Question

I’m currently attempting to create an HtmlHelper which takes in the same kind of expression as the built-in helpers LabelFor<>, DisplayFor<>, EditorFor<>, etc. but specifically for enumerated types:

e.g. model => model.MyEnumProperty

I’m new to the whole lambda expression thing and although I have been doing more or less okay so far (with a lot of help from other answers by the SackOverflow community) I’m now getting the following exception while trying to retrieve the object (i.e., model) in the expression:

“variable ‘model’ of type ‘WCSFAMembershipDatabase.Models.Address’ referenced from scope ”, but it is not defined”

public static MvcHtmlString EnumDisplayFor<TModel, TEnum>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TEnum>> expression)
{
    // memberExp represents "model.MyEnumProperty"
    MemberExpression memberExp = (MemberExpression)expression.Body;
    if (memberExp == null)
    {
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            expression.ToString()));
    }

    // modelExp represents "model"
    Expression modelExp = memberExp.Expression;

    // Convert modelExp to a Lambda Expression that can be compiled into a delegate that returns a 'model' object
    Expression<Func<TModel>> modelLambda = Expression.Lambda<Func<TModel>>(modelExp);

    // Compile modelLambda into the delegate
    // The next line is where the exception occurs...
    Func<TModel> modelDel = modelLambda.Compile();

    // Retrieve the 'model' object
    TModel modelVal = modelDel();

    // Compile the original expression into a delegate that accepts a 'model' object and returns the value of 'MyEnumProperty'
    Func<TModel, TEnum> valueDel = expression.Compile();

    // Retrieve 'MyEnumProperty' value
    TEnum value = valueDel(modelVal);

    // return the description or string value of 'MyEnumProperty'
    return MvcHtmlString.Create(GetEnumDescription(value));
}

// Function returns the Description Attribute (if one exists) or the string 
// representation for the specified enum value.
private static string GetEnumDescription<TEnum>(TEnum value)
{
    FieldInfo fi = value.GetType().GetField(value.ToString());

    DescriptionAttribute[] attributes = (DescriptionAttribute[])fi.GetCustomAttributes(typeof(DescriptionAttribute), false);

    if ((attributes != null) && (attributes.Length > 0))
        return attributes[0].Description;
    else
        return value.ToString();
}

The expression related code in EnumDisplayFor was cobbled together from details found at the following locations:

• http://blogs.msdn.com/b/csharpfaq/archive/2010/03/11/how-can-i-get-objects-and-property-values-from-expression-trees.aspx
• https://stackoverflow.com/a/672212

I did locate a few other questions that mention the same exception in relation to lambda expressions but they were all in a context where someone was manually crafting the expression tree and I couldn’t figure out how the information in the answers might apply to my case.

I would really appreciate if anyone can explain (a) why the exception is occurring and (b) how I can fix it. 🙂

Thanks, in advance.

Answer 1

As Jon has said, trying to extract the model parameter from the expression doesn’t work, as the expression simply does not have that parameter.

When you create the lambda model => model.Property you are only saying how do you want your method body to be. That lambda compiles to a delegate Func, or, in other words, a method that needs one parameter and returns a value.

So in order to call a Func you need to pass one parameter, in this case, the model parameter.

In your example, your method needs to get the model from somewhere before calling “expression” to get the return value. You can get the current model from HtmlHelper parameter, like this:

public static MvcHtmlString EnumDisplayFor<TModel, TEnum>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TEnum>> expression)
    {
        //Get the model
        TModel model = htmlHelper.ViewData.Model;

        //Compile expression as Func
        Func<TModel, TEnum> method = expression.Compile();

        //Calling compiled expression return TEnum
        TEnum enumValue = method(model);

        return MvcHtmlString.Create(GetEnumDescription(enumValue));
    }