complete the branch

HandyControlDemo/Tools/Helper/ExternDllHelper.cs

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+using System.Runtime.InteropServices;
+
+namespace HandyControlDemo.Tools
+{
+    internal class ExternDllHelper
+    {
+        private const string Winmm = "winmm.dll";
+
+        [DllImport(Winmm, EntryPoint = "mciSendString", CharSet = CharSet.Auto)]
+        public static extern int MciSendString(string lpstrCommand, string lpstrReturnString, int uReturnLength, int hwndCallback);
+    }
+}