C# Get type of fixed field in unsafe struct with r

Question

I'm trying to get the field types of an unsafe struct using some fixed fields. The fixed fields FieldType do not return the actual type.

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public unsafe struct MyStruct
{
   public UInt32 Field1;
   public fixed sbyte Field2[10];
   public UInt64 Field3;
}

void Test() 
{
   var theStruct = new MyStruct();
   string output = "";
   foreach (FieldInfo fi in theStruct.GetType().GetFields(BindingFlags.Public | BindingFlags.Instance))
   {
      output += fi.Name + ": " + fi.FieldType.ToString() + "\r\n";
   }

}

Output:

Field1: System.UInt32

Field2: TestProjectNS.MyStruct+<Field2>e__FixedBuffer0

Field3: System.UInt64

---

I'm looking for Field2 to tell me it's sbyte instead of TestProjectNS.MyStruct+<Field2>e__FixedBuffer0

Answer 1

The Fixed Size Buffer's underlying type can be retrieved through the FixedBufferAttribute, which is applied on the fixed size buffer statement.

foreach (FieldInfo fi in typeof(MyStruct).GetFields(BindingFlags.Public | BindingFlags.Instance))
{
    var attr = fi.GetCustomAttributes(typeof(FixedBufferAttribute), false);
    if(attr.Length > 0)
        output += fi.Name + ": " + ((FixedBufferAttribute)attr[0]).ElementType + "\r\n";
    else
        output += fi.Name + ": " + fi.FieldType + "\r\n";
}

Or the single field short version:

var type = typeof (MyStruct)
            .GetField("Field2")
            .GetCustomAttributes(typeof (FixedBufferAttribute), false)
            .Cast<FixedBufferAttribute>()
            .Single()
            .ElementType;

As CodeInChaos, I also needed to reflect it, but I do have the FixedBufferAttribute:

[StructLayout(LayoutKind.Sequential, Pack=1)]
public struct MyStruct
{
    public uint Field1;
    [FixedBuffer(typeof(sbyte), 10)]
    public <Field2>e__FixedBuffer0 Field2;
    public ulong Field3;
    // Nested Types
    [StructLayout(LayoutKind.Sequential, Size=10), CompilerGenerated, UnsafeValueType]
    public struct <Field2>e__FixedBuffer0
    {
       public sbyte FixedElementField;
    }
}

Awesome question!

Answer 2

CodeInChaos has it right. Another way you can do it:

foreach (FieldInfo fi in theStruct.GetType().GetFields(BindingFlags.Public | BindingFlags.Instance))
{
   if (fi.FieldType.IsNested)
   {
      output += fi.Name + ": " + 
          fi.FieldType.GetFields()[0].FieldType.ToString() + "\r\n";
   }
   else
   {
      output += fi.Name + ": " + fi.FieldType.ToString() + "\r\n";
   }
}

Answer 3

TestProjectNS.MyStruct+<Field2>e__FixedBuffer0 is a nested type. It contains a single field of the underlying type you want.

So given the FieldInfo of your fixed size array you can do:

Type bufferFieldType=fixedBufferFieldInfo.FieldType.GetFields(BindingFlags.Public | BindingFlags.Instance).Single().FieldType;

What the C# compiler generates looks similar to the following:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public unsafe struct MyStruct
{
   [StructLayout(LayoutKind.Sequential, Pack = 1, Size=10)]
   public struct Field2e__FixedBuffer0
    {
        public sbyte FixedElementField;
    }
   public UInt32 Field1;
   public Field2e__FixedBuffer0 Field2;
   public UInt64 Field3;
}

Except that the name of the generated struct contains a few special characters and the both the field and the nested type are marked as security critical.