What is the datatype of "value" in "set" accessor?

Question

I'm just wondering what is the datatype of value variable in C#'s set accessor?

Because I want to implement type-hinting in C#'s set accessor.

For example, I have a setter method:

public User
{

    private string username;

    public void setUsername(SingleWord username)
    {
        this.username = username.getValue(); // getValue() method from "SingleWord" class returns "string"
    }

}

Now how do I implement this in C#'s accessor syntax?

public User
{
    public string Username
    {
        get ;
        set {
            // How do I implement type-hinting here for class "SingleWord"?
            // Is it supposed to be:
            // this.Username = ((SingleWord)value).getValue();    ???
        }
    }

}

So that I can call it this way:

User newuser = new User() {
    Username = new SingleWord("sam023")
};

Thanks in advance!

EDIT: Here's the source code of SingleWord:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using Guitar32.Exceptions;
using Guitar32.Common;

namespace Guitar32.Validations
{
    public class SingleWord : Validator, IStringDatatype
    {
        public static String expression = "^[\\w\\S]+$";
        public static String message = "Spaces are not allowed";
        private String value;

        public SingleWord(String value, bool throwException = false) {
            this.value = value;
            if (throwException && value != null) {
                if (!this.isValid()) {
                    throw new InvalidSingleWordException();
                }
                //if (this.getValue().Length > 0) {
                //    if (!this.isWithinRange()) {
                //        throw new Guitar32.Exceptions.OutOfRangeLengthException();
                //    }
                //}
            }
        }

        public int getMaxLength() {
            return 99999;
        }

        public int getMinLength() {
            return 1;
        }

        public String getValue() {
            return this.value;
        }

        public bool isWithinRange() {
            return this.getValue().Length >= this.getMinLength() && this.getValue().Length <= this.getMaxLength();
        }

        public override bool isValid() {
            return this.getValue().Length > 0 ? Regex.IsMatch(this.getValue(), expression) : true;
        }
    }

    public class InvalidSingleWordException : Exception {
        public InvalidSingleWordException() : base("Value didn't comply to Single Word format")
        { }
    }
}

I used this class to provide back-end validation by adding SingleWord as the datatype required from the setter.