How can I get object instance from ()=>foo.Title expression

Question

I have a simple class with a property

class Foo 
{ 
    string Title { get; set; } 
}

I am trying to simplify data binding by calling a fun

Answer 1

Small LINQPad sample of what you want :

void Foo<T>(Expression<Func<T>> prop)
{
    var propertyGetExpression = prop.Body as MemberExpression;

    // Display the property you are accessing, here "Height"
    propertyGetExpression.Member.Name.Dump();

    // "s" is replaced by a field access on a compiler-generated class from the closure
    var fieldOnClosureExpression = propertyGetExpression.Expression as MemberExpression;

    // Find the compiler-generated class
    var closureClassExpression = fieldOnClosureExpression.Expression as ConstantExpression;
    var closureClassInstance = closureClassExpression.Value;

    // Find the field value, in this case it's a reference to the "s" variable
    var closureFieldInfo = fieldOnClosureExpression.Member as FieldInfo;
    var closureFieldValue = closureFieldInfo.GetValue(closureClassInstance);

    closureFieldValue.Dump();

    // We know that the Expression is a property access so we get the PropertyInfo instance
    // And even access the value (yes compiling the expression would have been simpler :D)
    var propertyInfo = propertyGetExpression.Member as PropertyInfo;
    var propertyValue = propertyInfo.GetValue(closureFieldValue, null);
    propertyValue.Dump();
}

void Main()
{
    string s = "Hello world";
    Foo(() => s.Length);
}

Answer 2

Well, this is similar in tone to Hangy's solution, but is I think rather comfortable to use and does not require much magic:

public static Binding CreateTextBinding<T>(this T source, Expression<Func<T,object>> access)
{
    var mex = access.Body as MemberExpression;
    string name = mex.Member.Name;
    return new Binding("Text", source, name);
}

This is basically an extension method that can be called on any object acting as source. It gives you back a Binding for a Text property which you can add to any Bindings collection.

Answer 3

Something like the following should work:

void BindToText<T>(Control control, Expression<Func<T>> property)
{
    var mex = property.Body as MemberExpression;
    string name = mex.Member.Name;

    var fooMember = mex.Expression as MemberExpression;
    var fooConstant = fooMember.Expression as ConstantExpression;
    var foo = fooConstant.Value;

    control.DataBindings.Add("Text", foo, name);
}

Let me know if that doesn't work for you.

Answer 4

Don't. Just modify the method to take another parameter, as described in #3444294. For your example, it may be something like this:

void BindToText<T>(Control control, T dataSource, Expression<Func<T>> property)
{
    var mex = property.Body as MemberExpression;
    string name = mex.Member.Name;

    control.DataBindings.Add("Text", dataSource, name);
}

and would be called like

BindToText(titleTextBox, foo, ()=>foo.Title );

Still nice, but easy to understand. There's no magic happening. ;)