Question

Given a simple piece of code which can return the name of a property in VB.NET:

Function NameForProperty(Of T)(ByVal field As Expression(Of Action(Of T))) As String

    Dim expression = DirectCast(field.Body, MemberExpression)

    Return expression.Member.Name

End Function

Which works like this:

NameForProperty(Of String)(Function (s) s.Length) ' ==> returns "Length"

And what I thought would have been the equivalent in C#:

string NameForProperty<T>(Expression<Action<T>> field)
{
    var expression = (MemberExpression)field.Body;

    return expression.Member.Name;
}

When I try to call the C# version:

NameForProperty<string>(s=>s.Length);

It returns a compiler error:

Only assignment, call, increment, decrement, and new object
expressions can be used as a statement

My question is: what is the difference between the two pieces of code?

EDIT

Ivan has provided an answer as to why the code does not work in C#. I am still curious as to why it does work in VB.NET.

EDIT#2

To be clear, I’m not looking for code which works — simply why the code would work in VB.NET and not C#.

Answer 1

public static class TypeMember<TClass>
{
    public static string PropertyName<TProp>(Expression<Func<TClass, TProp>> expression)
    {
        var body = expression.Body as MemberExpression;

        if (body == null)
            throw new ArgumentException("'expression' should be a property expression");

        return body.Member.Name;
    }
}

var propName = TypeMember<string>.PropertyName(s => s.Length);

The problem with your version lies in the fact that you’re trying to use Action<T>. It returns nothing and thus lambda’s body (s.Length) should be a statement. And it is not a statement actually. So compiler complains about it.

It would complain in the same way if you’d wrote

public void A()
{
    string s = "abc";
    s.Length;
}

It is the same thing.

I’m not an expert in VB.NET though, so I can’t explain why it is working in VB, sorry.