Fix source generator failing with @ verbatim identifier prefix in parameter type names

Agent-Logs-Url: https://github.com/EFNext/EntityFrameworkCore.Projectables/sessions/c8748592-1460-4fb8-bd4d-b8eb20d15ce3

Co-authored-by: PhenX <42170+PhenX@users.noreply.github.com>

Author: Copilot

src/EntityFrameworkCore.Projectables/Services/ProjectionExpressionClassNameGenerator.cs

@@ -140,6 +140,16 @@ private static void AppendSanitizedTypeName(StringBuilder sb, string typeName)
                     continue;
                 }
 
+                // Skip the verbatim identifier prefix '@' — it is a C# syntactic escape for
+                // reserved keywords (e.g. '@event') and has no meaning at the CLR level.
+                // The CLR type name is just 'event', so stripping '@' keeps the generated name
+                // consistent with the runtime resolver's output.
+                if (typeName[i] == '@')
+                {
+                    i++;
+                    continue;
+                }
+
                 var c = typeName[i];
                 sb.Append(IsInvalidIdentifierChar(c) ? '_' : c);
                 i++;

tests/EntityFrameworkCore.Projectables.Generator.Tests/ExtensionMethodTests.ProjectableExtensionMethod_WithVerbatimKeywordParameterType.verified.txt

@@ -0,0 +1,17 @@
+// <auto-generated/>
+#nullable disable
+using System;
+using EntityFrameworkCore.Projectables;
+using Foo;
+
+namespace EntityFrameworkCore.Projectables.Generated
+{
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    static class Foo_EventExtensions_GetId_P0_Foo_event
+    {
+        static global::System.Linq.Expressions.Expression<global::System.Func<global::Foo.@event, int>> Expression()
+        {
+            return (global::Foo.@event e) => e.Id;
+        }
+    }
+}

tests/EntityFrameworkCore.Projectables.Generator.Tests/ExtensionMethodTests.cs

@@ -105,6 +105,32 @@ static class C {
         return Verifier.Verify(result.GeneratedTrees[0].ToString());
     }
 
+    [Fact]
+    public Task ProjectableExtensionMethod_WithVerbatimKeywordParameterType()
+    {
+        var compilation = CreateCompilation(@"
+using System;
+using EntityFrameworkCore.Projectables;
+namespace Foo {
+    class @event {
+        public int Id { get; set; }
+    }
+    
+    static class EventExtensions {
+        [Projectable]
+        public static int GetId(this @event e) => e.Id;
+    }
+}
+");
+
+        var result = RunGenerator(compilation);
+
+        Assert.Empty(result.Diagnostics);
+        Assert.Single(result.GeneratedTrees);
+
+        return Verifier.Verify(result.GeneratedTrees[0].ToString());
+    }
+
     [Fact]
     public Task ProjectableExtensionMethod_WithExpressionPropertyBody()
     {

tests/EntityFrameworkCore.Projectables.Tests/Services/ProjectionExpressionClassNameGeneratorTests.cs

@@ -50,6 +50,31 @@ public void GenerateName_WithGlobalPrefixInGenericArgs_StripsAllGlobalPrefixes(
             Assert.Equal(expected, result);
         }
 
+        /// <summary>
+        /// Verifies that the verbatim identifier prefix <c>@</c> is stripped from parameter type
+        /// names. Roslyn's <c>FullyQualifiedFormat</c> includes it for types whose names are
+        /// reserved C# keywords (e.g. <c>@event</c>), but the CLR runtime name never includes
+        /// <c>@</c> — so both sides must agree on the sanitised name.
+        /// </summary>
+        [Theory]
+        [InlineData(
+            "global::Foo.Storage.@event",
+            "ns_a_m_P0_Foo_Storage_event")]
+        [InlineData(
+            "@event",
+            "ns_a_m_P0_event")]
+        [InlineData(
+            "global::Foo.@delegate",
+            "ns_a_m_P0_Foo_delegate")]
+        public void GenerateName_WithVerbatimAtPrefixInParamType_StripsAtSign(
+            string paramTypeName, string expected)
+        {
+            var result = ProjectionExpressionClassNameGenerator.GenerateName(
+                "ns", new[] { "a" }, "m", new[] { paramTypeName });
+
+            Assert.Equal(expected, result);
+        }
+
         [Fact]
         public void GeneratedFullName()
         {