Question

This is easier to explain with an example. Suppose I have a person class

Public Person
{
string firstName;
string SocialSecurityNumber;
}

When some changes are made by the user in a web page, the Person object is posted back to a controller that accepts Person as the input parameter. The SocialSecurity number is encrypted. We have many pages that post back objects (Not Necessarily Person class) that have encrypted Social security as a parameter. Now I want to modify the model binding, so that if the posted object has SocialSecurityNumber as a property, it should be automatically decrypted. How can I do this?

Answer 1

You could use a custom model binder. Some examples:

• http://www.dotnetcurry.com/ShowArticle.aspx?ID=584
• http://odetocode.com/blogs/scott/archive/2009/04/27/6-tips-for-asp-net-mvc-model-binding.aspx
• http://www.markeverard.com/blog/2011/07/18/creating-a-custom-modelbinder-allowing-validation-of-injected-composite-models/

This is an example of a simple one I’ve used before that you could modify for your needs:

public class FormatterModelBinder : DefaultModelBinder
{
    internal static string TrimValue([CanBeNull] string value, [CanBeNull] FormatAttribute formatter)
    {
        if (string.IsNullOrEmpty(value)) return value;
        return ((formatter == null) || formatter.Trim) ? value.Trim() : value;
    }

    protected override void SetProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, object value)
    {
        if ((propertyDescriptor != null) && (propertyDescriptor.PropertyType == typeof(string)))
        {
            var stringValue = value as string;
            var formatter = propertyDescriptor.Attributes.OfType<FormatAttribute>().FirstOrDefault();
            stringValue = TrimValue(stringValue, formatter);
            value = stringValue;
        }

        base.SetProperty(controllerContext, bindingContext, propertyDescriptor, value);
    }
}

You could then create an attribute to decorate the property as required:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public sealed class FormatAttribute : Attribute
{
    public FormatAttribute()
    {
        Trim = true;
    }

    public bool Trim { get; set; }
}

This is “activated” by an attribute on the property in the ViewModel

Public Person
{
    string firstName;
    [Format(Trim = true)]
    string SocialSecurityNumber;
}

It should be fairly simple to modify this to allow for encryption.