Improve SimConnect struct writing and allow empty unit with SetAsync

src/SimConnect.NET/SimVar/Internal/SimVarFieldReaderFactory.cs

@@ -23,12 +23,12 @@ public static List<IFieldReader<T>> Build<T>(
         {
             var t = typeof(T);
 
-            // Collect and order fields with [SimVar]
+            // Collect and order fields with [SimConnect]
             var fields = GetOrderedSimVarFields(t);
 
             if (fields.Count == 0)
             {
-                throw new InvalidOperationException($"Type {t.FullName} has no fields with [SimVar].");
+                throw new InvalidOperationException($"Type {t.FullName} has no fields with [SimConnect].");
             }
 
             var readers = new List<IFieldReader<T>>(fields.Count);

src/SimConnect.NET/SimVar/Internal/SimVarFieldWriter.cs

@@ -67,13 +67,26 @@ public void WriteFrom(in T source, IntPtr basePtr)
                     {
                         var getter = (Func<T, string>)this.Extractor;
                         string s = getter(source) ?? string.Empty;
-                        var bytes = System.Text.Encoding.ASCII.GetBytes(s);
 
-                        // zero-initialize then copy up to Size
+                        // zero-initialize then encode into span up to Size-1, ensure explicit null-termination
+                        // SimConnect expects fixed-size, null-terminated ANSI strings.
+                        // We reserve the last byte for '\0' when Size > 0 to avoid losing the terminator.
                         Span<byte> tmp = stackalloc byte[this.Size];
-                        var copyLen = Math.Min(bytes.Length, this.Size);
-                        bytes.AsSpan(0, copyLen).CopyTo(tmp);
-                        Marshal.Copy(tmp.ToArray(), 0, addr, this.Size);
+                        if (this.Size > 0)
+                        {
+                            var dest = tmp[..(this.Size - 1)];
+                            _ = System.Text.Encoding.Latin1.GetBytes(s.AsSpan(), dest);
+
+                            // Explicitly set terminator even though tmp is zeroed by default
+                            tmp[this.Size - 1] = 0;
+                        }
+
+                        // Copy without allocating an intermediate array
+                        for (int i = 0; i < this.Size; i++)
+                        {
+                            Marshal.WriteByte(addr, i, tmp[i]);
+                        }
+
                         break;
                     }
 

src/SimConnect.NET/SimVar/Internal/SimVarFieldWriterFactory.cs

@@ -25,7 +25,7 @@ public static (List<IFieldWriter<T>> Writers, int TotalSize) Build<T>(
             var fields = GetOrderedSimVarFields(t);
             if (fields.Count == 0)
             {
-                throw new InvalidOperationException($"Type {t.FullName} has no fields with [SimVar].");
+                throw new InvalidOperationException($"Type {t.FullName} has no fields with [SimConnect].");
             }
 
             var writers = new List<IFieldWriter<T>>(fields.Count);

src/SimConnect.NET/SimVar/SimVarManager.cs

@@ -25,6 +25,7 @@ public sealed class SimVarManager : IDisposable
         private readonly ConcurrentDictionary<(string Name, string Unit, SimConnectDataType DataType), uint> dataDefinitions = new();
         private readonly ConcurrentDictionary<Type, uint> typeToDefIndex = new();
         private readonly ConcurrentDictionary<uint, Action<IntPtr, ISimVarRequest>> defToParser = new();
+        private readonly ConcurrentDictionary<uint, (Delegate Write, int TotalSize)> defToWriter = new();
         private readonly ConcurrentDictionary<uint, SimVarSubscription> subscriptions = new();
         private readonly object typeDefinitionSync = new();
 
@@ -100,7 +101,7 @@ public async Task SetAsync<T>(string simVarName, string unit, T value, uint obje
         {
             ObjectDisposedException.ThrowIf(this.disposed, nameof(SimVarManager));
             ArgumentException.ThrowIfNullOrEmpty(simVarName);
-            ArgumentException.ThrowIfNullOrEmpty(unit);
+            ArgumentNullException.ThrowIfNull(unit);
 
             // Try to get definition from registry first
             var definition = SimVarRegistry.Get(simVarName);
@@ -430,6 +431,18 @@ internal uint EnsureTypeDefinition<T>(CancellationToken cancellationToken)
                     }
                 };
                 this.typeToDefIndex[typeof(T)] = definitionId;
+
+                var writerBuild = SimVarFieldWriterFactory.Build<T>(addToDefinition: null);
+                Action<IntPtr, T> write = (basePtr, v) =>
+                {
+                    foreach (var w in writerBuild.Writers)
+                    {
+                        w.WriteFrom(in v, basePtr);
+                    }
+                };
+
+                this.defToWriter[definitionId] = (write, writerBuild.TotalSize);
+
                 return definitionId;
             }
         }
@@ -484,6 +497,7 @@ private static SimConnectDataType InferDataType<T>()
                 Type t when t == typeof(float) => SimConnectDataType.FloatSingle,
                 Type t when t == typeof(double) => SimConnectDataType.FloatDouble,
                 Type t when t == typeof(string) => SimConnectDataType.String256, // Default string size
+                Type t when t == typeof(SimConnectDataInitPosition) => SimConnectDataType.InitPosition,
                 Type t when t == typeof(SimConnectDataLatLonAlt) => SimConnectDataType.LatLonAlt,
                 Type t when t == typeof(SimConnectDataXyz) => SimConnectDataType.Xyz,
                 _ => throw new ArgumentException($"Unsupported type for SimVar: {type.Name}"),
@@ -938,28 +952,33 @@ private async Task SetStructAsync<T>(uint definitionId, T value, uint objectId,
         {
             cancellationToken.ThrowIfCancellationRequested();
 
-            // Build writers without re-adding to definition; EnsureTypeDefinition already registered it.
-            var (writers, totalSize) = SimVarFieldWriterFactory.Build<T>(addToDefinition: null);
+            // Use cached write delegate/layout keyed by definitionId (must exist if EnsureTypeDefinition was used)
+            if (!this.defToWriter.TryGetValue(definitionId, out var cache))
+            {
+                throw new InvalidOperationException($"No struct writer found for DefinitionId={definitionId}. EnsureTypeDefinition must be called first.");
+            }
 
             await Task.Run(
                 () =>
                 {
-                    var dataPtr = Marshal.AllocHGlobal(totalSize);
+                    var dataPtr = Marshal.AllocHGlobal(cache.TotalSize);
                     try
                     {
-                        // Fill the buffer in the same order/sizes as the definition
-                        foreach (var w in writers)
+                        // Fill the buffer using the cached writer delegate for this definition
+                        if (cache.Write is not Action<IntPtr, T> write)
                         {
-                            w.WriteFrom(in value, dataPtr);
+                            throw new InvalidOperationException($"Cached writer has unexpected type for DefinitionId={definitionId} and T={typeof(T).Name}.");
                         }
 
+                        write(dataPtr, value);
+
                         var hr = SimConnectNative.SimConnect_SetDataOnSimObject(
                             this.simConnectHandle,
                             definitionId,
                             objectId,
                             0,
                             1,
-                            (uint)totalSize,
+                            (uint)cache.TotalSize,
                             dataPtr);
 
                         if (hr != (int)SimConnectError.None)