Member Expression cannot convert to object from nullable decimal

Question

I am working on an MVC project and would like to pass the Html.TextboxFor method the name of a property. This is my viewmodel

public class RuleViewModel<T> where T : class, IValidatableObject
{
    private T _rule;
    public T RuleModel
    {
        get
        {
            return _rule
                ?? (_rule = Activator.CreateInstance<T>());
        }
    }

    public RuleMetadata Metadata { get; set; }

    public Expression<Func<RuleViewModel<T>, Object>> GetParameterByName(PropertyInfo pi)
    {
        var fieldName = Expression.Parameter(typeof(RuleViewModel<T>), "x");
        var fieldExpression = Expression.PropertyOrField(Expression.Property(fieldName, "RuleModel"), pi.Name);
        var exp = Expression.Lambda<Func<RuleViewModel<T>, Object>>(fieldExpression, fieldName);
        return exp;
    }

}

Then in the view I do this

  @foreach (var prop in Model.RuleModel.GetType().GetProperties())
    {
        var result = Model.Metadata.Columns.SingleOrDefault(m => m.ColumnName == prop.Name);
        if (result != null)
        {
            <td>
                @Html.TextBoxFor(Model.GetParameterByName(prop))
            </td>
        }
    }

The problem is that when the property is of type decimal?, I get a cannot convert nullable decimal to object error. I looked around and found that you can use Expression.Convert to fix this but when I do that I get an error on the view

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

Any help would be appreciated. This is a proof of concept project that I'm working on and without this piece it is dead in the water.

Mike Strobel Mike Strobel · Accepted Answer · 2014-02-13T16:22:10

The problem is that you can't just use object as TProperty when calling TextBoxFor<TModel, TProperty>(). It expects a lambda expression of the form Func<TModel, TProperty>, and the variance rules of C# are such that a Func<TModel, decimal?> is not assignment compatible with Func<TModel, object>. You also can't simply use Convert(), because the MVC internals won't accept a lambda whose body is a Convert expression.

What you can do is use dynamic binding to invoke TextBoxFor<TModel, TProperty>() with the correct type arguments:

public Expression GetParameterByName(PropertyInfo pi)
{
    var fieldName = Expression.Parameter(typeof(RuleViewModel<T>), "x");
    var fieldExpression = Expression.PropertyOrField(
        Expression.Property(fieldName, "RuleModel"),
        pi.Name);
    var exp = Expression.Lambda(
        typeof(Func<,>).MakeGenericType(typeof(RuleViewModel<T>), fieldExpression.Type),
        fieldExpression,
        fieldName);
    return exp;
}

// ...

@InputExtensions.TextBoxFor(Html, (dynamic)Model.GetParameterByName(prop))

Member Expression cannot convert to object from nullable decimal

1 Answers