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

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

Have you tried treating `TestProjectNS.MyStruct+e__FixedBuffer0` as a type and checked it's members? It wouldn't surprise me if fixed buffers were implemented as nested structs. — CodesInChaos, Mar 23 '11 at 18:54

Mikael Östberg · Accepted Answer · 2011-03-23T19:23:04.360

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!

CodesInChaos · Answer 2 · 2011-03-23T19:14:46.517

4

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.

edited Mar 23 '11 at 19:14

answered Mar 23 '11 at 19:04

CodesInChaos

106,488
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+1 Nice solution. Did you write that up, or did you get it from Reflector? Have a look at the code I got from Reflector. – Mikael Östberg Mar 23 '11 at 19:24
Code is from the `FieldInfo` I dumped in LinqPad. So your code from reflector is certainly more accurate. – CodesInChaos Mar 23 '11 at 20:11
Good answer, Since the field is nested one must dive deeper into the nested fields. – Joe Mar 23 '11 at 20:40

score -1 · Answer 3 · answered Mar 23 '11 at 19:14

-1

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";
   }
}

answered Mar 23 '11 at 19:14

SwDevMan81

48,814
22
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Your if condition is a bit over eager. It thinks every nested type is a fixed array. – CodesInChaos Mar 23 '11 at 19:17
@Code - Yeah, probably not the best way to do it. Your answer is good :) – SwDevMan81 Mar 23 '11 at 19:19

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

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