[Rgen] Add method to get the pointer for the exception handler. (#22135)

Author: mandel-macaque

src/rgen/Microsoft.Macios.Generator/Emitters/BindingSyntaxFactory.ObjCRuntime.cs

@@ -650,6 +650,29 @@ internal static LocalDeclarationStatementSyntax GetAutoreleasePoolVariable ()
 			IdentifierName ("EnsureUIThread").WithTrailingTrivia (Space))));
 	}
 
+	/// <summary>
+	/// Generate the declaration needed for the exception marhsaling.
+	/// </summary>
+	/// <returns>The local declaration needed for the exception marshaling.</returns>
+	internal static LocalDeclarationStatementSyntax GetExceptionHandleAuxVariable ()
+	{
+		const string handleVariableName = "exception_gchandle";
+		const string intPtr = "IntPtr";
+		//  object creation
+		var zeroPtr = MemberAccessExpression (
+				SyntaxKind.SimpleMemberAccessExpression,
+				IdentifierName (intPtr),
+				IdentifierName ("Zero"));
+
+		var declarator = VariableDeclarator (Identifier (handleVariableName))
+				.WithInitializer (EqualsValueClause (zeroPtr));
+
+		return LocalDeclarationStatement (
+			VariableDeclaration (IdentifierName (intPtr))
+				.WithVariables (SingletonSeparatedList (declarator)))
+			.NormalizeWhitespace (); // no special mono style
+	}
+
 	static string? GetObjCMessageSendMethodName<T> (ExportData<T> exportData,
 		TypeInfo returnType, ImmutableArray<Parameter> parameters, bool isSuper = false, bool isStret = false)
 		where T : Enum

tests/rgen/Microsoft.Macios.Generator.Tests/Emitters/BindingSyntaxFactoryObjCRuntimeTests.cs

@@ -5,7 +5,6 @@
 using System.Collections.Generic;
 using Microsoft.CodeAnalysis;
 using Microsoft.Macios.Generator.DataModel;
-using Xamarin.Utils;
 using Xunit;
 using static Microsoft.Macios.Generator.Emitters.BindingSyntaxFactory;
 using static Microsoft.Macios.Generator.Tests.TestDataFactory;
@@ -655,4 +654,12 @@ void EnsureUiThreadTests (PlatformName platform, string? expectedDeclaration)
 			Assert.Equal (expectedDeclaration, declaration.ToString ());
 		}
 	}
+
+	[Fact]
+	void GetExceptionHandleAuxVariableTests ()
+	{
+		var expected = "IntPtr exception_gchandle = IntPtr.Zero;";
+		var declaration = GetExceptionHandleAuxVariable ();
+		Assert.Equal (expected, declaration.ToString ());
+	}
 }