[Core] Avoid creating TaskCompletionSource on UI thread (#9392)

We don't need a completion source when already on the UI thread, so optimize the implementation a bit

Author: Therzok

main/src/core/MonoDevelop.Core/MonoDevelop.Core/Runtime.cs

@@ -330,15 +330,15 @@ public static SynchronizationContext MainSynchronizationContext {
 		/// </summary>
 		public static Task RunInMainThread (Action action)
 		{
-			var ts = new TaskCompletionSource<int> ();
 			if (IsMainThread) {
 				try {
 					action ();
-					ts.SetResult (0);
+					return Task.CompletedTask;
 				} catch (Exception ex) {
-					ts.SetException (ex);
+					return Task.FromException (ex);
 				}
 			} else {
+				var ts = new TaskCompletionSource<int> ();
 				MainSynchronizationContext.Post (state => {
 					var (act, tcs) = (ValueTuple<Action, TaskCompletionSource<int>>)state;
 					try {
@@ -348,23 +348,23 @@ public static Task RunInMainThread (Action action)
 						tcs.SetException (ex);
 					}
 				}, (action, ts));
+				return ts.Task;
 			}
-			return ts.Task;
 		}
 
 		/// <summary>
 		/// Runs a function in the main thread (usually the UI thread). The method returns a task, so it can be awaited.
 		/// </summary>
 		public static Task<T> RunInMainThread<T> (Func<T> func)
 		{
-			var ts = new TaskCompletionSource<T> ();
 			if (IsMainThread) {
 				try {
-					ts.SetResult (func ());
+					return Task.FromResult (func ());
 				} catch (Exception ex) {
-					ts.SetException (ex);
+					return Task.FromException<T> (ex);
 				}
 			} else {
+				var ts = new TaskCompletionSource<T> ();
 				MainSynchronizationContext.Post (state => {
 					var (fun, tcs) = (ValueTuple<Func<T>, TaskCompletionSource<T>>)state;
 					try {
@@ -373,8 +373,8 @@ public static Task<T> RunInMainThread<T> (Func<T> func)
 						tcs.SetException (ex);
 					}
 				}, (func, ts));
+				return ts.Task;
 			}
-			return ts.Task;
 		}
 
 		/// <summary>