How to inject a struct with UnmanagedType field?

[13316642000]

can anyone tell me how to inject a struct like this
public struct test
{
public int cbS;
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = 255)]
public string Path;
}
my code is:
var test = new TypeDefinition("namespace_1", "test", TypeAttributes.Sealed | TypeAttributes.SequentialLayout | TypeAttributes.Public |
TypeAttributes.BeforeFieldInit, hMainMod.ImportReference(typeof(System.ValueType)));
var test_cb = new FieldDefinition("cb", Mono.Cecil.FieldAttributes.Public, hMainMod.TypeSystem.Int32);
var test_Path = new FieldDefinition("Path", Mono.Cecil.FieldAttributes.Public, hMainMod.TypeSystem.String);
hMainMod.Types.Add(test);
var FixedAttr = new FixedSysStringMarshalInfo();
FixedAttr.NativeType = NativeType.FixedSysString;
FixedAttr.Size = 255;
test_Path.DeclaringType = test;
test_Path.Attributes = Mono.Cecil.FieldAttributes.Assembly | Mono.Cecil.FieldAttributes.PInvokeImpl;
test_Path.InitialValue = new Byte[0];
test.Fields.Add(test_cb);
test.Fields.Add(test_Path);
the IL is:
.class public sequential ansi sealed beforefieldinit WindowsApplication1.test
extends [mscorlib]System.ValueType
{
// Fields
.field public int32 cb
.field public marshal(fixed sysstring[255]) string Path

} // end of class WindowsApplication1.test

but the IL not work well ,can anyone help me?