How to get the child declaring type from an expres

Question

I have a Parent / Child class hierarchy where the Parent abstractly declares a string property and the Child class implements it:

abstract class Parent
{
   public abstract string Value { get; }
}

class Child : Parent
{
   public override string Value { get { return null; } }
}

When I use an expression that explicitly (or implicitly) uses the Child class, I expect the Expressions's MemberInfo's DeclaringType to be 'Child', but instead it is Parent:

Child child = new Child();
Expression<Func<string>> expression = (() => child.Value);
MemberInfo memberInfo = expression.GetMemberInfo();
Assert.AreEqual(typeof(Child), memberInfo.DeclaringType); // FAILS!

The assertion fails because the DeclaringType is Parent.

Is there something I can do in declaring my expression or consuming it to reveal the actual use of the Child type?

NOTE: GetMemberInfo() above as an extension method (I even forgot we had written this!):

public static class TypeExtensions
{
    /// <summary>
    /// Gets the member info represented by an expression.
    /// </summary>
    /// <param name="expression">The member expression.</param>
    /// <returns>The member info represeted by the expression.</returns>
    public static MemberInfo GetMemberInfo(this Expression expression)
    {
        var lambda = (LambdaExpression)expression;

        MemberExpression memberExpression;
        if (lambda.Body is UnaryExpression)
        {
            var unaryExpression = (UnaryExpression)lambda.Body;
            memberExpression = (MemberExpression)unaryExpression.Operand;
        }
        else memberExpression = (MemberExpression)lambda.Body;

        return memberExpression.Member;
    }
}

Answer 1

If you don't want the method of the static type you work on, but rather the latest override, then it is possible. I didn't test, but something similar to the following should do the job:

public bool FindOverride(MethodInfo baseMethod, Type type)
{
    if(baseMethod==null)
      throw new ArgumentNullException("baseMethod");
    if(type==null)
      throw new ArgumentNullException("type");
    if(!type.IsSubclassOf(baseMethod.ReflectedType))
        throw new ArgumentException(string.Format("Type must be subtype of {0}",baseMethod.DeclaringType));
    while(true)
    {
        var methods=type.GetMethods(BindingFlags.Instance|
                                    BindingFlags.DeclaredOnly|
                                    BindingFlags.Public|
                                    BindingFlags.NonPublic);
        var method=methods.FirstOrDefault(m=>m.GetBaseDefinition()==baseMethod))
        if(method!=null)
          return method;
        type=type.BaseType;
    }
}

Where you pass the MemberInfo as the first param, and the runtime type of the object as second. Note that this is likely slow, so you might want to add some caching.

Answer 2

No - this is an accurate representation of what gets emitted by the C# compiler. The override is effectively ignored when looking for the member - the compiler only cares about the type that originally declared the member. You can see this for yourself by compiling code and then looking at the IL. This method:

static void Main()
{
    Child c = new Child();
    string x = c.Value;
}

is compiled into this IL:

IL_0000:  nop
IL_0001:  newobj     instance void Child::.ctor()
IL_0006:  stloc.0
IL_0007:  ldloc.0
IL_0008:  callvirt   instance string Parent::get_Value()
IL_000d:  stloc.1
IL_000e:  ret

One point of trivia: the VB compiler doesn't work the same way, so this method:

Public Shared Sub Main(Args As String())
    Dim x As Child = New Child()
    Dim y As String = x.Value
End Sub

is compiled as:

IL_0000:  newobj     instance void [lib]Child::.ctor()
IL_0005:  stloc.0
IL_0006:  ldloc.0
IL_0007:  callvirt   instance string [lib]Child::get_Value()
IL_000c:  stloc.1
IL_000d:  ret

Answer 3

My solution, based on information from @JonSkeet and @CodeInChaos is to not look purely at the PropertyInfo in the Expression, but also the Type of the MemberExpression's Member component:

/// <summary>
/// Extracts the PropertyInfo for the propertybeing accessed in the given expression.
/// </summary>
/// <remarks>
/// If possible, the actual owning type of the property is used, rather than the declaring class (so if "x" in "() => x.Foo" is a subclass overriding "Foo", then x's PropertyInfo for "Foo" is returned rather than the declaring base class's PropertyInfo for "Foo").
/// </remarks>
/// <typeparam name="T"></typeparam>
/// <param name="propertyExpression"></param>
/// <returns></returns>
internal static PropertyInfo ExtractPropertyInfo<T>(Expression<Func<T>> propertyExpression)
{
    if (propertyExpression == null)
    {
        throw new ArgumentNullException("propertyExpression");
    }

    var memberExpression = propertyExpression.Body as MemberExpression;
    if (memberExpression == null)
    {
        throw new ArgumentException(string.Format("Expression not a MemberExpresssion: {0}", propertyExpression), "propertyExpression");
    }

    var property = memberExpression.Member as PropertyInfo;
    if (property == null)
    {
        throw new ArgumentException(string.Format("Expression not a Property: {0}", propertyExpression), "propertyExpression");
    }

    var getMethod = property.GetGetMethod(true);
    if (getMethod.IsStatic)
    {
        throw new ArgumentException(string.Format("Expression cannot be static: {0}", propertyExpression), "propertyExpression");
    }

    Type realType = memberExpression.Expression.Type;
    if(realType == null) throw new ArgumentException(string.Format("Expression has no DeclaringType: {0}", propertyExpression), "propertyExpression");

    return realType.GetProperty(property.Name);
}