`ModelExpression` equivalent of `m => m`: Better support `asp-for="@Model"`

[alex-novikov-1990]

How to use label tag helper in the following scenario in a display template? asp-for="@Model" leads to invalid label text ("Model").

@Html.LabelFor(m => m, new { @class = "control-label col-md-2" })

[NTaylorMullen]

<label asp-for="@Model" class="control-label col-md-2"></label>

[alex-novikov-1990]

In a display template (e.g. @Html.DisplayFor(m => m.UserName)):

• <label asp-for="@Model"></label> results in <label for="UserName">Model</label>
• @Html.LabelFor(m => m) results in <label for="UserName">UserName</label>

[dougbu]

I'm able to reproduce this as well. Haven't debugged what is going South but it does look strange, especially given the for attribute has the correct value.

[dougbu]

FYI problem relates to how the ModelMetadata for the property is found. In the @Html.LabelFor(m => m) case, PropertyName is "UserName". But in the <label asp-for="@Model"></label> case, PropertyName is "Model". Looking a bit deeper...

[dougbu]

🆗 the issue here is that ExpressionMetadataProvider.FromLambdaExpression() correctly handles m => m and other identity expressions (ExpressionType.Parameter) but does not understand that m => {generated class}.Model needs special handling. That is, @Html.LabelFor(m => Model) hits exactly the same naming issue.

Unfortunately tag helpers do not support lambda identity expressions and always hit this issue if the @Model "fallback" is used. We probably didn't see this before because (a) the ModelMetadata.PropertyName property isn't used extensively in HTML generation and (b) @Model.UserName, @Model[i].UserName, and other lambda RHS that start with @Model work fine.

One caveat on (a): strings generated for client-side validation rules can be messed up when PropertyName is incorrect e.g. data-val-required="The Model field is required.".

[dougbu]

And, no, I don't know of an RC1 workaround other than to use HTML helpers for this case.