diff --git a/nanoFramework.CoreLibrary/System/String.cs b/nanoFramework.CoreLibrary/System/String.cs
index c9372ef..9d28c0a 100644
--- a/nanoFramework.CoreLibrary/System/String.cs
+++ b/nanoFramework.CoreLibrary/System/String.cs
@@ -718,212 +718,14 @@ public static string IsInterned(string str)
 #if NANOCLR_REFLECTION
 
 #nullable enable
-        public static string Format(string format, params object?[] args)
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        public static extern string Format(string format, params object?[] args);
 #nullable restore
 
 #else
-        public static string Format(string format, params object[] args)
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        public static extern string Format(string format, params object[] args);
 #endif
-        {
-            var index = 0;
-            var alignment = 0;
-            var chr = '\0';
-            var len = format.Length;
-            var fmt = Empty;
-            var token = Empty;
-            var output = Empty;
-
-            if (format is null)
-            {
-                throw new ArgumentNullException("format can't be null");
-            }
-
-            for (var i = 0; i < len; i++)
-            {
-                token = Empty;
-                chr = format[i];
-
-                if (chr == '{')
-                {
-                    if (i + 1 == len)
-                    {
-                        throw new ArgumentException("Format error: no closed brace, column " + i);
-                    }
-
-                    if (format[i + 1] == '{')
-                    {
-                        output += chr;
-                        i++;
-                        continue;
-                    }
-                    else
-                    {
-                        alignment = 0;
-                        fmt = Empty;
-
-                        for (i++; i < len; i++)
-                        {
-                            chr = format[i];
-
-                            if (chr >= '0' && chr <= '9')
-                            {
-                                token += chr;
-                            }
-                            else if (chr == ',' || chr == ':' || chr == '}')
-                            {
-                                break;
-                            }
-                            else
-                            {
-                                throw new ArgumentException("Format error: wrong symbol at {}, column " + i);
-                            }
-                        }
-
-                        if (token.Length > 0)
-                        {
-                            index = int.Parse(token);
-                        }
-                        else
-                        {
-                            throw new ArgumentException("Format error: empty {}, column " + i);
-                        }
-
-                        if (chr == ',')
-                        {
-                            if (format[i + 1] == '-')
-                            {
-                                token = "-";
-                                i++;
-                            }
-                            else
-                            {
-                                token = Empty;
-                            }
-
-                            for (i++; i < len; i++)
-                            {
-                                chr = format[i];
-
-                                if (chr >= '0' && chr <= '9')
-                                {
-                                    token += chr;
-                                }
-                                else if (chr == ':' || chr == '}')
-                                {
-                                    break;
-                                }
-                                else
-                                {
-                                    throw new ArgumentException("Format error: wrong symbol at alignment, column " + i);
-                                }
-                            }
-
-                            if (token.Length > 0)
-                            {
-                                alignment = int.Parse(token);
-                            }
-                            else
-                            {
-                                throw new ArgumentException("Format error: empty alignment, column " + i);
-                            }
-                        }
-
-                        if (chr == ':')
-                        {
-                            token = Empty;
-                            for (i++; i < len; i++)
-                            {
-                                chr = format[i];
-
-                                if (chr == '}')
-                                {
-                                    break;
-                                }
-                                else
-                                {
-                                    token += chr;
-                                }
-                            }
-
-                            if (token.Length > 0)
-                            {
-                                fmt = token;
-                            }
-                            else
-                            {
-                                throw new ArgumentException("Format error: empty format after ':', column " + i);
-                            }
-                        }
-                    }
-
-                    if (chr != '}')
-                    {
-                        throw new ArgumentException("Format error: no closed brace, column " + i);
-                    }
-
-                    if (fmt.Length > 0)
-                    {
-#if NANOCLR_REFLECTION
-
-                        if (args[index] is null)
-                        {
-                            token = "";
-                        }
-                        else
-                        {
-                            var method = args[index].GetType().GetMethod("ToString", new Type[] { typeof(string) });
-                            token = (method is null)
-                                ? args[index].ToString()
-                                : method.Invoke(args[index], new object[] { token }).ToString();
-                        }
-#else
-                        throw new NotImplementedException();
-#endif // NANOCLR_REFLECTION
-
-                    }
-                    else
-                    {
-                        token = args[index] == null ? "" : args[index].ToString();
-                    }
-
-                    if (alignment > 0)
-                    {
-                        output += token.PadLeft(alignment);
-                    }
-                    else if (alignment < 0)
-                    {
-                        output += token.PadRight(MathInternal.Abs(alignment));
-                    }
-                    else
-                    {
-                        output += token;
-                    }
-                }
-                else if (chr == '}')
-                {
-                    if (i + 1 == len)
-                    {
-                        throw new ArgumentException("Format error: no closed brace, column " + i);
-                    }
-
-                    if (format[i + 1] == '}')
-                    {
-                        output += chr;
-                        i++;
-                    }
-                    else
-                    {
-                        throw new ArgumentException("Format error: no closed brace, column " + i);
-                    }
-                }
-                else
-                {
-                    output += chr;
-                }
-            }
-
-            return output;
-        }
 
         /// <summary>
         /// Returns a new string that right-aligns the characters in this instance by padding them on the left with a specified Unicode character, for a specified total length.