Write item metadata the way a non-SDK project keeps it

Metadata as attributes on an item element is MSBuild 15 syntax. The
non-SDK project format is what an export falls back to for toolchains
that predate the SDK, and those reject an unknown attribute on an item
element, so a Page item carrying Generator and SubType as attributes
undoes the reason to write that format at all. Every non-SDK project
written by anything else keeps metadata in child elements.

The SDK-style writer keeps attributes: there the syntax is a given and
it is what the format's own tooling produces.

Assisted-by: Claude:claude-opus-5:Claude Code

ICSharpCode.Decompiler.Tests/ProjectDecompiler/ProjectFileWriterDefaultTests.cs

@@ -0,0 +1,86 @@
+// Copyright (c) 2026 Siegfried Pammer
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+using System.IO;
+
+using ICSharpCode.Decompiler.CSharp;
+using ICSharpCode.Decompiler.CSharp.ProjectDecompiler;
+using ICSharpCode.Decompiler.Metadata;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.Decompiler.Tests.ProjectDecompiler;
+
+[TestFixture]
+public sealed class ProjectFileWriterDefaultTests
+{
+	/// <summary>
+	/// Item metadata as attributes is MSBuild 15 syntax. The non-SDK format exists for the
+	/// toolchains that came before it, and those reject an unknown attribute on an item element,
+	/// so metadata has to be written the way every non-SDK project writes it: as child elements.
+	/// </summary>
+	[Test]
+	public void ItemMetadataIsWrittenAsChildElements()
+	{
+		ProjectItemInfo[] files = [
+			new ProjectItemInfo("Page", "Themes/Generic.xaml")
+				.With("Generator", "MSBuild:Compile")
+				.With("SubType", "Designer"),
+		];
+
+		string project = WriteProjectFile(files);
+
+		using (Assert.EnterMultipleScope())
+		{
+			Assert.That(project, Does.Contain(@"<Page Include=""Themes/Generic.xaml"">"), project);
+			Assert.That(project, Does.Contain(@"<Generator>MSBuild:Compile</Generator>"), project);
+			Assert.That(project, Does.Contain(@"<SubType>Designer</SubType>"), project);
+			Assert.That(project, Does.Not.Contain(@"Generator="""), "metadata does not belong in an attribute");
+		}
+	}
+
+	[Test]
+	public void AnItemWithoutMetadataStaysOnOneLine()
+	{
+		ProjectItemInfo[] files = [new ProjectItemInfo("Compile", "Program.cs")];
+
+		string project = WriteProjectFile(files);
+
+		Assert.That(project, Does.Contain(@"<Compile Include=""Program.cs"" />"), project);
+	}
+
+	static string WriteProjectFile(ProjectItemInfo[] files)
+	{
+		StringWriter output = new();
+		ProjectFileWriterDefault.Instance.Write(output, new TestProjectInfoProvider(), files,
+			new PEFile("ICSharpCode.Decompiler.dll"));
+		return output.ToString();
+	}
+
+	sealed class TestProjectInfoProvider : IProjectInfoProvider
+	{
+		public IAssemblyResolver AssemblyResolver { get; } = new UniversalAssemblyResolver(null, false, null);
+		public IAssemblyReferenceClassifier AssemblyReferenceClassifier { get; } = new AssemblyReferenceClassifier();
+		public LanguageVersion LanguageVersion => LanguageVersion.Latest;
+		public bool CheckForOverflowUnderflow => false;
+		public Guid ProjectGuid { get; } = Guid.NewGuid();
+		public string TargetDirectory { get; } = Environment.CurrentDirectory;
+		public string StrongNameKeyFile => null;
+	}
+}

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterDefault.cs

@@ -173,8 +173,11 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 						w.WriteAttributeString("Include", item.FileName);
 						if (item.AdditionalProperties != null)
 						{
+							// Item metadata as attributes is MSBuild 15 syntax. This format is what an
+							// export targets when the toolchain predates that, so the metadata goes where
+							// every non-SDK project keeps it: in child elements.
 							foreach (var (key, value) in item.AdditionalProperties)
-								w.WriteAttributeString(key, value);
+								w.WriteElementString(key, value);
 						}
 						w.WriteEndElement();
 					}