Merge branch '6-nullable-t-for-optional-fields-are-call' into 'master'

Resolve "`Nullable<T>` for optional fields are call"

Closes kaitai-io#6

See merge request marta/kaitai_struct_csharp_runtime!14

Kaitai.Struct.Runtime.Async/KaitaiAsyncStream.cs

@@ -58,6 +58,7 @@ public KaitaiAsyncStream(byte[] bytes) : this(new MemoryStream(bytes))
     public ValueTask<long> GetSizeAsync() => _readerContext.GetSizeAsync();
 
     public virtual async Task SeekAsync(long position) => await _readerContext.SeekAsync(position);
+    public virtual async Task SeekAsync(ulong position) => await SeekAsync((long)position);
 
     public override long Pos => _readerContext.Position;
 

Kaitai.Struct.Runtime/ImplicitNullable.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Kaitai
+{
+    public struct ImplicitNullable<T> where T : struct
+    {
+        public bool HasValue { get { return this._value.HasValue; } }
+        public T Value { get { return this._value.Value; } }
+
+        public ImplicitNullable(T value) : this() { this._value = value; }
+        public ImplicitNullable(T? value) : this() { this._value = value; }
+
+        public static implicit operator ImplicitNullable<T>(T value) { return new ImplicitNullable<T>(value); }
+        public static implicit operator ImplicitNullable<T>(T? value) { return new ImplicitNullable<T>(value); }
+
+        public static implicit operator T(ImplicitNullable<T> value) { return value._value ?? default(T); }
+        public static implicit operator T?(ImplicitNullable<T> value) { return value._value; }
+
+        private T? _value { get; set; }
+
+        // Should define other Nullable<T> members, especially 
+        // Equals and GetHashCode to avoid boxing
+    }
+}

Kaitai.Struct.Runtime/KaitaiStream.cs

@@ -49,6 +49,7 @@ protected BinaryReader BinaryReader
         public override bool IsEof => BaseStream.Position >= BaseStream.Length && BitsLeft == 0;
 
         public void Seek(long position) => BaseStream.Seek(position, SeekOrigin.Begin);
+        public void Seek(ulong position) => Seek((long)position);
 
         public override long Pos => BaseStream.Position;