Add TypeName APIs to simplify metadata lookup.

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

@@ -231,15 +231,15 @@ internal static RuntimeType GetTypeReferencedByCustomAttribute(string typeName,
                         }
                         return null;
                     }
-                    return GetTypeFromDefaultAssemblies(TypeNameHelpers.Unescape(escapedTypeName), nestedTypeNames, parsedName);
+                    return GetTypeFromDefaultAssemblies(TypeName.Unescape(escapedTypeName), nestedTypeNames, parsedName);
                 }
 
                 if (assembly is RuntimeAssembly runtimeAssembly)
                 {
                     // Compat: Non-extensible parser allows ambiguous matches with ignore case lookup
                     bool useReflectionForNestedTypes = _extensibleParser && _ignoreCase;
 
-                    type = runtimeAssembly.GetTypeCore(TypeNameHelpers.Unescape(escapedTypeName), useReflectionForNestedTypes ? default : nestedTypeNames,
+                    type = runtimeAssembly.GetTypeCore(TypeName.Unescape(escapedTypeName), useReflectionForNestedTypes ? default : nestedTypeNames,
                         throwOnFileNotFound: _throwOnError, ignoreCase: _ignoreCase);
 
                     if (type is null)
@@ -282,7 +282,7 @@ internal static RuntimeType GetTypeReferencedByCustomAttribute(string typeName,
                     if (_throwOnError)
                     {
                         throw new TypeLoadException(SR.Format(SR.TypeLoad_ResolveNestedType,
-                            nestedTypeNames[i], (i > 0) ? nestedTypeNames[i - 1] : TypeNameHelpers.Unescape(escapedTypeName)),
+                            nestedTypeNames[i], (i > 0) ? nestedTypeNames[i - 1] : TypeName.Unescape(escapedTypeName)),
                             typeName: parsedName.FullName);
                     }
                     return null;

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Reflection/TypeNameResolver.NativeAot.cs

@@ -158,7 +158,7 @@ internal partial struct TypeNameResolver
                 {
                     if (assembly is RuntimeAssemblyInfo runtimeAssembly)
                     {
-                        type = runtimeAssembly.GetTypeCore(TypeNameHelpers.Unescape(escapedTypeName), throwOnError: _throwOnError, ignoreCase: _ignoreCase);
+                        type = runtimeAssembly.GetTypeCore(TypeName.Unescape(escapedTypeName), throwOnError: _throwOnError, ignoreCase: _ignoreCase);
                     }
                     else
                     {
@@ -173,7 +173,7 @@ internal partial struct TypeNameResolver
                 }
                 else
                 {
-                    string? unescapedTypeName = TypeNameHelpers.Unescape(escapedTypeName);
+                    string? unescapedTypeName = TypeName.Unescape(escapedTypeName);
 
                     RuntimeAssemblyInfo? defaultAssembly = null;
                     if (_defaultAssemblyName != null)
@@ -235,7 +235,7 @@ internal partial struct TypeNameResolver
                     if (_throwOnError)
                     {
                         throw new TypeLoadException(SR.Format(SR.TypeLoad_ResolveNestedType,
-                            nestedTypeNames[i], (i > 0) ? nestedTypeNames[i - 1] : TypeNameHelpers.Unescape(escapedTypeName)),
+                            nestedTypeNames[i], (i > 0) ? nestedTypeNames[i - 1] : TypeName.Unescape(escapedTypeName)),
                             typeName: parsedName.FullName);
                     }
                     return null;

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -1489,9 +1489,6 @@
     <Compile Include="$(LibrariesProjectRoot)System.Reflection.Metadata\src\System\Reflection\Metadata\TypeName.cs">
       <Link>Common\System\Reflection\Metadata\TypeName.cs</Link>
     </Compile>
-    <Compile Include="$(CommonPath)System\Reflection\Metadata\TypeNameHelpers.cs">
-      <Link>Common\System\Reflection\Metadata\TypeNameHelpers.cs</Link>
-    </Compile>
     <Compile Include="$(LibrariesProjectRoot)System.Reflection.Metadata\src\System\Reflection\Metadata\TypeNameParser.cs">
       <Link>Common\System\Reflection\Metadata\TypeNameParser.cs</Link>
     </Compile>

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

@@ -80,7 +80,7 @@ internal partial struct TypeNameResolver
                 current = typeName;
                 while (current.IsNested)
                 {
-                    nestedTypeNames[--nestingDepth] = TypeNameHelpers.Unescape(current.Name);
+                    nestedTypeNames[--nestingDepth] = TypeName.Unescape(current.Name);
                     current = current.DeclaringType!;
                 }
 

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

@@ -2437,6 +2437,7 @@ internal TypeName() { }
         public bool IsSZArray { get { throw null; } }
         public bool IsVariableBoundArrayType { get { throw null; } }
         public string Name { get { throw null; } }
+        public string Namespace { get { throw null; } }
         public int GetArrayRank() { throw null; }
         public System.Reflection.Metadata.TypeName GetElementType() { throw null; }
         public System.Collections.Immutable.ImmutableArray<System.Reflection.Metadata.TypeName> GetGenericArguments() { throw null; }
@@ -2449,6 +2450,7 @@ internal TypeName() { }
         public System.Reflection.Metadata.TypeName MakeSZArrayTypeName() { throw null; }
         public static System.Reflection.Metadata.TypeName Parse(System.ReadOnlySpan<char> typeName, System.Reflection.Metadata.TypeNameParseOptions? options = null) { throw null; }
         public static bool TryParse(System.ReadOnlySpan<char> typeName, [System.Diagnostics.CodeAnalysis.NotNullWhenAttribute(true)] out System.Reflection.Metadata.TypeName? result, System.Reflection.Metadata.TypeNameParseOptions? options = null) { throw null; }
+        public static string Unescape(string name) { throw null; }
         public System.Reflection.Metadata.TypeName WithAssemblyName(System.Reflection.Metadata.AssemblyNameInfo? assemblyName) { throw null; }
     }
     public sealed partial class TypeNameParseOptions

src/libraries/System.Reflection.Metadata/src/Resources/Strings.resx

@@ -429,6 +429,9 @@
   <data name="InvalidOperation_NoElement" xml:space="preserve">
     <value>This operation is only valid on arrays, pointers and references.</value>
   </data>
+  <data name="InvalidOperation_NestedTypeNamespace" xml:space="preserve">
+    <value>Cannot retrieve the namespace of a nested type.</value>
+  </data>
   <data name="InvalidAssemblyName" xml:space="preserve">
     <value>The given assembly name was invalid.</value>
   </data>

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

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
@@ -39,7 +39,7 @@ sealed class TypeName
 #else
         private readonly ImmutableArray<TypeName> _genericArguments;
 #endif
-        private string? _name, _fullName, _assemblyQualifiedName;
+        private string? _name, _namespace, _fullName, _assemblyQualifiedName;
 
         internal TypeName(string? fullName,
             AssemblyNameInfo? assemblyName,
@@ -217,6 +217,7 @@ public string FullName
         /// This is because determining whether a type truly is a generic type requires loading the type
         /// and performing a runtime check.</para>
         /// </remarks>
+        [MemberNotNullWhen(false, nameof(_elementOrGenericType))]
         public bool IsSimple => _elementOrGenericType is null;
 
         /// <summary>
@@ -229,6 +230,7 @@ public string FullName
         /// Returns true if this is a nested type (e.g., "Namespace.Declaring+Nested").
         /// For nested types <seealso cref="DeclaringType"/> returns their declaring type.
         /// </summary>
+        [MemberNotNullWhen(true, nameof(_declaringType))]
         public bool IsNested => _declaringType is not null;
 
         /// <summary>
@@ -262,28 +264,82 @@ public string Name
                 {
                     if (IsConstructedGenericType)
                     {
-                        _name = TypeNameParserHelpers.GetName(GetGenericTypeDefinition().FullName.AsSpan()).ToString();
+                        _name = GetGenericTypeDefinition().Name;
                     }
                     else if (IsPointer || IsByRef || IsArray)
                     {
                         ValueStringBuilder builder = new(stackalloc char[64]);
                         builder.Append(GetElementType().Name);
                         _name = TypeNameParserHelpers.GetRankOrModifierStringRepresentation(_rankOrModifier, ref builder);
                     }
-                    else if (_nestedNameLength > 0 && _fullName is not null)
-                    {
-                        _name = TypeNameParserHelpers.GetName(_fullName.AsSpan(0, _nestedNameLength)).ToString();
-                    }
                     else
                     {
-                        _name = TypeNameParserHelpers.GetName(FullName.AsSpan()).ToString();
+                        // _fullName can be null only in constructed generic or modified types, which we handled above.
+                        Debug.Assert(_fullName is not null);
+                        ReadOnlySpan<char> name = _fullName.AsSpan();
+                        if (_nestedNameLength > 0)
+                        {
+                            name = name.Slice(0, _nestedNameLength);
+                        }
+                        if (IsNested)
+                        {
+                            // If the type is nested, we know the length of the declaring type's full name.
+                            // Get the characters after that plus one for the '+' separator.
+                            name = name.Slice(_declaringType._nestedNameLength + 1);
+                        }
+                        else if (name.LastIndexOf('.') is int dotIndex && dotIndex >= 0)
+                        {
+                            // If the type is not nested, find the last dot in the full name and and return the substring after it.
+                            name = name.Slice(dotIndex + 1);
+                        }
+                        _name = name.ToString();
                     }
                 }
 
                 return _name;
             }
         }
 
+        /// <summary>
+        /// The namespace of this type; e.g., "System".
+        /// </summary>
+        /// <exception cref="InvalidOperationException">This instance is a nested type.</exception>
+        public string Namespace
+        {
+            get
+            {
+                if (IsNested)
+                {
+                    TypeNameParserHelpers.ThrowInvalidOperation_NestedTypeNamespace();
+                }
+
+                if (_namespace is null)
+                {
+                    TypeName rootTypeName = this;
+                    while (!rootTypeName.IsSimple)
+                    {
+                        rootTypeName = rootTypeName._elementOrGenericType;
+                    }
+
+                    // By setting the namespace field at the root type name, we avoid recomputing it for all derived names.
+                    if (rootTypeName._namespace is null)
+                    {
+                        // At this point the type does not have a modifier applied to it, so it should have its full name set.
+                        Debug.Assert(rootTypeName._fullName is not null);
+                        ReadOnlySpan<char> rootFullName = rootTypeName._fullName.AsSpan();
+                        if (rootTypeName._nestedNameLength > 0)
+                        {
+                            rootFullName = rootFullName.Slice(0, rootTypeName._nestedNameLength);
+                        }
+                        rootTypeName._namespace = TypeNameParserHelpers.GetNamespace(rootFullName).ToString();
+                    }
+                    _namespace = rootTypeName._namespace;
+                }
+
+                return _namespace;
+            }
+        }
+
         /// <summary>
         /// Represents the total number of <see cref="TypeName"/> instances that are used to describe
         /// this instance, including any generic arguments or underlying types.
@@ -401,6 +457,25 @@ public static bool TryParse(ReadOnlySpan<char> typeName, [NotNullWhen(true)] out
             return result is not null;
         }
 
+        /// <summary>
+        /// Converts any escaped characters in the input type name or namespace.
+        /// </summary>
+        /// <param name="name">The input string containing the name to convert.</param>
+        /// <returns>A string of characters with any escaped characters converted to their unescaped form.</returns>
+        /// <remarks>
+        /// <para>The unescaped string can be used for looking up the type name or namespace in metadata.</para>
+        /// <para>This method removes escape characters even if they precede a character that does not require escaping.</para>
+        /// </remarks>
+        public static string Unescape(string name)
+        {
+            if (name is null)
+            {
+                TypeNameParserHelpers.ThrowArgumentNullException(nameof(name));
+            }
+
+            return TypeNameParserHelpers.Unescape(name);
+        }
+
         /// <summary>
         /// Gets the number of dimensions in an array.
         /// </summary>

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

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Buffers;
@@ -102,36 +102,48 @@ static int GetUnescapedOffset(ReadOnlySpan<char> input, int startOffset)
             static bool NeedsEscaping(char c) => c is '[' or ']' or '&' or '*' or ',' or '+' or EscapeCharacter;
         }
 
-        internal static ReadOnlySpan<char> GetName(ReadOnlySpan<char> fullName)
+        internal static ReadOnlySpan<char> GetNamespace(ReadOnlySpan<char> fullName)
         {
-            // The two-value form of MemoryExtensions.LastIndexOfAny does not suffer
-            // from the behavior mentioned in the comment at the top of GetFullTypeNameLength.
-            // It always takes O(m * i) worst-case time and is safe to use here.
-
-            int offset = fullName.LastIndexOfAny('.', '+');
+            int offset = fullName.LastIndexOf('.');
+            return offset < 0 ? [] : fullName.Slice(0, offset);
+        }
 
-            if (offset > 0 && fullName[offset - 1] == EscapeCharacter) // this should be very rare (IL Emit & pure IL)
+        internal static string Unescape(string input)
+        {
+            int indexOfEscapeCharacter = input.IndexOf(EscapeCharacter);
+            if (indexOfEscapeCharacter < 0)
             {
-                offset = GetUnescapedOffset(fullName, startIndex: offset);
+                // Nothing to escape, just return the original value.
+                return input;
             }
 
-            return offset < 0 ? fullName : fullName.Slice(offset + 1);
+            return UnescapeToBuilder(input, indexOfEscapeCharacter);
 
-            static int GetUnescapedOffset(ReadOnlySpan<char> fullName, int startIndex)
+            static string UnescapeToBuilder(string name, int indexOfEscapeCharacter)
             {
-                int offset = startIndex;
-                for (; offset >= 0; offset--)
+                // this code path is executed very rarely (IL Emit or pure IL with chars not allowed in C# or F#)
+                var sb = new ValueStringBuilder(stackalloc char[64]);
+                sb.EnsureCapacity(name.Length);
+                sb.Append(name.AsSpan(0, indexOfEscapeCharacter));
+
+                for (int i = indexOfEscapeCharacter; i < name.Length;)
                 {
-                    if (fullName[offset] is '.' or '+')
+                    char c = name[i++];
+
+                    if (c != EscapeCharacter || i == name.Length)
                     {
-                        if (offset == 0 || fullName[offset - 1] != EscapeCharacter)
-                        {
-                            break;
-                        }
-                        offset--; // skip the escaping character
+                        sb.Append(c);
+                    }
+                    else if (name[i] == EscapeCharacter) // escaped escape character ;)
+                    {
+                        sb.Append(c);
+                        // Consume the escaped escape character, it's important for edge cases
+                        // like escaped escape character followed by another escaped char (example: "\\\\\\+")
+                        i++;
                     }
                 }
-                return offset;
+
+                return sb.ToString();
             }
         }
 
@@ -350,6 +362,12 @@ internal static bool TryStripFirstCharAndTrailingSpaces(ref ReadOnlySpan<char> s
             return false;
         }
 
+        [DoesNotReturn]
+        internal static void ThrowArgumentNullException(string paramName)
+        {
+            throw new ArgumentNullException(paramName);
+        }
+
         [DoesNotReturn]
         internal static void ThrowArgumentException_InvalidTypeName(int errorIndex)
         {
@@ -411,6 +429,17 @@ internal static void ThrowInvalidOperation_HasToBeArrayClass()
 #endif
         }
 
+        [DoesNotReturn]
+        internal static void ThrowInvalidOperation_NestedTypeNamespace()
+        {
+#if SYSTEM_REFLECTION_METADATA
+            throw new InvalidOperationException(SR.InvalidOperation_NestedTypeNamespace);
+#else
+            Debug.Fail("Expected to be unreachable");
+            throw new InvalidOperationException();
+#endif
+        }
+
         internal static bool IsMaxDepthExceeded(TypeNameParseOptions options, int depth)
 #if SYSTEM_PRIVATE_CORELIB
             => false; // CoreLib does not enforce any limits

src/libraries/System.Reflection.Metadata/tests/Metadata/TypeNameParserHelpersTests.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
@@ -38,16 +38,12 @@ public void GetFullTypeNameLengthReturnsExpectedValue(string input, int expected
         }
 
         [Theory]
-        [InlineData("JustTypeName", "JustTypeName")]
-        [InlineData("Namespace.TypeName", "TypeName")]
-        [InlineData("Namespace1.Namespace2.TypeName", "TypeName")]
-        [InlineData("Namespace.NotNamespace\\.TypeName", "NotNamespace\\.TypeName")]
-        [InlineData("Namespace1.Namespace2.Containing+Nested", "Nested")]
-        [InlineData("Namespace1.Namespace2.Not\\+Nested", "Not\\+Nested")]
-        [InlineData("NotNamespace1\\.NotNamespace2\\.TypeName", "NotNamespace1\\.NotNamespace2\\.TypeName")]
-        [InlineData("NotNamespace1\\.NotNamespace2\\.Not\\+Nested", "NotNamespace1\\.NotNamespace2\\.Not\\+Nested")]
-        public void GetNameReturnsJustName(string fullName, string expected)
-            => Assert.Equal(expected, TypeNameParserHelpers.GetName(fullName.AsSpan()).ToString());
+        [InlineData("JustTypeName", "")]
+        [InlineData("Namespace.TypeName", "Namespace")]
+        [InlineData("Namespace1.Namespace2.TypeName", "Namespace1.Namespace2")]
+        [InlineData("Namespace1.Namespace2.Not\\+Nested", "Namespace1.Namespace2")]
+        public void GetNamespaceReturnsJustNamespace(string fullName, string expected)
+            => Assert.Equal(expected, TypeNameParserHelpers.GetNamespace(fullName.AsSpan()).ToString());
 
         [Theory]
         [InlineData("simple", "simple")]