{"id":33,"date":"2010-04-17T14:04:01","date_gmt":"2010-04-17T14:04:01","guid":{"rendered":"http:\/\/blog.joefield.co.uk\/?p=33"},"modified":"2010-05-02T23:51:41","modified_gmt":"2010-05-02T23:51:41","slug":"digging-around-textboxfor-in-asp-net-mvc-2","status":"publish","type":"post","link":"https:\/\/blog.joefield.co.uk\/?p=33","title":{"rendered":"Digging around: TextBoxFor in ASP.NET MVC 2"},"content":{"rendered":"

This is a companion post to Scott Guthrie’s ASP.NET MVC 2: Strongly Typed Html Helpers<\/a>.<\/p>\n

I was interested in what made this code:<\/p>\n

<%= Html.TextBoxFor(model => model.ProductName) %><\/pre>\n
in an aspx view template produce this html:<\/p>\n
<input id="ProductName" name="ProductName" type="text" value="Aniseed Syrup" \/><\/pre>\n
In order to do it ASP.NET MVC has performed two distinct operations:<\/p>\n
    \n
  1. parsed the expression tree produced by the compiler for model => model.ProductName<\/strong> to extract the name of the property ("ProductName") to use in the id and name attributes of the input element<\/li>\n
  2. compiled the expression and invoked the resulting delegate to produce the "Aniseed Syrup" value<\/li>\n<\/ol>\n
    TextBoxFor extension method<\/h4>\n
    The call to Html.TextBoxFor ends up in the following extension method in the InputExtensions class:<\/p>\n
    public static MvcHtmlString TextBoxFor<TModel, TProperty>( \n    this HtmlHelper<TModel> htmlHelper, \n    Expression<Func<TModel, TProperty>> expression, \n    IDictionary<string, object> htmlAttributes) \n{ \n    return TextBoxHelper( \n        htmlHelper, \n        ModelMetadata.FromLambdaExpression(\n            expression, \n            htmlHelper.ViewData).Model, \n        ExpressionHelper.GetExpressionText(expression), \n        htmlAttributes); \n}<\/pre>\n
    I’ve highlighted the second and third parameters. The call to TextBoxHelper() does the final rendering by:<\/p>\n
      \n
    • calling Convert.ToString() on the second parameter to populate the value attribute of the input element<\/li>\n
    • using the third parameter to populate the name and id attributes of the input element<\/li>\n<\/ul>\n
      But let’s backtrack a bit first and look at the TextBoxFor signature itself.<\/p>\n
      Quick detour to Expression<Func<TModel, TProperty>><\/h4>\n
      Here’s the signature of the TextBoxFor method in the previous section:<\/p>\n
      public static MvcHtmlString TextBoxFor<TModel, TProperty>( \n    this HtmlHelper<TModel> htmlHelper, \n    Expression<Func<TModel, TProperty>> expression, \n    IDictionary<string, object> htmlAttributes) \n{ \n... \n... \n} <\/pre>\n
      By declaring a parameter of type Expression<Func<TModel, TProperty>><\/strong> the TextBoxFor method has asked the compiler to generate an expression tree rather than a delegate. <\/p>\n
      Expression<TDelegate><\/strong> is derived from LambdaExpression, which represents an expression tree which can be compiled into a delegate by calling its Compile() method.<\/p>\n
      Interestingly, the implementation of Compile() in Expression<TDelegate><\/strong> calls Compile() on its base and casts the resulting delegate to TDelegate<\/strong>, so its a pretty thin wrapper for LambdaExpression.<\/p>\n
      All these uses of the term "lambda expression" can be mighty confusing. We call<\/p>\n
      model => model.ProductName<\/pre>\n
      a lambda expression when we see it in code. The compiler however will treat it as an anonymous function or an expression tree, depending on the context<\/p>\n
      If it’s being assigned to Func<TModel, TProperty><\/strong> it is treated as an anonymous function.<\/p>\n
      But if it’s being assigned to Expression<Func<TModel, TProperty>><\/strong> then it is treated as an expression tree.<\/p>\n
      Anyway, back to the TextBoxFor method.<\/p>\n
      ExpressionHelper.GetExpressionText<\/h4>\n
      public static MvcHtmlString TextBoxFor<TModel, TProperty>( \n    this HtmlHelper<TModel> htmlHelper, \n    Expression<Func<TModel, TProperty>> expression, \n    IDictionary<string, object> htmlAttributes) \n{ \n    return TextBoxHelper( \n        htmlHelper, \n        ModelMetadata.FromLambdaExpression(\n            expression, \n            htmlHelper.ViewData).Model, \n        ExpressionHelper.GetExpressionText(expression), \n        htmlAttributes); \n} <\/pre>\n
      We want to get the text with which to populate the id and name attributes of our text box.<\/p>\n
      Because MVC has an expression tree rather than a delegate it can can pick it apart to find out, in this case, the name of the property being accessed.<\/p>\n
      Here’s my stripped down version of the implementation of GetExpressionText()<\/strong>, which covers the case of our simple model.ProductName property access:<\/p>\n
      public static string GetExpressionText(LambdaExpression expression) \n{ \n    MemberExpression memberExpression = (MemberExpression)expression.Body; \n    return memberExpression.Member.Name; \n}<\/pre>\n
      The body of the expression is cast to a MemberExpression so that its Member.Name property can be accessed.<\/p>\n
      This returns "ProductName".<\/p>\n
      That was easy. Now on to the evaluation of the expression itself.<\/p>\n
      ModelMetadata.FromLambdaExpression<\/h4>\n
      public static MvcHtmlString TextBoxFor<TModel, TProperty>( \n    this HtmlHelper<TModel> htmlHelper, \n    Expression<Func<TModel, TProperty>> expression, \n    IDictionary<string, object> htmlAttributes) \n{ \n    return TextBoxHelper( \n        htmlHelper, \n        ModelMetadata.FromLambdaExpression( \n            expression, \n            htmlHelper.ViewData).Model, \n        ExpressionHelper.GetExpressionText(expression), \n        htmlAttributes); \n} <\/pre>\n
      OK so we’ve got hold of of the string <\/p>\n","protected":false},"excerpt":{"rendered":"
      This is a companion post to Scott Guthrie’s ASP.NET MVC 2: Strongly Typed Html Helpers. I was interested in what made this code: <%= Html.TextBoxFor(model => model.ProductName) %> in an aspx view template produce this html: <input id="ProductName" name="ProductName" type="text" value="Aniseed Syrup" \/> In order to do it ASP.NET MVC has performed two distinct operations: […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[6],"tags":[],"_links":{"self":[{"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/33"}],"collection":[{"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=33"}],"version-history":[{"count":65,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/33\/revisions"}],"predecessor-version":[{"id":108,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/33\/revisions\/108"}],"wp:attachment":[{"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=33"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=33"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.joefield.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=33"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}