GetType: keep throwing same exceptions as .NET 8

src/coreclr/System.Private.CoreLib/src/System/Reflection/TypeNameResolver.CoreCLR.cs

@@ -80,7 +80,7 @@ internal partial struct TypeNameResolver
             bool ignoreCase,
             Assembly topLevelAssembly)
         {
-            TypeName? parsed = TypeNameParser.Parse(typeName, throwOnError);
+            TypeName? parsed = TypeNameParser.Parse(typeName, throwOnError, new() { TopLevelAssemblyWasProvided = topLevelAssembly is not null });
 
             if (parsed is null)
             {

src/libraries/System.Private.CoreLib/src/System/Reflection/TypeNameResolver.cs

@@ -20,6 +20,8 @@ public int MaxNodes
             }
         }
 #pragma warning restore CA1822
+
+        internal bool TopLevelAssemblyWasProvided { get; set; }
     }
 }
 

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/TypeNameParser.cs

@@ -188,14 +188,22 @@ private TypeNameParser(ReadOnlySpan<char> name, bool throwOnError, TypeNameParse
             if (allowFullyQualifiedName && !TryParseAssemblyName(ref assemblyName))
             {
 #if SYSTEM_PRIVATE_CORELIB
-                // backward compat: throw FileLoadException for non-empty invalid strings
-                if (_throwOnError || !_inputString.TrimStart().StartsWith(","))
+                // Backward compatibility: throw for non-empty invalid assembly names.
+                if (!_inputString.TrimStart().StartsWith(","))
                 {
-                    throw new IO.FileLoadException(SR.InvalidAssemblyName, _inputString.ToString());
+                    // No matter what throwOnError is set to, if top level assembly was not provided, CLR throws FileLoadException for invalid assembly names.
+                    if (!_parseOptions.TopLevelAssemblyWasProvided)
+                    {
+                        throw new IO.FileLoadException(SR.InvalidAssemblyName, _inputString.ToString());
+                    }
+                    // If top level was provided and user has requested for error, we throw ArgumentException.
+                    else if (_throwOnError)
+                    {
+                        throw new ArgumentException(SR.Argument_AssemblyGetTypeCannotSpecifyAssembly);
+                    }
                 }
-#else
-                return null;
 #endif
+                return null;
             }
 
             // No matter what was parsed, the full name string is allocated only once.

src/libraries/System.Runtime/tests/System.Reflection.Tests/GetTypeTests.cs

@@ -276,6 +276,8 @@ public void GetType_CoreAssembly()
         {
             Assert.Equal(typeof(int), Type.GetType("System.Int32", throwOnError: true));
             Assert.Equal(typeof(int), Type.GetType("system.int32", throwOnError: true, ignoreCase: true));
+            Assert.Null(typeof(int).Assembly.GetType("a,b", throwOnError: false, ignoreCase: false));
+            Assert.Null(typeof(int).Assembly.GetType("a,b,c", throwOnError: false, ignoreCase: false));
         }
 
         [Fact]
@@ -294,7 +296,15 @@ public void GetType_InvalidAssemblyName()
             Assert.Null(Type.GetType("MissingAssemblyName, "));
             Assert.Null(Type.GetType("ExtraComma, ,"));
             Assert.Null(Type.GetType("ExtraComma, , System.Runtime"));
-            Assert.Throws<FileLoadException>(() => Type.GetType("System.Object, System.Runtime, Version=x.y"));
+
+            // Backward compat: no matter what throwOnError is set to, CLR throws FileLoadException for invalid assembly names.
+            Assert.Throws<FileLoadException>(() => Type.GetType("System.Object, System.Runtime, Version=x.y", throwOnError: false));
+            Assert.Throws<FileLoadException>(() => Type.GetType("System.Object, System.Runtime, Version=x.y", throwOnError: true));
+
+            // Backward compat: when using Assembly.GetType, throwOnError is not ignored and ArgumentException can be thrown.
+            Assembly coreLib = typeof(int).Assembly;
+            Assert.Null(coreLib.GetType("System.Object, System.Runtime, Version=x.y", throwOnError: false));
+            Assert.Throws<ArgumentException>(() => coreLib.GetType("System.Object, System.Runtime, Version=x.y", throwOnError: true));
         }
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsTypeEquivalenceSupported))]