Is there any restriction for writing property names C#

I've written a class definition like 

 public class Item
        {
            public double? 30dhi { get; set; }
            public double? 30dlo { get; set; }
            public double? 7dhi { get; set; }
            public double? 7dlo { get; set; }
        }

Why it's not allowing me to add properties having alphanumeric keys The error is like below

invalid token '30d' for class , struct or interface member declaration

Jon Skeet
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Why it's not allowing me to add properties having alphanumeric keys

A property name has to be an identifier. Identifiers in C# can't start with a digit. You can have a digit after the first character, but not as the first character.

See section 2.4.2 of the C# 5 specification for the precise details of what is allowed for an identifier. The full grammar is too long to post here usefully, but the crucial part is:

identifier-or-keyword:
   identifier-start-character identifier-part-charactersopt

identifier-start-character:
  letter-character
  _ (the underscore character U+005F)

identifier-part-characters:
  identifier-part-character
  identifier-part-characters identifier-part-character

identifier-part-character:
  letter-character
  decimal-digit-character
  connecting-character
  combining-character
  formatting-character

letter-character:
  A Unicode character of classes Lu, Ll, Lt, Lm, Lo, or Nl
  A unicode-escape-sequence representing a character of classes Lu, Ll, Lt, Lm, Lo, or Nl

Note that the example of 30d is a great one for why it's not allowed - that's actually a numeric literal, of the value 30 and type double. (You've actually got 30dhi of course, but the parser has parsed 30d as a token.)

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