TypeName.FullName: reduce allocations

src/libraries/Common/src/System/Reflection/AssemblyNameFormatter.cs

@@ -11,11 +11,17 @@ namespace System.Reflection
     internal static class AssemblyNameFormatter
     {
         public static string ComputeDisplayName(string name, Version? version, string? cultureName, byte[]? pkt, AssemblyNameFlags flags = 0, AssemblyContentType contentType = 0, byte[]? pk = null)
+        {
+            ValueStringBuilder vsb = new(stackalloc char[256]);
+            AppendDisplayName(ref vsb, name, version, cultureName, pkt, flags, contentType, pk);
+            return vsb.ToString();
+        }
+
+        public static void AppendDisplayName(ref ValueStringBuilder vsb, string name, Version? version, string? cultureName, byte[]? pkt, AssemblyNameFlags flags = 0, AssemblyContentType contentType = 0, byte[]? pk = null)
         {
             const int PUBLIC_KEY_TOKEN_LEN = 8;
             Debug.Assert(name.Length != 0);
 
-            var vsb = new ValueStringBuilder(stackalloc char[256]);
             vsb.AppendQuoted(name);
 
             if (version != null)
@@ -89,8 +95,6 @@ public static string ComputeDisplayName(string name, Version? version, string? c
                 vsb.Append(", ContentType=WindowsRuntime");
 
             // NOTE: By design (desktop compat) AssemblyName.FullName and ToString() do not include ProcessorArchitecture.
-
-            return vsb.ToString();
         }
 
         private static void AppendQuoted(this ref ValueStringBuilder vsb, string s)

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

@@ -112,23 +112,37 @@ public string FullName
             {
                 if (_fullName is null)
                 {
-                    bool isPublicKey = (Flags & AssemblyNameFlags.PublicKey) != 0;
+                    ValueStringBuilder vsb = new(stackalloc char[256]);
+                    AppendFullName(ref vsb);
+                    _fullName = vsb.ToString();
+                }
 
-                    byte[]? publicKeyOrToken =
+                return _fullName;
+            }
+        }
+
+        internal void AppendFullName(ref ValueStringBuilder vsb)
+        {
+            if (_fullName is not null)
+            {
+                vsb.Append(_fullName);
+            }
+            else
+            {
+                bool isPublicKey = (Flags & AssemblyNameFlags.PublicKey) != 0;
+
+                byte[]? publicKeyOrToken =
 #if SYSTEM_PRIVATE_CORELIB
                     PublicKeyOrToken;
 #elif NET8_0_OR_GREATER
                     !PublicKeyOrToken.IsDefault ? Runtime.InteropServices.ImmutableCollectionsMarshal.AsArray(PublicKeyOrToken) : null;
 #else
                     !PublicKeyOrToken.IsDefault ? PublicKeyOrToken.ToArray() : null;
 #endif
-                    _fullName = AssemblyNameFormatter.ComputeDisplayName(Name, Version, CultureName,
-                        pkt: isPublicKey ? null : publicKeyOrToken,
-                        ExtractAssemblyNameFlags(_flags), ExtractAssemblyContentType(_flags),
-                        pk: isPublicKey ? publicKeyOrToken : null);
-                }
-
-                return _fullName;
+                AssemblyNameFormatter.AppendDisplayName(ref vsb, Name, Version, CultureName,
+                    pkt: isPublicKey ? null : publicKeyOrToken,
+                    ExtractAssemblyNameFlags(_flags), ExtractAssemblyContentType(_flags),
+                    pk: isPublicKey ? publicKeyOrToken : null);
             }
         }
 

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

@@ -95,7 +95,39 @@ private TypeName(string? fullName,
         /// If <see cref="AssemblyName"/> returns null, simply returns <see cref="FullName"/>.
         /// </remarks>
         public string AssemblyQualifiedName
-            => _assemblyQualifiedName ??= AssemblyName is null ? FullName : $"{FullName}, {AssemblyName.FullName}"; // see recursion comments in FullName
+        {
+            get
+            {
+                if (_assemblyQualifiedName is null)
+                {
+                    if (_fullName is not null && AssemblyName is null)
+                    {
+                        // _fullName may carry more information than FullName property, so we need to use FullName property.
+                        _assemblyQualifiedName = FullName;
+                    }
+                    else
+                    {
+                        ValueStringBuilder builder = new(stackalloc char[256]);
+                        AppendFullName(ref builder); // see recursion comments in AppendFullName
+                        if (AssemblyName is not null)
+                        {
+                            builder.Append(", ");
+                            AssemblyName.AppendFullName(ref builder);
+                        }
+                        _assemblyQualifiedName = builder.ToString();
+
+                        if (AssemblyName is null)
+                        {
+                            // If the type name was not created from a assembly-qualified name,
+                            // the FullName and AssemblyQualifiedName are the same.
+                            _fullName = _assemblyQualifiedName;
+                        }
+                    }
+                }
+
+                return _assemblyQualifiedName;
+            }
+        }
 
         /// <summary>
         /// Returns assembly name which contains this type, or null if this <see cref="TypeName"/> was not
@@ -142,38 +174,11 @@ public string FullName
         {
             get
             {
-                // This is a recursive method over potentially hostile input. Protection against DoS is offered
-                // via the [Try]Parse method and TypeNameParserOptions.MaxNodes property at construction time.
-                // This FullName property getter and related methods assume that this TypeName instance has an
-                // acceptable node count.
-                //
-                // The node count controls the total amount of work performed by this method, including:
-                // - The max possible stack depth due to the recursive methods calls; and
-                // - The total number of bytes allocated by this function. For a deeply-nested TypeName
-                //   object, the total allocation across the full object graph will be
-                //   O(FullName.Length * GetNodeCount()).
-
                 if (_fullName is null)
                 {
-                    if (IsConstructedGenericType)
-                    {
-                        _fullName = TypeNameParserHelpers.GetGenericTypeFullName(GetGenericTypeDefinition().FullName.AsSpan(),
-#if SYSTEM_PRIVATE_CORELIB
-                            CollectionsMarshal.AsSpan(_genericArguments));
-#else
-                            _genericArguments.AsSpan());
-#endif
-                    }
-                    else if (IsArray || IsPointer || IsByRef)
-                    {
-                        ValueStringBuilder builder = new(stackalloc char[128]);
-                        builder.Append(GetElementType().FullName);
-                        _fullName = TypeNameParserHelpers.GetRankOrModifierStringRepresentation(_rankOrModifier, ref builder);
-                    }
-                    else
-                    {
-                        Debug.Fail("Pre-allocated full name should have been provided in the ctor");
-                    }
+                    ValueStringBuilder builder = new(stackalloc char[128]);
+                    AppendFullName(ref builder);
+                    _fullName = builder.ToString();
                 }
                 else if (_nestedNameLength > 0 && _fullName.Length > _nestedNameLength) // Declaring types
                 {
@@ -186,6 +191,58 @@ public string FullName
             }
         }
 
+        private void AppendFullName(ref ValueStringBuilder builder)
+        {
+            // This is a recursive method over potentially hostile input. Protection against DoS is offered
+            // via the [Try]Parse method and TypeNameParserOptions.MaxNodes property at construction time.
+            // This FullName property getter and related methods assume that this TypeName instance has an
+            // acceptable node count.
+            //
+            // The node count controls the total amount of work performed by this method, including:
+            // - The max possible stack depth due to the recursive methods calls.
+
+            if (_fullName is null)
+            {
+                if (IsConstructedGenericType)
+                {
+                    GetGenericTypeDefinition().AppendFullName(ref builder);
+                    builder.Append('[');
+                    foreach (TypeName genericArg in GetGenericArguments())
+                    {
+                        builder.Append('[');
+                        genericArg.AppendFullName(ref builder);
+                        // Generic arguments need to be always fully qualified.
+                        if (genericArg.AssemblyName is not null)
+                        {
+                            builder.Append(", ");
+                            genericArg.AssemblyName.AppendFullName(ref builder);
+                        }
+                        builder.Append("],");
+                    }
+                    builder[builder.Length - 1] = ']'; // replace ',' with ']'
+                }
+                else if (IsArray || IsPointer || IsByRef)
+                {
+                    GetElementType().AppendFullName(ref builder);
+                    TypeNameParserHelpers.AppendRankOrModifierStringRepresentation(_rankOrModifier, ref builder);
+                }
+                else
+                {
+                    Debug.Fail("Pre-allocated full name should have been provided in the ctor");
+                }
+            }
+            else if (_nestedNameLength > 0 && _fullName.Length > _nestedNameLength) // Declaring types
+            {
+                // Stored fullName represents the full name of the nested type.
+                // Example: Namespace.Declaring+Nested
+                builder.Append(_fullName.AsSpan(0, _nestedNameLength));
+            }
+            else
+            {
+                builder.Append(_fullName);
+            }
+        }
+
         /// <summary>
         /// Returns true if this type represents any kind of array, regardless of the array's
         /// rank or its bounds.
@@ -258,48 +315,54 @@ public string Name
         {
             get
             {
-                // Lookups to Name and FullName might be recursive. See comments in FullName property getter.
-
                 if (_name is null)
                 {
-                    if (IsConstructedGenericType)
-                    {
-                        _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
-                    {
-                        // _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 (TypeNameParserHelpers.IndexOfNamespaceDelimiter(name) is int idx && idx >= 0)
-                        {
-                            // If the type is not nested, find the namespace delimiter in the full name and return the substring after it.
-                            name = name.Slice(idx + 1);
-                        }
-                        _name = name.ToString();
-                    }
+                    ValueStringBuilder builder = new(stackalloc char[64]);
+                    AppendName(ref builder);
+                    _name = builder.ToString();
                 }
 
                 return _name;
             }
         }
 
+        private void AppendName(ref ValueStringBuilder builder)
+        {
+            // Lookups to Name and FullName might be recursive. See comments in AppendFullName method.
+
+            if (IsConstructedGenericType)
+            {
+                GetGenericTypeDefinition().AppendName(ref builder);
+            }
+            else if (IsPointer || IsByRef || IsArray)
+            {
+                GetElementType().AppendName(ref builder);
+                TypeNameParserHelpers.AppendRankOrModifierStringRepresentation(_rankOrModifier, ref builder);
+            }
+            else
+            {
+                // _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 (TypeNameParserHelpers.IndexOfNamespaceDelimiter(name) is int idx && idx >= 0)
+                {
+                    // If the type is not nested, find the namespace delimiter in the full name and return the substring after it.
+                    name = name.Slice(idx + 1);
+                }
+                builder.Append(name);
+            }
+        }
+
         /// <summary>
         /// The namespace of this type; e.g., "System".
         /// </summary>

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

@@ -19,27 +19,6 @@ internal static class TypeNameParserHelpers
         // Keep this in sync with GetFullTypeNameLength/NeedsEscaping
         private static readonly SearchValues<char> s_endOfFullTypeNameDelimitersSearchValues = SearchValues.Create("[]&*,+\\");
 #endif
-
-        internal static string GetGenericTypeFullName(ReadOnlySpan<char> fullTypeName, ReadOnlySpan<TypeName> genericArgs)
-        {
-            Debug.Assert(genericArgs.Length > 0);
-
-            ValueStringBuilder result = new(stackalloc char[128]);
-            result.Append(fullTypeName);
-
-            result.Append('[');
-            foreach (TypeName genericArg in genericArgs)
-            {
-                result.Append('[');
-                result.Append(genericArg.AssemblyQualifiedName); // see recursion comments in TypeName.FullName
-                result.Append(']');
-                result.Append(',');
-            }
-            result[result.Length - 1] = ']'; // replace ',' with ']'
-
-            return result.ToString();
-        }
-
         /// <returns>Positive length or negative value for invalid name</returns>
         internal static int GetFullTypeNameLength(ReadOnlySpan<char> input, out bool isNestedType)
         {
@@ -186,7 +165,7 @@ static int GetUnescapedOffset(ReadOnlySpan<char> input, int startIndex)
             }
         }
 
-        internal static string GetRankOrModifierStringRepresentation(int rankOrModifier, ref ValueStringBuilder builder)
+        internal static void AppendRankOrModifierStringRepresentation(int rankOrModifier, ref ValueStringBuilder builder)
         {
             if (rankOrModifier == ByRef)
             {
@@ -219,8 +198,6 @@ internal static string GetRankOrModifierStringRepresentation(int rankOrModifier,
                 builder.Append(',', rankOrModifier - 1);
                 builder.Append(']');
             }
-
-            return builder.ToString();
         }
 
         /// <summary>

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

@@ -66,22 +66,8 @@ public void GetAssemblyNameCandidateReturnsExpectedValue(string input, string ex
         public void AppendRankOrModifierStringRepresentationAppendsExpectedString(int input, string expected)
         {
             ValueStringBuilder builder = new ValueStringBuilder(initialCapacity: 10);
-            Assert.Equal(expected, TypeNameParserHelpers.GetRankOrModifierStringRepresentation(input, ref builder));
-        }
-
-        [Theory]
-        [InlineData(typeof(List<int>))]
-        [InlineData(typeof(int?))]
-        [InlineData(typeof(List<string>))]
-        [InlineData(typeof(Dictionary<string, DateTime>))]
-        [InlineData(typeof(ValueTuple<bool, short, int, DateTime>))]
-        [InlineData(typeof(ValueTuple<bool, short, int, DateTime, char, ushort, long, sbyte>))]
-        public void GetGenericTypeFullNameReturnsSameStringAsTypeAPI(Type genericType)
-        {
-            TypeName openGenericTypeName = TypeName.Parse(genericType.GetGenericTypeDefinition().FullName.AsSpan());
-            ReadOnlySpan<TypeName> genericArgNames = genericType.GetGenericArguments().Select(arg => TypeName.Parse(arg.AssemblyQualifiedName.AsSpan())).ToArray();
-
-            Assert.Equal(genericType.FullName, TypeNameParserHelpers.GetGenericTypeFullName(openGenericTypeName.FullName.AsSpan(), genericArgNames));
+            TypeNameParserHelpers.AppendRankOrModifierStringRepresentation(input, ref builder);
+            Assert.Equal(expected, builder.ToString());
         }
 
         [Theory]