
 				}
 			];
+
+			const string customMarshallingProperty = @"
+using ObjCBindings;
+
+namespace NS;
+
+[BindingType<Class>]
+public partial class MyClass {
+	[Export<Property> (""name"", Flags = Property.CustomMarshalDirective, NativePrefix = ""xamarin_"", Library = ""__Internal"")]
+	public partial string Name { get; set; } = string.Empty;
+}
+";
+
+			yield return [
+				customMarshallingProperty,
+				new CodeChanges (
+					bindingInfo: new (new BindingTypeData<Class> ()),
+					name: "MyClass",
+					@namespace: ["NS"],
+					fullyQualifiedSymbol: "NS.MyClass",
+					symbolAvailability: new ()
+				) {
+					Base = "object",
+					Interfaces = ImmutableArray<string>.Empty,
+					Attributes = [
+						new ("ObjCBindings.BindingTypeAttribute<ObjCBindings.Class>")
+					],
+					UsingDirectives = new HashSet<string> { "ObjCBindings" },
+					Modifiers = [
+						SyntaxFactory.Token (SyntaxKind.PublicKeyword),
+						SyntaxFactory.Token (SyntaxKind.PartialKeyword)
+					],
+					Properties = [
+						new (
+							name: "Name",
+							returnType: ReturnTypeForString (),
+							symbolAvailability: new (),
+							attributes: [
+								new ("ObjCBindings.ExportAttribute<ObjCBindings.Property>", ["name", "ObjCBindings.Property.CustomMarshalDirective", "xamarin_", "__Internal"])
+							],
+							modifiers: [
+								SyntaxFactory.Token (SyntaxKind.PublicKeyword),
+								SyntaxFactory.Token (SyntaxKind.PartialKeyword),
+							],
+							accessors: [
+								new (
+									accessorKind: AccessorKind.Getter,
+									symbolAvailability: new (),
+									exportPropertyData: null,
+									attributes: [],
+									modifiers: []
+								),
+								new (
+									accessorKind: AccessorKind.Setter,
+									symbolAvailability: new (),
+									exportPropertyData: null,
+									attributes: [],
+									modifiers: []
+								),
+							]
+						) {
+							ExportPropertyData = new (
+								selector: "name", 
+								argumentSemantic: ArgumentSemantic.None, 
+								flags: Property.Default | Property.CustomMarshalDirective) {
+								NativePrefix = "xamarin_",
+								Library = "__Internal"
+							}
+						}
+					]
+				}
+			];
 
 			const string multiPropertyClass = @"
 using ObjCBindings;