fix #166

Author: neuecc

README.md

@@ -335,6 +335,20 @@ public partial class MyStringDictionary<TValue> : Dictionary<string, TValue>
 }
 ```
 
+Static constructor
+---
+MemoryPackable class can not define static constructor because generated partial class uses it. Instead, you can define a `static partial void StaticConstructor()` to do the same thing.
+
+```csharp
+[MemoryPackable]
+public partial class CctorSample
+{
+    static partial void StaticConstructor()
+    {
+    }
+}
+```
+
 Polymorphism (Union)
 ---
 MemoryPack supports serializing interface and abstract class objects for polymorphism serialization. In MemoryPack this feature is called Union. Only interfaces and abstracts classes are allowed to be annotated with `[MemoryPackUnion]` attributes. Unique union tags are required.

sandbox/SandboxConsoleApp/Program.cs

@@ -47,11 +47,26 @@
 
 var span = CollectionsMarshal.AsSpan(value.Payload);
 
+
+[MemoryPackable]
+public partial class CctorSample
+{
+    static partial void StaticConstructor()
+    {
+    }
+}
+
 [MemoryPackable]
 public partial class ListBytesSample
 {
     public int Id { get; set; }
     public List<byte> Payload { get; set; }
+
+    static partial void StaticConstructor()
+    {
+        Console.WriteLine("foo");
+        // throw new NotImplementedException();
+    }
 }
 
 [MemoryPackable]

src/MemoryPack.Generator/MemoryPackGenerator.Emitter.cs

@@ -350,10 +350,12 @@ public void Emit(StringBuilder writer, IGeneratorContext context)
 partial {{classOrStructOrRecord}} {{TypeName}} : IMemoryPackable<{{TypeName}}>{{fixedSizeInterface}}
 {
 {{EmitCustomFormatters()}}
+    static partial void StaticConstructor();
 
     static {{Symbol.Name}}()
     {
         {{registerT}}
+        StaticConstructor();
     }
 {{fixedSizeMethod}}
     [global::MemoryPack.Internal.Preserve]
@@ -1194,9 +1196,12 @@ string EmitGenericCollectionTemplate(IGeneratorContext context)
         var code = $$"""
 partial class {{TypeName}} : IMemoryPackFormatterRegister
 {
+    static partial void StaticConstructor();
+
     static {{Symbol.Name}}()
     {
         {{register}}
+        StaticConstructor();
     }
 
     {{staticRegisterFormatterMethod}}RegisterFormatter()

src/MemoryPack.Generator/TypeScriptRuntime.cs

@@ -283,6 +283,11 @@ public writeString(value: string | null): void {
             return;
         }
 
+        if (value.length == 0) {
+            this.writeCollectionHeader(0);
+            return;
+        }
+
         // (int ~utf8-byte-count, int utf16-length, utf8-bytes)
         this.ensureCapacity(8 + ((value.length + 1) * 3));