diff --git a/lib/system/widestrs.nim b/lib/system/widestrs.nim
index e7b7f3972..77310b289 100644
--- a/lib/system/widestrs.nim
+++ b/lib/system/widestrs.nim
@@ -114,41 +114,47 @@ proc newWideCString*(s: cstring): WideCString =
 proc newWideCString*(s: string): WideCString =
   result = newWideCString(s, s.len)
 
-proc `$`*(w: WideCString, estimate: int): string =
+proc `$`*(w: WideCString, estimate: int, replacement: int = 0xFFFD): string =
   result = newStringOfCap(estimate + estimate shr 2)
 
   var i = 0
   while w[i].int16 != 0'i16:
-    var ch = w[i].int
+    var ch = int(cast[uint16](w[i]))
     inc i
-    if ch >=% UNI_SUR_HIGH_START and ch <=% UNI_SUR_HIGH_END:
+    if ch >= UNI_SUR_HIGH_START and ch <= UNI_SUR_HIGH_END:
       # If the 16 bits following the high surrogate are in the source buffer...
-      let ch2 = w[i].int
+      let ch2 = int(cast[uint16](w[i]))
+      
       # If it's a low surrogate, convert to UTF32:
-      if ch2 >=% UNI_SUR_LOW_START and ch2 <=% UNI_SUR_LOW_END:
-        ch = ((ch -% UNI_SUR_HIGH_START) shr halfShift) +%
-              (ch2 -% UNI_SUR_LOW_START) +% halfBase
+      if ch2 >= UNI_SUR_LOW_START and ch2 <= UNI_SUR_LOW_END:
+        ch = (((ch and halfMask) shl halfShift) + (ch2 and halfMask)) + halfBase
         inc i
-        
-    if ch <=% 127:
+      else:
+        #invalid UTF-16
+        ch = replacement
+    elif ch >= UNI_SUR_LOW_START and ch <= UNI_SUR_LOW_END:
+      #invalid UTF-16
+      ch = replacement
+      
+    if ch < 0x80:
       result.add chr(ch)
-    elif ch <=% 0x07FF:
-      result.add chr((ch shr 6) or 0b110_00000)
-      result.add chr((ch and ones(6)) or 0b10_000000)
-    elif ch <=% 0xFFFF:
-      result.add chr(ch shr 12 or 0b1110_0000)
-      result.add chr(ch shr 6 and ones(6) or 0b10_0000_00)
-      result.add chr(ch and ones(6) or 0b10_0000_00)
-    elif ch <=% 0x0010FFFF:
-      result.add chr(ch shr 18 or 0b1111_0000)
-      result.add chr(ch shr 12 and ones(6) or 0b10_0000_00)
-      result.add chr(ch shr 6 and ones(6) or 0b10_0000_00)
-      result.add chr(ch and ones(6) or 0b10_0000_00)
+    elif ch < 0x800:
+      result.add chr((ch shr 6) or 0xc0)
+      result.add chr((ch and 0x3f) or 0x80)
+    elif ch < 0x10000:
+      result.add chr((ch shr 12) or 0xe0)
+      result.add chr(((ch shr 6) and 0x3f) or 0x80)
+      result.add chr((ch and 0x3f) or 0x80)
+    elif ch <= 0x10FFFF:
+      result.add chr((ch shr 18) or 0xf0)
+      result.add chr(((ch shr 12) and 0x3f) or 0x80)
+      result.add chr(((ch shr 6) and 0x3f) or 0x80)
+      result.add chr((ch and 0x3f) or 0x80)
     else:
-      # replacement char:
+      # replacement char(in case user give very large number):
       result.add chr(0xFFFD shr 12 or 0b1110_0000)
       result.add chr(0xFFFD shr 6 and ones(6) or 0b10_0000_00)
       result.add chr(0xFFFD and ones(6) or 0b10_0000_00)
-
+      
 proc `$`*(s: WideCString): string =
   result = s $ 80
diff --git a/tests/stdlib/twchartoutf8.nim b/tests/stdlib/twchartoutf8.nim
new file mode 100644
index 000000000..9838bbfe7
--- /dev/null
+++ b/tests/stdlib/twchartoutf8.nim
@@ -0,0 +1,106 @@
+#assume WideCharToMultiByte always produce correct result
+#windows only
+
+when not defined(windows):
+  {.error: "windows only".}
+  
+{.push gcsafe.}
+
+const CP_UTF8 = 65001'i32
+
+type
+  LPBOOL = ptr int32
+  LPWCSTR = ptr uint16
+  
+proc WideCharToMultiByte*(CodePage: int32, dwFlags: int32,
+                          lpWideCharStr: LPWCSTR, cchWideChar: int32,
+                          lpMultiByteStr: cstring, cchMultiByte: int32,
+                          lpDefaultChar: cstring, lpUsedDefaultChar: LPBOOL): int32{.
+    stdcall, dynlib: "kernel32", importc: "WideCharToMultiByte".}
+
+{.pop.}
+
+proc convertToUTF8(wc: WideCString, wclen: int32): string =
+  let size = WideCharToMultiByte(CP_UTF8, 0'i32, cast[LPWCSTR](addr(wc[0])), wclen, 
+    cstring(nil), 0'i32, cstring(nil), LPBOOL(nil))
+  result = newString(size)
+  let res = WideCharToMultiByte(CP_UTF8, 0'i32, cast[LPWCSTR](addr(wc[0])), wclen, 
+    cstring(result), size, cstring(nil), LPBOOL(nil))
+  result[size] = chr(0)
+  assert size == res
+   
+proc testCP(wc: WideCString, lo, hi: int) =
+  var x = 0
+  let chunk = 1024
+  for i in lo..hi:
+    wc[x] = cast[Utf16Char](i)
+    if (x >= chunk) or (i >= hi):
+      wc[x] = Utf16Char(0)
+      var a = convertToUTF8(wc, int32(x))
+      var b = wc $ chunk
+      assert a == b
+      x = 0
+    inc x
+
+proc testCP2(wc: WideCString, lo, hi: int) =
+  assert((lo >= 0x10000) and (hi <= 0x10FFFF))
+  var x = 0
+  let chunk = 1024
+  for i in lo..hi:
+    let ch = i - 0x10000
+    let W1 = 0xD800 or (ch shr 10)
+    let W2 = 0xDC00 or (0x3FF and ch)
+    wc[x] = cast[Utf16Char](W1)
+    wc[x+1] = cast[Utf16Char](W2)
+    inc(x, 2)
+    
+    if (x >= chunk) or (i >= hi):
+      wc[x] = Utf16Char(0)
+      var a = convertToUTF8(wc, int32(x))
+      var b = wc $ chunk
+      assert a == b
+      x = 0
+
+#RFC-2781 "UTF-16, an encoding of ISO 10646"
+    
+var wc: WideCString
+unsafeNew(wc, 1024 * 4 + 2)
+
+#U+0000 to U+D7FF
+#skip the U+0000
+wc.testCP(1, 0xD7FF)
+
+#U+E000 to U+FFFF
+wc.testCP(0xE000, 0xFFFF)
+
+#U+10000 to U+10FFFF
+wc.testCP2(0x10000, 0x10FFFF)
+
+#invalid UTF-16
+const 
+  b = "\xEF\xBF\xBD"
+  c = "\xEF\xBF\xBF"
+
+wc[0] = cast[Utf16Char](0xDC00)
+wc[1] = Utf16Char(0)
+var a = $wc
+assert a == b
+
+wc[0] = cast[Utf16Char](0xFFFF)
+wc[1] = cast[Utf16Char](0xDC00)
+wc[2] = Utf16Char(0)
+a = $wc
+assert a == c & b
+
+wc[0] = cast[Utf16Char](0xD800)
+wc[1] = Utf16Char(0)
+a = $wc
+assert a == b
+
+wc[0] = cast[Utf16Char](0xD800)
+wc[1] = cast[Utf16Char](0xFFFF)
+wc[2] = Utf16Char(0)
+a = $wc
+assert a == b & c
+
+echo "OK"
\ No newline at end of file