Question

I’m wondering if following code is “safe”. By “safe” I mean that I don’t depend on some specific compiler version or undocumented feature.
I want to get string with property / field name, but I want to declare it using strong typing (I want the compiler to check if specific field / property exists).
My method looks like this:

string GetPropertyName<T>(Expression<Func<T, object>> expression)
{
    if (expression.Body is UnaryExpression)
    {
        var operand = ((UnaryExpression)expression.Body).Operand.ToString();
        return operand.Substring(operand.IndexOf(".") + 1);
    }
    else if (expression.Body is MemberExpression)
    {
        return ((MemberExpression)expression.Body).Member.Name;
    }
    else
    {
        throw new NotImplementedException();
    }            
}

And here is how I want to use it:

class Foo
{
    public string A { get; set; }
    public Bar B { get; set; }
}

class Bar
{
    public int C { get; set; }
    public Baz D { get; set; }
}

class Baz
{
    public int E { get; set; }
}


GetPropertyName<Foo>(x => x.A)
GetPropertyName<Foo>(x => x.B)
GetPropertyName<Foo>(x => x.B.C)
GetPropertyName<Foo>(foo => foo.B.D.E)

Thanks in advance for help.

Answer 1

I’m not sure that the output of the ToString method is guaranteed in any way. The documentation just says that it “returns a textual representation of the Expression“.

(I suspect that the output is unlikely to change across different platforms/versions, but I’d be a bit reluctant to rely on it when your aim is to use strong typing, compile-time checks etc.)

Here’s my method that does something similar without using ToString:

public static string GetPropertyName<T>(Expression<Func<T, object>> e)
{
    MemberExpression me;
    switch (e.Body.NodeType)
    {
        case ExpressionType.Convert:
        case ExpressionType.ConvertChecked:
            var ue = e.Body as UnaryExpression;
            me = ((ue != null) ? ue.Operand : null) as MemberExpression;
            break;
        default:
            me = e.Body as MemberExpression;
            break;
    }

    if (me == null)
        throw new ArgumentException("Expression must represent field or property access.", "e");

    var stack = new Stack<string>();

    do
    {
        stack.Push(me.Member.Name);
        me = me.Expression as MemberExpression;
    } while (me != null);

    return string.Join(".", stack);    // use "stack.ToArray()" on .NET 3.5
}