1AbdullahButt · Nov 8, 2026
diff --git a/src/MessagePack.SourceGenerator/CodeAnalysis/CodeAnalysisUtilities.cs b/src/MessagePack.SourceGenerator/CodeAnalysis/CodeAnalysisUtilities.cs
@@ -76,7 +76,7 @@ internal static int GetArity(ITypeSymbol dataType)
         {
             INamedTypeSymbol namedType => namedType.Arity,
             IArrayTypeSymbol arrayType => GetArity(arrayType.ElementType),
-            ITypeParameterSymbol => 0,
+            ITypeParameterSymbol or IDynamicTypeSymbol => 0,
             _ => throw new NotSupportedException(),
         };
 
@@ -85,7 +85,7 @@ internal static ImmutableArray<GenericTypeParameterInfo> GetTypeParameters(IType
         {
             INamedTypeSymbol namedType => namedType.TypeParameters.Select(t => GenericTypeParameterInfo.Create(t, recursionGuard)).ToImmutableArray(),
             IArrayTypeSymbol arrayType => GetTypeParameters(arrayType.ElementType, recursionGuard),
-            ITypeParameterSymbol => ImmutableArray<GenericTypeParameterInfo>.Empty,
+            ITypeParameterSymbol or IDynamicTypeSymbol => ImmutableArray<GenericTypeParameterInfo>.Empty,
             _ => throw new NotSupportedException(),
         };
 
@@ -94,7 +94,7 @@ internal static ImmutableArray<QualifiedTypeName> GetTypeArguments(ITypeSymbol d
         {
             INamedTypeSymbol namedType => namedType.TypeArguments.Select(t => QualifiedTypeName.Create(t, recursionGuard)).ToImmutableArray(),
             IArrayTypeSymbol arrayType => GetTypeArguments(arrayType.ElementType, recursionGuard),
-            ITypeParameterSymbol => ImmutableArray<QualifiedTypeName>.Empty,
+            ITypeParameterSymbol or IDynamicTypeSymbol => ImmutableArray<QualifiedTypeName>.Empty,
             _ => throw new NotSupportedException(),
         };
 }
diff --git a/src/MessagePack.SourceGenerator/CodeAnalysis/QualifiedTypeName.cs b/src/MessagePack.SourceGenerator/CodeAnalysis/QualifiedTypeName.cs
@@ -11,6 +11,15 @@ namespace MessagePack.SourceGenerator.CodeAnalysis;
 /// </summary>
 public abstract record QualifiedTypeName : IComparable<QualifiedTypeName>
 {
+    /// <summary>
+    /// The name used for <c>dynamic</c>, which is <see cref="object"/> at runtime.
+    /// </summary>
+    private static readonly QualifiedNamedTypeName DynamicTypeName = new(TypeKind.Class)
+    {
+        Name = "Object",
+        Container = new NamespaceTypeContainer("System"),
+    };
+
     /// <summary>
     /// Initializes a new instance of the <see cref="QualifiedTypeName"/> class.
     /// </summary>
@@ -23,6 +32,7 @@ public static QualifiedTypeName Create(ITypeSymbol symbol, ImmutableStack<Generi
             IArrayTypeSymbol arrayType => new QualifiedArrayTypeName(arrayType),
             INamedTypeSymbol namedType => new QualifiedNamedTypeName(namedType, recursionGuard),
             ITypeParameterSymbol typeParam => new TypeParameter(typeParam),
+            IDynamicTypeSymbol => DynamicTypeName,
             _ => throw new NotSupportedException(),
         };
     }

diff --git a/src/MessagePack.SourceGenerator/CodeAnalysis/TypeCollector.cs b/src/MessagePack.SourceGenerator/CodeAnalysis/TypeCollector.cs
@@ -1,4 +1,4 @@
-// Copyright (c) All contributors. All rights reserved.
+// Copyright (c) All contributors. All rights reserved.
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 #pragma warning disable SA1402 // File may only contain a single type
@@ -48,6 +48,7 @@ public class TypeCollector
         "char",
         "string",
         "object",
+        "dynamic",
         "System.Guid",
         "System.TimeSpan",
         "System.DateTime",
@@ -71,6 +72,7 @@ public class TypeCollector
         "char[]",
         "string[]",
         "object[]",
+        "dynamic[]",
         "System.DateTime[]",
         "System.ArraySegment<byte>",
         "System.Memory<byte>",

diff --git a/tests/MessagePack.SourceGenerator.Tests/MsgPack00xAnalyzerTests.cs b/tests/MessagePack.SourceGenerator.Tests/MsgPack00xAnalyzerTests.cs
@@ -1109,4 +1109,26 @@ public class {|MsgPack003:Foo|}
             MarkupOptions = MarkupOptions.UseFirstDescriptor,
         }.RunAsync();
     }
+
+    [Fact]
+    public async Task DynamicTypedMembers()
+    {
+        string input = Preamble + @"
+using System.Collections.Generic;
+
+[MessagePackObject(keyAsPropertyName: true)]
+public class AnObject
+{
+    public dynamic Value;
+
+    public dynamic[] Values { get; set; }
+
+    public List<dynamic> ValueList { get; set; }
+
+    public Dictionary<string, dynamic> ValueMap { get; set; }
+}
+";
+
+        await VerifyCS.VerifyAnalyzerAsync(input);
+    }
 }