In MVC we use model for defining entities and attributes .

Models are also used for providing validations into the attributes that will be used in our views.

public class ChangePasswordModel
{

[Required]
[DataType(DataType.Password)]
[Display(Name = "Current password")]
public string OldPassword { get; set; }

[Required]
[StringLength(100, ErrorMessage = "The {0} must be at least {2}      characters long.", MinimumLength = 6)]
[DataType(DataType.Password)]
[Display(Name = "New password")]
public string NewPassword { get; set; }

[DataType(DataType.Password)]
[Display(Name = "Confirm new password")]
[Compare("NewPassword", ErrorMessage = "The new password and confirmation password do not match.")]
public string ConfirmPassword { get; set; }

}

public class LogOnModel
{

[Required]
[Display(Name = "User name")]
public string UserName { get; set; }

[Required]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }

[Display(Name = "Remember me?")]
public bool RememberMe { get; set; }

}

public class RegisterModel
{

[Required]
[Display(Name = "User name")]
public string UserName { get; set; }

[Required]
[DataType(DataType.EmailAddress)]
[Display(Name = "Email address")]
public string Email { get; set; }

[Required]
[StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long.", MinimumLength = 6)]
[DataType(DataType.Password)]
[Display(Name = "Password")]
public string Password { get; set; }

[DataType(DataType.Password)]
[Display(Name = "Confirm password")]
[Compare("Password", ErrorMessage = "The password and confirmation password do not match.")]
public string ConfirmPassword { get; set; }

}

These validations will only allowed valid data to enter while making registeration or while doing login