just adding a control I made for Xamarin Forms. still in beta testing but seems to work ok on Android. just giving back for all the great code I have seen from github projects. 
https://github.com/amccorma/xamarin-amccorma
For now, only Android. But, IOS be up soon.
Mask Entry Box that will format the text based on the rules supplied to it.
entry = new MyEntry ();
entry.Text = "";
entry.Keyboard = Keyboard.Numeric;
entry.FormatCharacters = "-";
Format Characters: (FormatCharacters)
"-" : dash
"-(/" : multiple characters
Rules definition:
new MaskRules { Start = 0, End = 3, Mask = "" }
new MaskRules { Start = 4, End = 6, Mask = "{0:3}-{3:}"}
new MaskRules { Start = 7, End = 10, Mask = "{0:3}-{3:3}-{6:}"}
new MaskRules { Start = 10, End = 20, Mask = "{0:}"}
Mask Format:
{0:3}: take the first 3 characters
{3:3}: start at character 3 and take 3 characters
{0:}: apply no mask. take all the characters and output them
{6:3}: start at character 6 and take 3 characters
Final mask:
999-999-9999 if less than 10 characters
if 10 characters to 20, use {0:}. just output the text
Text Length from 0 to 3 characters. apply the Mask "". Mask = "", no formatting
new MaskRules { Start = 0, End = 3, Mask = "" }
Text Length from 0 to 3 characters. apply the Mask "{0:3}-{3:}".
{0:3}: first 3 characters
followed by a "-"
next three characters {3:0}
new MaskRules { Start = 4, End = 6, Mask = "{0:3}-{3:}"}
Text Length from 0 to 3 characters. apply the Mask "{0:3}-{3:}".
{0:3}: first 3 characters
followed by a "-"
next three characters {3:0}
add a "-"
next characters use (6:0}
new MaskRules { Start = 7, End = 10, Mask = "{0:3}-{3:3}-{6:}"}
Text Length from 10 to 20 characters.
20 characters MAX Length
apply the Mask "{0:} is the same as no Mask.
new MaskRules { Start = 10, End = 20, Mask = "{0:}"}