Better error handling for name conflicts

readme.md

@@ -402,6 +402,36 @@ string envVars = File.ReadAllText(ProjectFiles._env);
 - `.editorconfig` → `_editorconfig`
 - `.txt` → `_txt`
 
+**Important:** Avoid naming conflicts. The generator detects and reports duplicate property names:
+
+**Example 1: Dot file conflict**
+```xml
+<!-- ❌ ERROR: Both generate property '_txt' -->
+<ItemGroup>
+  <None Update=".txt">
+    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+  </None>
+  <None Update="_txt">
+    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+  </None>
+</ItemGroup>
+```
+
+**Example 2: Dots vs underscores conflict**
+```xml
+<!-- ❌ ERROR: Both generate property 'config_json' -->
+<ItemGroup>
+  <None Update="config.json">
+    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+  </None>
+  <None Update="config_json">
+    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+  </None>
+</ItemGroup>
+```
+
+Error: `PROJFILES004: Files 'config.json' and 'config_json' both generate the same property name 'config_json'. Rename one of the files to avoid the conflict.`
+
 
 ## Glob Pattern Support
 

src/ProjectFiles/Diagnostics.cs

@@ -24,4 +24,12 @@ public static class Diagnostics
         category: "ProjectFiles",
         DiagnosticSeverity.Error,
         isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor DuplicatePropertyName = new(
+        id: "PROJFILES004",
+        title: "Duplicate property name generated",
+        messageFormat: "Files '{0}' and '{1}' both generate the same property name '{2}'. Rename one of the files to avoid the conflict.",
+        category: "ProjectFiles",
+        DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
 }

src/ProjectFiles/Generator.cs

@@ -98,6 +98,21 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                     context.ReportDiagnostic(diagnostic);
                 }
 
+                // Check for duplicate property names
+                var duplicateConflicts = FindDuplicatePropertyNames(fileList);
+                foreach (var (file1, file2, propertyName) in duplicateConflicts)
+                {
+                    conflictingFiles.Add(file1);
+                    conflictingFiles.Add(file2);
+                    var diagnostic = Diagnostic.Create(
+                        Diagnostics.DuplicatePropertyName,
+                        Location.None,
+                        file1,
+                        file2,
+                        propertyName);
+                    context.ReportDiagnostic(diagnostic);
+                }
+
                 // Filter out conflicting files before generating source
                 var filteredFiles = fileList.Where(_ => !conflictingFiles.Contains(_)).ToImmutableArray();
 
@@ -152,6 +167,48 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         return conflicts;
     }
 
+    static List<(string File1, string File2, string PropertyName)> FindDuplicatePropertyNames(ImmutableArray<string> files)
+    {
+        var conflicts = new List<(string, string, string)>();
+
+        // Group files by their directory (same scope)
+        var filesByDirectory = new Dictionary<string, List<string>>();
+
+        foreach (var file in files)
+        {
+            var directory = Path.GetDirectoryName(file) ?? string.Empty;
+            if (!filesByDirectory.TryGetValue(directory, out var filesInDir))
+            {
+                filesInDir = [];
+                filesByDirectory[directory] = filesInDir;
+            }
+
+            filesInDir.Add(file);
+        }
+
+        // Check for duplicates within each directory
+        foreach (var filesInDir in filesByDirectory.Values)
+        {
+            var propertyToFile = new Dictionary<string, string>();
+
+            foreach (var file in filesInDir)
+            {
+                var propertyName = ToFilePropertyName(file);
+
+                if (propertyToFile.TryGetValue(propertyName, out var existingFile))
+                {
+                    conflicts.Add((existingFile, file, propertyName));
+                }
+                else
+                {
+                    propertyToFile[propertyName] = file;
+                }
+            }
+        }
+
+        return conflicts;
+    }
+
     static string GenerateSource(ImmutableArray<string> files, MsBuildProperties properties, Cancel cancel)
     {
         var (tree, rootFiles) = BuildFileTree(files, cancel);
@@ -417,4 +474,4 @@ static string ToFilePropertyName(string filePath)
 
         return (topLevelDirectories.Values, rootFiles);
     }
-}
+}

src/Tests/GeneratorTest.ConflictBetweenDotFileAndRegularFile#ProjectFiles.ProjectDirectory.g.verified.cs

@@ -0,0 +1,34 @@
+//HintName: ProjectFiles.ProjectDirectory.g.cs
+namespace ProjectFilesGenerator;
+
+using System.IO;
+using System.Collections.Generic;
+
+partial class ProjectDirectory(string path)
+{
+    public string Path { get; } = path;
+
+    public string FullPath => System.IO.Path.GetFullPath(Path);
+
+    public override string ToString() => Path;
+
+    public static implicit operator string(ProjectDirectory temp) =>
+        temp.Path;
+
+    public static implicit operator FileInfo(ProjectDirectory temp) =>
+        new(temp.Path);
+
+    public IEnumerable<string> EnumerateDirectories() =>
+        Directory.EnumerateDirectories(Path);
+
+    public IEnumerable<string> EnumerateFiles() =>
+        Directory.EnumerateFiles(Path);
+
+    public IEnumerable<string> GetFiles() =>
+        Directory.GetFiles(Path);
+
+    public IEnumerable<string> GetDirectories() =>
+        Directory.GetDirectories(Path);
+
+    public DirectoryInfo Info => new(Path);
+}

src/Tests/GeneratorTest.ConflictBetweenDotFileAndRegularFile#ProjectFiles.ProjectFile.g.verified.cs

@@ -0,0 +1,50 @@
+//HintName: ProjectFiles.ProjectFile.g.cs
+namespace ProjectFilesGenerator;
+
+using System.IO;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+partial class ProjectFile(string path)
+{
+    public string Path { get; } = path;
+
+    public string FullPath => System.IO.Path.GetFullPath(Path);
+
+    public override string ToString() => Path;
+
+    public static implicit operator string(ProjectFile temp) =>
+        temp.Path;
+
+    public static implicit operator FileInfo(ProjectFile temp) =>
+        new(temp.Path);
+
+    public FileStream OpenRead() =>
+        File.OpenRead(Path);
+
+    public StreamReader OpenText() =>
+        File.OpenText(Path);
+
+    public string ReadAllText() =>
+        File.ReadAllText(Path);
+
+    public string ReadAllText(Encoding encoding) =>
+        File.ReadAllText(Path, encoding);
+
+    public FileInfo Info => new(Path);
+
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_0_OR_GREATER
+    public Task<string> ReadAllTextAsync(CancellationToken cancel = default) =>
+        File.ReadAllTextAsync(Path, cancel);
+
+    public Task<string> ReadAllTextAsync(Encoding encoding, CancellationToken cancel = default) =>
+        File.ReadAllTextAsync(Path, encoding,cancel);
+#else
+    public Task<string> ReadAllTextAsync(CancellationToken cancel = default) =>
+        Task.FromResult(File.ReadAllText(Path));
+
+    public Task<string> ReadAllTextAsync(Encoding encoding, CancellationToken cancel = default) =>
+        Task.FromResult(File.ReadAllText(Path, encoding));
+#endif
+}

src/Tests/GeneratorTest.ConflictBetweenDotFileAndRegularFile#ProjectFiles.g.verified.cs

@@ -0,0 +1,17 @@
+//HintName: ProjectFiles.g.cs
+// <auto-generated/>
+#nullable enable
+
+namespace ProjectFilesGenerator
+{
+    using ProjectFilesGenerator.Types;
+
+    /// <summary>Provides strongly-typed access to project files marked with CopyToOutputDirectory.</summary>
+    static partial class ProjectFiles
+    {
+    }
+}
+
+namespace ProjectFilesGenerator.Types
+{
+}

src/Tests/GeneratorTest.ConflictBetweenDotFileAndRegularFile.verified.txt

@@ -0,0 +1,16 @@
+{
+  Diagnostics: [
+    {
+      Message: Files '.txt' and '_txt' both generate the same property name '_txt'. Rename one of the files to avoid the conflict.,
+      Severity: Error,
+      Descriptor: {
+        Id: PROJFILES004,
+        Title: Duplicate property name generated,
+        MessageFormat: Files '{0}' and '{1}' both generate the same property name '{2}'. Rename one of the files to avoid the conflict.,
+        Category: ProjectFiles,
+        DefaultSeverity: Error,
+        IsEnabledByDefault: true
+      }
+    }
+  ]
+}

src/Tests/GeneratorTest.ConflictBetweenFileWithDotsAndUnderscores#ProjectFiles.ProjectDirectory.g.verified.cs

@@ -0,0 +1,34 @@
+//HintName: ProjectFiles.ProjectDirectory.g.cs
+namespace ProjectFilesGenerator;
+
+using System.IO;
+using System.Collections.Generic;
+
+partial class ProjectDirectory(string path)
+{
+    public string Path { get; } = path;
+
+    public string FullPath => System.IO.Path.GetFullPath(Path);
+
+    public override string ToString() => Path;
+
+    public static implicit operator string(ProjectDirectory temp) =>
+        temp.Path;
+
+    public static implicit operator FileInfo(ProjectDirectory temp) =>
+        new(temp.Path);
+
+    public IEnumerable<string> EnumerateDirectories() =>
+        Directory.EnumerateDirectories(Path);
+
+    public IEnumerable<string> EnumerateFiles() =>
+        Directory.EnumerateFiles(Path);
+
+    public IEnumerable<string> GetFiles() =>
+        Directory.GetFiles(Path);
+
+    public IEnumerable<string> GetDirectories() =>
+        Directory.GetDirectories(Path);
+
+    public DirectoryInfo Info => new(Path);
+}

src/Tests/GeneratorTest.ConflictBetweenFileWithDotsAndUnderscores#ProjectFiles.ProjectFile.g.verified.cs

@@ -0,0 +1,50 @@
+//HintName: ProjectFiles.ProjectFile.g.cs
+namespace ProjectFilesGenerator;
+
+using System.IO;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+partial class ProjectFile(string path)
+{
+    public string Path { get; } = path;
+
+    public string FullPath => System.IO.Path.GetFullPath(Path);
+
+    public override string ToString() => Path;
+
+    public static implicit operator string(ProjectFile temp) =>
+        temp.Path;
+
+    public static implicit operator FileInfo(ProjectFile temp) =>
+        new(temp.Path);
+
+    public FileStream OpenRead() =>
+        File.OpenRead(Path);
+
+    public StreamReader OpenText() =>
+        File.OpenText(Path);
+
+    public string ReadAllText() =>
+        File.ReadAllText(Path);
+
+    public string ReadAllText(Encoding encoding) =>
+        File.ReadAllText(Path, encoding);
+
+    public FileInfo Info => new(Path);
+
+#if NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_0_OR_GREATER
+    public Task<string> ReadAllTextAsync(CancellationToken cancel = default) =>
+        File.ReadAllTextAsync(Path, cancel);
+
+    public Task<string> ReadAllTextAsync(Encoding encoding, CancellationToken cancel = default) =>
+        File.ReadAllTextAsync(Path, encoding,cancel);
+#else
+    public Task<string> ReadAllTextAsync(CancellationToken cancel = default) =>
+        Task.FromResult(File.ReadAllText(Path));
+
+    public Task<string> ReadAllTextAsync(Encoding encoding, CancellationToken cancel = default) =>
+        Task.FromResult(File.ReadAllText(Path, encoding));
+#endif
+}

src/Tests/GeneratorTest.ConflictBetweenFileWithDotsAndUnderscores#ProjectFiles.g.verified.cs

@@ -0,0 +1,17 @@
+//HintName: ProjectFiles.g.cs
+// <auto-generated/>
+#nullable enable
+
+namespace ProjectFilesGenerator
+{
+    using ProjectFilesGenerator.Types;
+
+    /// <summary>Provides strongly-typed access to project files marked with CopyToOutputDirectory.</summary>
+    static partial class ProjectFiles
+    {
+    }
+}
+
+namespace ProjectFilesGenerator.Types
+{
+}

src/Tests/GeneratorTest.ConflictBetweenFileWithDotsAndUnderscores.verified.txt

@@ -0,0 +1,16 @@
+{
+  Diagnostics: [
+    {
+      Message: Files 'config.json' and 'config_json' both generate the same property name 'config_json'. Rename one of the files to avoid the conflict.,
+      Severity: Error,
+      Descriptor: {
+        Id: PROJFILES004,
+        Title: Duplicate property name generated,
+        MessageFormat: Files '{0}' and '{1}' both generate the same property name '{2}'. Rename one of the files to avoid the conflict.,
+        Category: ProjectFiles,
+        DefaultSeverity: Error,
+        IsEnabledByDefault: true
+      }
+    }
+  ]
+}