Get reference to object from c# expression

Question

I have an extension generic method

public static void AddError<TModel>(
    this ModelStateDictionary modelState, 
    Expression<Func<TModel, object>> expression, 
    string resourceKey, 
    string defaultValue)
{
    // How can I get a reference to TModel object from expression here?
}

I need to get the reference to TModel object from expression. This method called by the following code:

ModelState.AddError<AccountLogOnModel>(
    x => x.Login, "resourceKey", "defaultValue")

Vibratory

There is no such object in an expression – x is a parameter of the expression, you're supposed to pass an object of the type into it. (Or I'm understanding what you want to achieve wrongly.) – Prostate

Do you really want the object or the text Login to use for the AddModelError(key, errorMessage) method? Use ExpressionHelper.GetExpressionText (built in to MVC) to get the property name from a lambda expression. – Christner

@subkamran you probably meant lambda expression... – Cohune

I need the reference to TModel object ) I don't want to pass another param with reference to object ) – Vibratory

Answer 1

You cannot get to the TModel object itself without passing it into the method. The expression you are passing in is only saying "take this property from a TModel". It isn't actually providing a TModel to operate on. So, I would refactor the code to something like this:

public static void AddError<TModel>(
    this ModelStateDictionary modelState, 
    TModel item,
    Expression<Func<TModel, object>> expression, 
    string resourceKey, 
    string defaultValue)
{
    // TModel's instance is accessible through `item`.
}

Then your calling code would look something like this:

ModelState.AddError<AccountLogOnModel>(
    currentAccountLogOnModel, x => x.Login, "resourceKey", "defaultValue")

Epigene

Thanks for reply, I don't see another way also. I done this already yesterday in my code ^) – Vibratory

Answer 2

I imagine you really want the text "Login" to use to add a new model error to the ModelStateDictionary.

public static void AddError<TModel>(this ModelStateDictionary modelState, 
  Expression<Func<TModel, object>> expression, string resourceKey, string defaultValue)
{
    var propName = ExpressionHelper.GetExpressionText(expression);

    modelState.AddModelError(propName, GetResource("resourceKey") ?? defaultValue);
}

Assume you have some resource factory/method that returns null if the resource isn't found, that's just for illustration.

Christner

Thanks for reply, but i need to save my error data such as resource key and default value into modelState object and TModel object. With expression i pass property to which error data related, that should be saved into TModel object. ) – Vibratory

To do that, you'll need to pass in the actual object as a parameter. A generic is not the object reference itself. – Christner