What is the easiest way to get the property value from a passed lambda expression in an extension method for HtmlHelper?

问题

I am writing a dirty little extension method for HtmlHelper so that I can say something like HtmlHelper.WysiwygFor(lambda) and display the CKEditor.

I have this working currently but it seems a bit more cumbersome than I would prefer. I am hoping that there is a more straight forward way of doing this.

Here is what I have so far.

public static MvcHtmlString WysiwygFor<TModel, TProperty>(this HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression)
{
    return MvcHtmlString.Create(string.Concat("<textarea class=\"ckeditor\" cols=\"80\" id=\"",
                                        expression.MemberName(), "\" name=\"editor1\" rows=\"10\">", 
                                        GetValue(helper, expression),
                                        "</textarea>"));
}

private static string GetValue<TModel, TProperty>(HtmlHelper<TModel> helper, Expression<Func<TModel, TProperty>> expression)
{
    MemberExpression body = (MemberExpression)expression.Body;
    string propertyName = body.Member.Name;
    TModel model = helper.ViewData.Model;
    string value = typeof(TModel).GetProperty(propertyName).GetValue(model, null).ToString();
    return value;
}

private static string MemberName<T, V>(this Expression<Func<T, V>> expression)
{
    var memberExpression = expression.Body as MemberExpression;
    if (memberExpression == null)
            throw new InvalidOperationException("Expression must be a member expression");

    return memberExpression.Member.Name;
}

Thanks!

回答1:

Try like this:

public static MvcHtmlString Try<TModel, TProperty>(
    this HtmlHelper<TModel> htmlHelper, 
    Expression<Func<TModel, TProperty>> expression
)
{
    var builder = new TagBuilder("textarea");
    builder.AddCssClass("ckeditor");
    builder.MergeAttribute("cols", "80");
    builder.MergeAttribute("name", "editor1");
    builder.MergeAttribute("id", expression.Name); // not sure about the id - verify
    var value = ModelMetadata.FromLambdaExpression(
        expression, htmlHelper.ViewData
    ).Model;
    builder.SetInnerText(value.ToString());
    return MvcHtmlString.Create(builder.ToString());
}

回答2:

ModelMetadata metadata = ModelMetadata.FromLambdaExpression(expression, htmlHelper.ViewData);
Object value = metadata.Model;
String name = metadata.PropertyName;

回答3:

I Know this is an old thread but just in case if someone is looking for it, the way to generate id / name attribute is also:

System.Web.Mvc.ExpressionHelper.GetExpressionText(expression);

I'm using this in my extensions and never had any issues with it. It also works great with nested properties.

回答4:

Simplest way is to wrap it all up in an extension method:

public static class ExtensionMethods
{   

    public static object Value<TModel, TProperty>(this Expression<Func<TModel, TProperty>> expression, ViewDataDictionary<TModel> viewData)
    {
        return ModelMetadata.FromLambdaExpression(expression, viewData).Model;
    }  

}

So the calling syntax is:

expression.Value(htmlHelper.ViewData)

回答5:

ASP.NET MVC 3 Futures includes a helper for that.

回答6:

This isn't addressed by either Peter or BigMomma's answer, but it combines both. If you're calling this from a controller method, where you don't have access to an HtmlHelper instance, just create a base controller method like this:

public ModelMetadata GetModelMetadata<TModel, TProperty>( TModel model, Expression<Func<TModel, TProperty>> expression )
{
    ViewData.Model = model; //model is null in Controller; you must set it here (or earlier) in order to extract values from the returned ModelMetadata.
    return ModelMetadata.FromLambdaExpression( expression, new ViewDataDictionary<TModel>( ViewData ) );
}

Then you can read what you need from the model metadata as usual;

var mm = GetModelMetaData( model, m => m.SomeProperty );
string name = mm.PropertyName;
object value = mm.Model;

来源：https://stackoverflow.com/questions/2846778/what-is-the-easiest-way-to-get-the-property-value-from-a-passed-lambda-expressio