Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions

Question

My model:

public class EmployeeModel
{
    [Required]
    [StringLength(50)]
    [Display(Name = "Employee Name")]
    public string EmpName { get; set; }

    [Required]
    [StringLength(150)]
    [Display(Name = "Email Id")]
    public string Email { get; set; }

    [Required]
    [Range(18, 150)]
    public int Age { get; set; }

}

In my view:

 @Html.MyEditFor(model=>model.EmpName)
 @Html.MyEditFor(model=>model.Email)
 @Html.MyEditFor(model=>model.Age)

My custom helper:

public static MvcHtmlString MyEditFor<TModel>(this HtmlHelper<TModel> html, Expression<Func<TModel, object>> expression)
    {
        var partial = html.Partial("Item", new LabelEditorValidation() { Label = html.LabelFor(expression), Editor = html.EditorFor(expression), Validation = html.ValidationMessageFor(expression) }).ToString();
        return MvcHtmlString.Create(partial);
    }

Item.cshtml - partial view:

 @model MyClientCustomValidation.Models.LabelEditorValidation 
        <tr>
            <td class="editor-label" style="border: 0;">
                @Model.Label
            </td>
            <td class="editor-field" style="border: 0">
                @Model.Editor
                @Model.Validation
            </td>
        </tr>

LabelEditorValidation - model for Item.cshtml:

      public class LabelEditorValidation
{
    public MvcHtmlString Validation { get; set; }
    public MvcHtmlString Label { get; set; }
    public MvcHtmlString Editor { get; set; }
}

I have exception

Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions

on line:

    var partial = html.Partial("Item", new LabelEditorValidation() { Label = html.LabelFor(expression), Editor = html.EditorFor(expression), Validation = html.ValidationMessageFor(expression) }).ToString();

The exception occures when @Html.MyEditFor is called for model.Age.

 @Html.MyEditFor(model=>model.Age) 

But it not occures when @Html.MyEditFor is called for model.EmpName and model.Email. That's because model.EmpName and model.Email are strings but model.Age is int

Answer 1

For the Google search users, Just don't call any Html.XyzFor methods like this

@Html.CheckBoxFor(model => model.Property***.MyMethod()***)

Use view models instead, to apply that MyMethod on the given property.

Answer 2

You could make your helper a little more generic and get rid of the object argument:

public static MvcHtmlString MyEditFor<TModel, TProperty>(
    this HtmlHelper<TModel> html, 
    Expression<Func<TModel, TProperty>> expression
)
{
    var partial = html.Partial(
        "Item", 
        new LabelEditorValidation 
        { 
            Label = html.LabelFor(expression), 
            Editor = html.EditorFor(expression), 
            Validation = html.ValidationMessageFor(expression) 
        }
    ).ToString();
    return MvcHtmlString.Create(partial);
}

Now your expression won't break as there won't be unnecessary boxing.