.cs

public class UserName : IEquatable<UserName> {
  private readonly string name;

  public UserName(string name) {
    if (string.IsNullOrEmpty(name)) {
        throw new ArgumentNullException(nameof(name));
    }
    if (name.Length > 50) {
        throw new ArgumentOutOfRangeException("It must be 50 characters or less", nameof(name));
    }
    this.name = name;
  }

  public string Value { get { return name; } }

  public bool Equals(UserName other) {
    if (ReferenceEquals(null, other)) return false;
    if (ReferenceEquals(this, other)) return true;
    return string.Equals(name, other.name);
  }

  public override bool Equals(object obj) {
    if (ReferenceEquals(null, obj)) return false;
    if (ReferenceEquals(this, obj)) return true;
    if (obj.GetType() != this.GetType()) return false;
    return Equals((UserName) obj);
  }

  public override int GetHashCode() {
    return (name != null ? name.GetHashCode() : 0);
  }
}