Secure + MVVM way of using PasswordBox. For more info: http://briannoyes.net/2015/03/04/wpf-using-passwordbox-in-mvvm/

PasswordBoxBindingBehavior.cs

/*
    How use in View?
    <PasswordBox>
        <i:Interaction.Behaviors>
            <local:PasswordBoxBindingBehavior Password="{Binding Password}"/>
        </i:Interaction.Behaviors>
    </PasswordBox>
    
    How access to orginal string?
    
    public bool Login(string username, SecureString password)
    {
        IntPtr passwordBSTR = default(IntPtr);
        string insecurePassword = "";
        try
        {
            passwordBSTR = Marshal.SecureStringToBSTR(password);
            insecurePassword = Marshal.PtrToStringBSTR(passwordBSTR);
        }
        catch
        {
            insecurePassword = "";
        }
        
        // immediately use insecurePassword (in local variable) value after decrypting it:
        return MockServiceProxyCall(username, insecurePassword);
    }

    NOTE: In associated ViewModel, use SecurePassword type for Password property.
*/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Security;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Interactivity;

namespace System.Windows.Controls
{
    public class PasswordBoxBindingBehavior : Behavior<PasswordBox>
    {
        protected override void OnAttached()
        {
            AssociatedObject.PasswordChanged += OnPasswordBoxValueChanged;
        }

        public SecureString Password
        {
            get { return (SecureString)GetValue(PasswordProperty); }
            set { SetValue(PasswordProperty, value); }
        }

        public static readonly DependencyProperty PasswordProperty =
            DependencyProperty.Register("Password", typeof(SecureString), typeof(PasswordBoxBindingBehavior), new PropertyMetadata(null));


        private void OnPasswordBoxValueChanged(object sender, RoutedEventArgs e)
        {
            var binding = BindingOperations.GetBindingExpression(this, PasswordProperty);
            if (binding != null)
            {
                PropertyInfo property = binding.DataItem.GetType().GetProperty(binding.ParentBinding.Path.Path);
                if (property != null)
                    property.SetValue(binding.DataItem, AssociatedObject.SecurePassword, null);
            }
        }

    }
}

I can not access the PasswordBoxBindingBehavior

I'm using xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

<i:Interaction.Behaviors>
<local:PasswordBoxBindingBehavior Password="{Binding Password}"/> <--------- Error
</i:Interaction.Behaviors>