Question

I have two viewmodel classes called ChangePwdViewModel.cs and ExpiringPwdViewModel.cs.
ChangPwd.xaml binds to ChangePwdViewModel and ExpiringPwd.xaml binds to ExpiringPwdViewModel.

Both have the property as below.

private string _message;
public string Message
{
    get { return _message; }
    set { _message = value; OnPropertyChanged("Message"); }
}

In each class, there’s a function called ValidatePwd() to validate the new password.

In this function, Message property is updated.
Eg.

if (IsAlphaNumeric(this.NewPassword) == false || IsAlphaNumeric(this.CfmPassword) == false)
{
    this.Message = "Invalid new password, only characters and numbers are accepted, password must contain at least one character and one number";
    this.ResetPasswordFields();
    return false;
}

I want to create a common class to write this function and used by two viewmodel. But, How can I update the Message Property of the viewmodels from this class?

Answer 1

How about putting it in a base class:

class ViewModelBase
{
    private string _message;
    public string Message 
    {
        get { return _message; }
        set { _message = value; OnPropertyChanged("Message"); }
    }

    public bool VerifyPassword(string newPassword)
    {
        ....
    }
}

class ChangePwdViewModel : ViewModelBase
{
}

class ExpiringPwdViewModel : ViewModelBase
{
}

Update:

If you can’t use a base class because your view models already have a base class then you could use an interface as suggested by others. However this means that you will still have to implement the interface in all your view model classes so you don’t gain that much in terms of avoiding multiple implementations (except that you have a contract for your view models then which is usually a good thing to have).

You can achieve some kind of “multiple inheritance” in C# by using a tool like Dynamic Proxy which allows you to create mixins. So you could implement the Message property and password verification in one class and then create a mixin proxy which merges the view model with that implementation. It’s not as nice as you will have to create all your view model instances via the proxy generator but it can be made to work. Have a look at this tutorial if it sounds like an option for you.