I am trying to use reflection to auto-generate a view. Html.DisplayFor and some of the other helpers take Expression<Func<,>> which derives from LambdaExpression. Seemed like I'd be able to manually generate my own lambda and then pass that in, but it's throwing this error:
The type arguments for method 'DisplayExtensions.DisplayFor<TModel, TValue>(HtmlHelper<TModel>, Expression<Func<TModel, TValue>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.`
Here's my markup:
<tr>
#foreach (var pi in Model.GetType().GetProperties())
{
<td>
#Html.DisplayFor(ExpressionHelpers.GetPropertyGetterLambda(pi))
</td>
}
</tr>
I am pretty sure what's happening is that .DisplayFor requires generic type arguments to infer the types for Func<TModel, TValue>, but I am using LambdaExpression which is hiding the types.
It seems like the only way to do what I want is to build/compile an expression that actually calls .DisplayFor using type-safe arguments, but that seems overly complicated.
Is there another way to achieve my goal or would I be better off just outputting the results directly to the HTML rather than calling the helpers?
Edit: Per request, here's the code for GetPropertyGetterLambda:
public static LambdaExpression GetPropertyGetterLambda(PropertyInfo pi, BindingTypeSafety TypeSafety)
{
if (pi.CanRead)
{
ParameterExpression entityParameter = Expression.Parameter(TypeSafety.HasFlag(BindingTypeSafety.TypeSafeEntity) ?
pi.ReflectedType : typeof(object));
LambdaExpression lambda = Expression.Lambda(GetPropertyReadExpression(entityParameter, pi, TypeSafety), entityParameter);
return lambda;
}
else
{
return null;
}
}
There is another solution for this that I recommend first. Change your for loop to an DisplayForModel call:
#Html.DisplayForModel()
That will render out all the properties, using DisplayFor internally. You can modify this by modifying the Object.cshtml in the DisplayTemplates folder (either in the Views folder your are working in, or in the Shared folder to apply globally).
If that is insufficient, or you really really want to use LambdaExpression, here is an alternative. It will require you adding a DisplayFor<TModel>(LambdaExpression expression) extension method. It would be something like (note that I haven't actually tested this, but this is close to what is needed):
public static IHtmlString DisplayFor<TModel>(this HtmlHelper<TModel> helper, LambdaExpression expression)
{
var wrapperClass = typeof(DisplayExtensions);
///find matching DisplayFor<TModel, TProperty> method
var matchingMethod = wrapperClass.GetMethods()
.Single( c => c.IsStatic
&& c.Name == "DisplayFor"
&& c.GetGenericArguments( ).Count() == 2
&& c.GetParameters( ).Count() == 2 //overloads don't have same # of parameters. This should be sufficient.
).MakeGenericMethod( typeof(TModel), expression.ReturnType ); //Make generic type from the TModel and the Return Type
//invoke the method. The result is a IHtmlString already, so just cast.
return (IHtmlString) matchingMethod.Invoke( null, new Object[] { helper, expression } );
}
Related
I've looked into this question and found questions such as:
SO-Link1
SO-Link2
So the problem seems rather common - but i really don't like the solution.
Question:
I've started digging into sources but couldn't find a convenient solution yet. Is there a built in solution to this by now using attributes?
Having to use dynamic parameters in place instead of using attributes on the model is very inconvenient.
Guess i'll see what i can achieve by modifying the ModelBinder in the meantime - there has to be some way i guess.
Update:
Why do i want to do this?
I want to reduce network traffic because properties in code may be long + nested. Imagine 200 Checkboxes x 40 bytes generated names.
I can already make my modelbinder work with aliases - however in order to fully automate it, i need the TextBoxFor etc methods to use alias names instead of the actual property names.
Personally I don't think you should be totally concerned with the size of the the name attribute's values. As long as you're using compression through something like IIS then you aren't going to be saving that much.
You can however, achieve what you're after by creating custom HTML helpers which will create the HTML markup you desire.
Example Model
public class UserModel
{
public string FullName { get; set; }
}
Helper
public static MvcHtmlString CustomTextBoxFor<TModel, TValue>(this HtmlHelper<TModel> html, Expression<Func<TModel, TValue>> expression, string name)
{
var fieldName = ExpressionHelper.GetExpressionText(expression);
//
// Pass in alias or call method to get alias here
//
var fullBindingName = String.IsNullOrWhiteSpace(name) ? html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(fieldName) : name;
var fieldId = TagBuilder.CreateSanitizedId(fullBindingName);
var metadata = ModelMetadata.FromLambdaExpression(expression, html.ViewData);
var value = metadata.Model;
var tag = new TagBuilder("input");
tag.Attributes.Add("name", fullBindingName);
tag.Attributes.Add("id", fieldId);
tag.Attributes.Add("type", "text");
tag.Attributes.Add("value", value == null ? "" : value.ToString());
var validationAttributes = html.GetUnobtrusiveValidationAttributes(fullBindingName, metadata);
foreach (var key in validationAttributes.Keys)
{
tag.Attributes.Add(key, validationAttributes[key].ToString());
}
return new MvcHtmlString(tag.ToString(TagRenderMode.SelfClosing));
}
Call the Method in your View
#Html.TextBoxFor(x => x.FullName)
#Html.CustomTextBoxFor(x => x.FullName, "FName")
Output
<input id="FullName" type="text" value="Heymega" name="FullName">
<input id="FName" type="text" value="Heymega" name="FName">
Is there a way to do something like the following:
public static IHtmlString TableFor(this HtmlHelper helper, IEnumerable<MaterialGroup> groups, Func<HtmlHelper, MaterialGroup, int, string> tableContentsFunc)
{
return MvcHtmlString.Create("#Html.TextBoxFor(x => this.Model.Something)");
}
Obviously this is a trivial example, but when ever I try something of the sort it renders the Helpers i.e. "#Html.TextBoxFor(x => this.Model.Something)" as text on the page instead of processing them as helpers.
Is there a way to achieve this?
public static MvcHtmlString TableFor(this HtmlHelper<IEnumerable<MaterialGroup>> helper, IEnumerable<MaterialGroup> groups, Func<HtmlHelper<MaterialGroup>, MaterialGroup, int, string> tableContentsFunc)
{
String html = "<table class='materials joists'>";
String endHtml = "</table>";
for (int i = 0; i < groups.Count(); ++i)
{
HtmlHelper<MaterialGroup> groupHelper = new HtmlHelper<MaterialGroup>(helper.ViewContext, helper.ViewDataContainer); // Crashes here with cannot convert IEnumerable<MaterialGroup> to MaterialGroup.
html += TbodyFor(groupHelper , groups.ElementAt(i), i);
html += tableContentsFunc(groupHelper , groups.ElementAt(i), i);
}
return MvcHtmlString.Create(html + endHtml);
}
public static string TbodyForJoists(this HtmlHelper<MaterialGroup> helper, MaterialGroup group, int index)
{
string html = string.Empty;
MvcHtmlString markTextbox = InputExtensions.TextBoxFor<MaterialGroup, String>(helper, x => group.Joists.ElementAt(i).Mark, new { Name = "MaterialGroups[" + index + "].Joists[" + i + "].Mark", Class = "auto-size first-column" });
html += martTextbox;
.
.
.
return html;
}
When I attempt the above I get issues with the HtmlHelper<> Types.
If I leave it with just HtmlHelper I get an error telling me to explicity state since it doesn't know what I doing with it. If I explicitly state is have conversion? issues I guess you could say.
How can I simply just use the TextBoxFor in this way?
Because that is very literally what you are telling it to do. The output of the helper itself is not processed by Razor. Whatever you return is what's going on the page.
However, you could always do:
MvcHtmlString textBox = Html.TextBoxFor(expression);
And, you'd have to feed the helper the expression to use. It perhaps would be easier in this situation to use Html.TextBox instead, but then you're going to have to do more work to try to figure out the right names and such for the fields.
It's going to be far easier and less convoluted to just use editor templates for this type of thing.
Is there a way to make a reflection for a view model property as an element with different name and id values on the html side.
That is the main question of what I want to achieve. So the basic introduction for the question is like:
1- I have a view model (as an example) which created for a filter operation in view side.
public class FilterViewModel
{
public string FilterParameter { get; set; }
}
2- I have a controller action which is created for GETting form values(here it is filter)
public ActionResult Index(FilterViewModel filter)
{
return View();
}
3- I have a view that a user can filter on some data and sends parameters via querystring over form submit.
#using (Html.BeginForm("Index", "Demo", FormMethod.Get))
{
#Html.LabelFor(model => model.FilterParameter)
#Html.EditorFor(model => model.FilterParameter)
<input type="submit" value="Do Filter" />
}
4- And what I want to see in rendered view output is
<form action="/Demo" method="get">
<label for="fp">FilterParameter</label>
<input id="fp" name="fp" type="text" />
<input type="submit" value="Do Filter" />
</form>
5- And as a solution I want to modify my view model like this:
public class FilterViewModel
{
[BindParameter("fp")]
[BindParameter("filter")] // this one extra alias
[BindParameter("param")] //this one extra alias
public string FilterParameter { get; set; }
}
So the basic question is about BindAttribute but the usage of complex type properties. But also if there is a built in way of doing this is much better.
Built-in pros:
1- Usage with TextBoxFor, EditorFor, LabelFor and other strongly typed view model helpers can understand and communicate better with each other.
2- Url routing support
3- No framework by desing problems :
In general, we recommend folks don’t write custom model binders
because they’re difficult to get right and they’re rarely needed. The
issue I’m discussing in this post might be one of those cases where
it’s warranted.
Link of quote
And also after some research I found these useful works:
Binding model property with different name
One step upgrade of first link
Here some informative guide
Result: But none of them give me my problems exact solution. I am looking for a strongly typed solution for this problem. Of course if you know any other way to go, please share.
Update
The underlying reasons why I want to do this are basically:
1- Everytime I want to change the html control name then I have to change PropertyName at compile time. (There is a difference Changing a property name between changing a string in code)
2- I want to hide (camouflage) real property names from end users. Most of times View Model property names same as mapped Entity Objects property names. (For developer readability reasons)
3- I don't want to remove the readability for developer. Think about lots of properties with like 2-3 character long and with mo meanings.
4- There are lots of view models written. So changing their names are going to take more time than this solution.
5- This is going to be better solution (in my POV) than others which are described in other questions until now.
Actually, there is a way to do it.
In ASP.NET binding metadata gathered by TypeDescriptor, not by reflection directly. To be more precious, AssociatedMetadataTypeTypeDescriptionProvider is used, which, in turn, simply calls TypeDescriptor.GetProvider with our model type as parameter:
public AssociatedMetadataTypeTypeDescriptionProvider(Type type)
: base(TypeDescriptor.GetProvider(type))
{
}
So, everything we need is to set our custom TypeDescriptionProvider for our model.
Let's implement our custom provider. First of all, let's define attribute for custom property name:
[AttributeUsage(AttributeTargets.Property)]
public class CustomBindingNameAttribute : Attribute
{
public CustomBindingNameAttribute(string propertyName)
{
this.PropertyName = propertyName;
}
public string PropertyName { get; private set; }
}
If you already have attribute with desired name, you can reuse it. Attribute defined above is just an example. I prefer to use JsonPropertyAttribute because in most cases I work with json and Newtonsoft's library and want to define custom name only once.
The next step is to define custom type descriptor. We will not implement whole type descriptor logic and use default implementation. Only property accessing will be overridden:
public class MyTypeDescription : CustomTypeDescriptor
{
public MyTypeDescription(ICustomTypeDescriptor parent)
: base(parent)
{
}
public override PropertyDescriptorCollection GetProperties()
{
return Wrap(base.GetProperties());
}
public override PropertyDescriptorCollection GetProperties(Attribute[] attributes)
{
return Wrap(base.GetProperties(attributes));
}
private static PropertyDescriptorCollection Wrap(PropertyDescriptorCollection src)
{
var wrapped = src.Cast<PropertyDescriptor>()
.Select(pd => (PropertyDescriptor)new MyPropertyDescriptor(pd))
.ToArray();
return new PropertyDescriptorCollection(wrapped);
}
}
Also custom property descriptor need to be implemented. Again, everything except property name will be handled by default descriptor. Note, NameHashCode for some reason is a separate property. As name changed, so it's hash code need to be changed too:
public class MyPropertyDescriptor : PropertyDescriptor
{
private readonly PropertyDescriptor _descr;
private readonly string _name;
public MyPropertyDescriptor(PropertyDescriptor descr)
: base(descr)
{
this._descr = descr;
var customBindingName = this._descr.Attributes[typeof(CustomBindingNameAttribute)] as CustomBindingNameAttribute;
this._name = customBindingName != null ? customBindingName.PropertyName : this._descr.Name;
}
public override string Name
{
get { return this._name; }
}
protected override int NameHashCode
{
get { return this.Name.GetHashCode(); }
}
public override bool CanResetValue(object component)
{
return this._descr.CanResetValue(component);
}
public override object GetValue(object component)
{
return this._descr.GetValue(component);
}
public override void ResetValue(object component)
{
this._descr.ResetValue(component);
}
public override void SetValue(object component, object value)
{
this._descr.SetValue(component, value);
}
public override bool ShouldSerializeValue(object component)
{
return this._descr.ShouldSerializeValue(component);
}
public override Type ComponentType
{
get { return this._descr.ComponentType; }
}
public override bool IsReadOnly
{
get { return this._descr.IsReadOnly; }
}
public override Type PropertyType
{
get { return this._descr.PropertyType; }
}
}
Finally, we need our custom TypeDescriptionProvider and way to bind it to our model type. By default, TypeDescriptionProviderAttribute is designed to perform that binding. But in this case we will not able to get default provider that we want to use internally. In most cases, default provider will be ReflectTypeDescriptionProvider. But this is not guaranteed and this provider is inaccessible due to it's protection level - it's internal. But we do still want to fallback to default provider.
TypeDescriptor also allow to explicitly add provider for our type via AddProvider method. That what we will use. But firstly, let's define our custom provider itself:
public class MyTypeDescriptionProvider : TypeDescriptionProvider
{
private readonly TypeDescriptionProvider _defaultProvider;
public MyTypeDescriptionProvider(TypeDescriptionProvider defaultProvider)
{
this._defaultProvider = defaultProvider;
}
public override ICustomTypeDescriptor GetTypeDescriptor(Type objectType, object instance)
{
return new MyTypeDescription(this._defaultProvider.GetTypeDescriptor(objectType, instance));
}
}
The last step is to bind our provider to our model types. We can implement it in any way we want. For example, let's define some simple class, such as:
public static class TypeDescriptorsConfig
{
public static void InitializeCustomTypeDescriptorProvider()
{
// Assume, this code and all models are in one assembly
var types = Assembly.GetExecutingAssembly().GetTypes()
.Where(t => t.GetProperties().Any(p => p.IsDefined(typeof(CustomBindingNameAttribute))));
foreach (var type in types)
{
var defaultProvider = TypeDescriptor.GetProvider(type);
TypeDescriptor.AddProvider(new MyTypeDescriptionProvider(defaultProvider), type);
}
}
}
And either invoke that code via web activation:
[assembly: PreApplicationStartMethod(typeof(TypeDescriptorsConfig), "InitializeCustomTypeDescriptorProvider")]
Or simply call it in Application_Start method:
public class MvcApplication : HttpApplication
{
protected void Application_Start()
{
TypeDescriptorsConfig.InitializeCustomTypeDescriptorProvider();
// rest of init code ...
}
}
But this is not the end of the story. :(
Consider following model:
public class TestModel
{
[CustomBindingName("actual_name")]
[DisplayName("Yay!")]
public string TestProperty { get; set; }
}
If we try to write in .cshtml view something like:
#model Some.Namespace.TestModel
#Html.DisplayNameFor(x => x.TestProperty) #* fail *#
We will get ArgumentException:
An exception of type 'System.ArgumentException' occurred in System.Web.Mvc.dll but was not handled in user code
Additional information: The property Some.Namespace.TestModel.TestProperty could not be found.
That because all helpers soon or later invoke ModelMetadata.FromLambdaExpression method. And this method take expression we provided (x => x.TestProperty) and takes member name directly from member info and have no clue about any of our attributes, metadata (who cares, huh?):
internal static ModelMetadata FromLambdaExpression<TParameter, TValue>(/* ... */)
{
// ...
case ExpressionType.MemberAccess:
MemberExpression memberExpression = (MemberExpression) expression.Body;
propertyName = memberExpression.Member is PropertyInfo ? memberExpression.Member.Name : (string) null;
// I want to cry here - ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// ...
}
For x => x.TestProperty (where x is TestModel) this method will return TestProperty, not actual_name, but model metadata contains actual_name property, have no TestProperty. That is why the property could not be found error thrown.
This is a design failure.
However despite this little inconvenience there are several workarounds, such as:
The easiest way is to access our members by theirs redefined names:
#model Some.Namespace.TestModel
#Html.DisplayName("actual_name") #* this will render "Yay!" *#
This is not good. No intellisense at all and as our model change we will have no any compilation errors. On any change anything can be broken and there is no easy way to detect that.
Another way is a bit more complex - we can create our own version of that helpers and forbid anybody from calling default helpers or ModelMetadata.FromLambdaExpression for model classes with renamed properties.
Finally, combination of previous two would be preferred: write own analogue to get property name with redefinition support, then pass that into default helper. Something like this:
#model Some.Namespace.TestModel
#Html.DisplayName(Html.For(x => x.TestProperty))
Compilation-time and intellisense support and no need to spend a lot of time for complete set of helpers. Profit!
Also everything described above work like a charm for model binding. During model binding process default binder also use metadata, gathered by TypeDescriptor.
But I guess binding json data is the best use case. You know, lots of web software and standards use lowercase_separated_by_underscores naming convention. Unfortunately this is not usual convention for C#. Having classes with members named in different convention looks ugly and can end up in troubles. Especially when you have tools that whining every time about naming violation.
ASP.NET MVC default model binder does not bind json to model the same way as it happens when you call newtonsoft's JsonConverter.DeserializeObject method. Instead, json parsed into dictionary. For example:
{
complex: {
text: "blabla",
value: 12.34
},
num: 1
}
will be translated into following dictionary:
{ "complex.text", "blabla" }
{ "complex.value", "12.34" }
{ "num", "1" }
And later these values along with others values from query string, route data and so on, collected by different implementations of IValueProvider, will be used by default binder to bind a model with help of metadata, gathered by TypeDescriptor.
So we came full circle from creating model, rendering, binding it back and use it.
The short answer is NO and long answer still NO. There is no built-in helper, attribute, model binder, whatever is it (Nothing out of box).
But what I did in before answer (I deleted it) was an awful solution that I realized yesterday. I am going to put it in github for who still wants to see (maybe it solves somebody problem) (I don't suggest it also!)
Now I searched it for again and I couldn't find anything helpful. If you are using something like AutoMapper or ValueInjecter like tool for mapping your ViewModel objects to Business objects and if you want to obfuscate that View Model parameters also, probably you are in some trouble. Of course you can do it but strongly typed html helpers are not going to help you alot. I even not talking about the if other developers taking branch and working over common view models.
Luckily my project (4 people working on it, and its commercial use for) not that big for now, so I decided to change View Model property names! (It is still lot work to do. Hundreds of view models to obfuscate their properties!!!) Thank you Asp.Net MVC !
There some ways in the links which I gave in question. But also if you still want to use the BindAlias attribute, I can only suggest you to use the following extension methods. At least you dont have to write same alias string which you write in BindAlias attribute.
Here it is:
public static string AliasNameFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression)
{
var memberExpression = ExpressionHelpers.GetMemberExpression(expression);
if (memberExpression == null)
throw new InvalidOperationException("Expression must be a member expression");
var aliasAttr = memberExpression.Member.GetAttribute<BindAliasAttribute>();
if (aliasAttr != null)
{
return MvcHtmlString.Create(aliasAttr.Alias).ToHtmlString();
}
return htmlHelper.NameFor(expression).ToHtmlString();
}
public static string AliasIdFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
Expression<Func<TModel, TProperty>> expression)
{
var memberExpression = ExpressionHelpers.GetMemberExpression(expression);
if (memberExpression == null)
throw new InvalidOperationException("Expression must be a member expression");
var aliasAttr = memberExpression.Member.GetAttribute<BindAliasAttribute>();
if (aliasAttr != null)
{
return MvcHtmlString.Create(TagBuilder.CreateSanitizedId(aliasAttr.Alias)).ToHtmlString();
}
return htmlHelper.IdFor(expression).ToHtmlString();
}
public static T GetAttribute<T>(this ICustomAttributeProvider provider)
where T : Attribute
{
var attributes = provider.GetCustomAttributes(typeof(T), true);
return attributes.Length > 0 ? attributes[0] as T : null;
}
public static MemberExpression GetMemberExpression<TModel, TProperty>(Expression<Func<TModel, TProperty>> expression)
{
MemberExpression memberExpression;
if (expression.Body is UnaryExpression)
{
var unaryExpression = (UnaryExpression)expression.Body;
memberExpression = (MemberExpression)unaryExpression.Operand;
}
else
{
memberExpression = (MemberExpression)expression.Body;
}
return memberExpression;
}
When you want to use it:
[ModelBinder(typeof(AliasModelBinder))]
public class FilterViewModel
{
[BindAlias("someText")]
public string FilterParameter { get; set; }
}
In html:
#* at least you dont write "someText" here again *#
#Html.Editor(Html.AliasNameFor(model => model.FilterParameter))
#Html.ValidationMessage(Html.AliasNameFor(model => model.FilterParameter))
So I am leaving this answer here like this. This is even not an answer (and there is no answer for MVC 5) but who searching in google for same problem might find useful this experience.
And here is the github repo: https://github.com/yusufuzun/so-view-model-bind-20869735
I'm writing an HTML Helper for an editor. The idea is to get properties from the Model with attributes AutoGenerateField and build a table, each line of which contains a name of a field (also from the attribute) and a TextBox or a CheckBox containing actual value of the field.
I have a problem with HTMLHelper. Since I send the whole model to the helper and not one value, I cannot use methods such as TextBoxFor, as they need parameter, such as
"Expression<Func<TModel, TValue>> expression".
I'm using reflexion and I tried to send the property instead, still VisualStudio considers this as incorrect usage.
Below is simplified method for my HtmlHelper:
public static MvcHtmlString GenerateEditor<TModel>(this HtmlHelper<TModel> htmlHelper)
{
var model = htmlHelper.ViewData.Model;
var result = String.Empty;
//generating container, etc ...
foreach (var property in model.GetType().GetProperties())
{
var attr = property.GetCustomAttributes(typeof (DisplayAttribute), true).FirstOrDefault();
if (attr == null) continue;
var autoGenerate = ((DisplayAttribute)attr).AutoGenerateField;
if(autoGenerate)
{
//here I'm building the html string
//My problem is in the line below:
var r = htmlHelper.TextBoxFor(property);
}
}
return MvcHtmlString.Create(result);
}
Any ideas?
How about just using the non-lambda overloads. : InputExtensions.TextBox()
if(autoGenerate)
{
//here I'm building the html string
//My problem is in the line below:
var r = htmlHelper.TextBox(property.Name);
}
//not sure what you do with r from here...
If I'm not mistaken the name attribute of the form element is set to the property name even when you use the lambda version of the function so this should do the same thing.
I will try and verify what the lambda function does, you might be able to do the same since you have the TModel with you.
Update
From a quick look of things inside the source code of InputExtensions.cs, TextBoxFor calls eventually calls InputHelper() which eventually calls ExpressionHelper.GetExpressionText(LambdaExpression expression) inside ExpressionHelper.cs which from the cursory looks of things gets the member.Name for the name html attribute on the input element.
I can't quite verify it right now because I'm not on windows but I think the non-lambda function should suit your need. Do tell me how it goes?
I've got some data, and sometimes that data is blank. Instead of making a bunch of crazy logic on my view I'd rather use a helper method that will render the data if it exists, and render some HTML that just says "N/A" when the string is empty/null.
Ideal syntax: #Helpers.RenderThisOrThat(Model.CustomerPhone)
If the Model.CustomerPhone (a string) is empty it will render this alternate HTML instead: <span class='muted'>N/A</span>
Here's what we have so far:
#helper RenderThisOrThat(string stringToRender, string methodToGetAlternateText = null)
{
#RenderThisOrThat(MvcHtmlString.Create(stringToRender), methodToGetAlternateText)
}
#helper RenderThisOrThat(MvcHtmlString stringToRender, string methodToGetAlternateText = null)
{
if (string.IsNullOrWhiteSpace(stringToRender.ToHtmlString()))
{
if (!string.IsNullOrWhiteSpace(methodToGetAlternateText)) {
#methodToGetAlternateText
}
<span class='muted'>N/A</span>
}
#stringToRender
}
This works just fine until we want to pass something other than a string into either parameter. For example when we have an email address we want it to be a link to that email and not just the string of the email.
#Helpers.RenderThisOrThat(##Html.DisplayFor(m => m.CustomerEmail))
It gives us the error: "Cannot convert lambda expression to type 'string' because it is not a delegate type"
We are at a loss for how to make this work... any help here?
You're looking for a helpers that will take a string and:
If the string is not empty, render that string.
If the string is not empty, render a given template.
If the string is empty, render "N/A" html.
If the string is empty, render a given template.
When passing a razor block to a function as a parameter, razor packages the block as Func. Change the parameters in the helper functions to take that type of delegate and don't forget to call those delegates (I chose to pass null).
These helpers should handle those scenarios.
Solution
#helper RenderThisOrThat(string stringToRender, Func<object, IHtmlString> leftTemplate = null, Func<object, IHtmlString> rightTemplate = null)
{
var shouldRenderLeft = !string.IsNullOrWhiteSpace(stringToRender);
leftTemplate = leftTemplate ?? (o => MvcHtmlString.Create(stringToRender));
#RenderThisOrThat(shouldRenderLeft, leftTemplate, rightTemplate)
}
#helper RenderThisOrThat(bool shouldRenderLeft, Func<object, IHtmlString> leftTemplate, Func<object, IHtmlString> rightTemplate = null)
{
var shouldRenderRight = !shouldRenderLeft;
if (shouldRenderRight)
{
if (rightTemplate != null)
{
#rightTemplate(null)
}
else
{
<span class='muted'>N/A</span>
}
}
else
{
#leftTemplate(null)
}
}
Examples
1. #Helpers.RenderThisOrThat(Model.StringWithBob)
2. #Helpers.RenderThisOrThat(Model.StringWithNull)
3. #Helpers.RenderThisOrThat(Model.StringWithBob, #<span>I'm #Model.StringWithBob</span>)
4. #Helpers.RenderThisOrThat(Model.StringWithNull, #<span>I'm #Model.StringWithBob</span>)
5. #Helpers.RenderThisOrThat(Model.StringWithBob, #<span>I'm #Model.StringWithBob</span>, #<span>What about Bob?</span>)
6. #Helpers.RenderThisOrThat(Model.StringWithNull, #<span>I'm #Model.StringWithBob</span>, #<span>What about Bob?</span>)
Will output:
Bob
<span class='muted'>N/A</span>
<span>I'm Bob</span>
<span class='muted'>N/A</span>
<span>I'm Bob</span>
<span>What about Bob?</span>
This is an awfully complex solution to a simple problem. You don't need to create complex views, in fact, you should be using an Editor/DisplayTemplate, then you put your logic in the template and it's done once, without all the need for extra inclusion of helper functions, or anything else.
You can also go a step further here, because in this case you're rendering an email address. You apply a DataType attribute to your model and then specify an Phone Number rendering type.
public class MyModel {
[DataType(DataType.PhoneNumber)]
public string PhoneNumber {get;set;}
}
Then you create a folder in ~/Views/Shared called DisplayTemplates and in that folder create a file called PhoneNumber.cshtml, in it you do this:
#model string
#if (string.IsEmptyOrWhiteSpace(Model)) {
#:<span class='muted'>N/A</span>
} else {
#: <span>#Model</span>
}
Then, in your view:
#Html.DisplayFor(x => x.PhoneNumber)
That's it, no complex logic in your view, no convluted helper functions everywhere. This is simple, easy, and maintainable. You can do the same for Email address as there is an EmailAddress datatype as well.
MVC has a lot of very good functionality built-in that most people simply do not use, because they haven't spent any real time learning it.