Cannot get this property name using this code and expression api

Question

I have the following class and I need to get its property names:

public class PMLButtonData
{
    public int BackgroundColorID
    {
        get;
        set;
    }

    public string Callback
    {
        get;
        set;
    }
}

To get the names I'm using this function

public static string GetPropertyName<T>(Expression<Func<T, object>> lambda)
{
    MemberExpression member = lambda.Body as MemberExpression;
    PropertyInfo property = member.Member as PropertyInfo;

    return property.Name;
}

I can get the Callback property name using this code:

string name = GetPropertyName<PMLButtonData>(x => x.Callback);

But the same code for the other property doesn't work:

string name = GetPropertyName<PMLButtonData>(x => x.BackgroundColorID);

The only difference between them is the data type, so I changed the Callback to int and the code does not work with this property anymore. Why can't I get the name of a property this way if it's an integer?

Answer 1

The problem is the type of your expression tree - you're trying to represent a delegate of type Func<T, object>, and if the property returns an int, that means it would need to be converted. You just need to make the method generic in both the source and target types:

public static string GetPropertyName<TSource, TTarget>
    (Expression<Func<TSource, TTarget>> lambda)

Now you should be able to do:

string name = GetPropertyName<PMLButtonData, int>(x => x.BackgroundColorID);

I realize that's slightly annoying, but you can trampoline via a generic type instead, so you only need to infer a single type parameter:

public static class PropertyName<TSource>
{
    public static string Get<TTarget>(Expression<Func<TSource, TTarget>> lambda)
    {           
        // Use casts instead of "as" to get more meaningful exceptions.
        var member = (MemberExpression) lambda.Body;
        var property = (PropertyInfo) member.Member;
       return property.Name;
    }
}

Then:

string name = PropertyName<PMLButtonData>.Get(x => x.BackgroundColorID);

Of course, in C# 6 you don't need any of this nonsense:

string name = nameof(PMLButtonData.BackgroundColorId);

:)