How to make Databinding type safe and support refactoring?

Question

When I wish to bind a control to a property of my object, I have to provide the name of the property as a string. This is not very good because:

1. If the property is removed or renamed, then I don’t get a compiler warning.
2. If a rename the property with a refactoring tool, then it is likely the data binding will not be updated.
3. If the type of the property is wrong, e.g. binding an integer to a date chooser, then I don’t get an error until runtime.

Is there a design-pattern that gets round this, but still has the ease of use of data-binding?

(This is a problem in WinForms, ASP.NET, and WPF and possibly other systems.)

I have now found "workarounds for nameof() operator in C#: typesafe databinding" that also has a good starting point for a solution.

If you are willing to use a post processor after compiling your code, then NotifyPropertyWeaver is worth looking at.

---

Does anyone know of a good solution for WPF when the bindings are done in XML rather than C#?

Answer 1

Note this answer uses WinForm and was written before C# had 'NameOf()'

Thanks to Oliver for getting me started I now have a solution that both supports refactoring and is type safe. It also let me implement INotifyPropertyChanged so it copes with properties being renamed.

It’s usage looks like:

checkBoxCanEdit.Bind(c => c.Checked, person, p => p.UserCanEdit);
textBoxName.BindEnabled(person, p => p.UserCanEdit);
checkBoxEmployed.BindEnabled(person, p => p.UserCanEdit);
trackBarAge.BindEnabled(person, p => p.UserCanEdit);

textBoxName.Bind(c => c.Text, person, d => d.Name);
checkBoxEmployed.Bind(c => c.Checked, person, d => d.Employed);
trackBarAge.Bind(c => c.Value, person, d => d.Age);

labelName.BindLabelText(person, p => p.Name);
labelEmployed.BindLabelText(person, p => p.Employed);
labelAge.BindLabelText(person, p => p.Age);

The person class shows how to implemented INotifyPropertyChanged in a type safe way (or see this answer for a other rather nice way of implementing INotifyPropertyChanged, ActiveSharp - Automatic INotifyPropertyChanged also looks good ):

public class Person : INotifyPropertyChanged
{
   private bool _employed;
   public bool Employed
   {
      get { return _employed; }
      set
      {
         _employed = value;
         OnPropertyChanged(() => c.Employed);
      }
   }
    
   // etc
    
   private void OnPropertyChanged(Expression<Func<object>> property)
   {
      if (PropertyChanged != null)
      {
         PropertyChanged(this, 
             new PropertyChangedEventArgs(BindingHelper.Name(property)));
      }
   }
    
   public event PropertyChangedEventHandler PropertyChanged;
}

The WinForms binding helper class has the meat in it that makes it all work:

namespace TypeSafeBinding
{
    public static class BindingHelper
    {
        private static string GetMemberName(Expression expression)
        {
            // The nameof operator was implemented in C# 6.0 with .NET 4.6
            // and VS2015 in July 2015. 
            // The following is still valid for C# < 6.0

            switch (expression.NodeType)
            {
                case ExpressionType.MemberAccess:
                    var memberExpression = (MemberExpression) expression;
                    var supername = GetMemberName(memberExpression.Expression);
                    if (String.IsNullOrEmpty(supername)) return memberExpression.Member.Name;
                    return String.Concat(supername, '.', memberExpression.Member.Name);
                case ExpressionType.Call:
                    var callExpression = (MethodCallExpression) expression;
                    return callExpression.Method.Name;
                case ExpressionType.Convert:
                    var unaryExpression = (UnaryExpression) expression;
                    return GetMemberName(unaryExpression.Operand);
                case ExpressionType.Parameter:
                case ExpressionType.Constant: //Change
                    return String.Empty;
                default:
                    throw new ArgumentException("The expression is not a member access or method call expression");
            }
        }

        public static string Name<T, T2>(Expression<Func<T, T2>> expression)
        {
            return GetMemberName(expression.Body);
        }

        //NEW
        public static string Name<T>(Expression<Func<T>> expression)
        {
           return GetMemberName(expression.Body);
        }

        public static void Bind<TC, TD, TP>(this TC control, Expression<Func<TC, TP>> controlProperty, TD dataSource, Expression<Func<TD, TP>> dataMember) where TC : Control
        {
            control.DataBindings.Add(Name(controlProperty), dataSource, Name(dataMember));
        }

        public static void BindLabelText<T>(this Label control, T dataObject, Expression<Func<T, object>> dataMember)
        {
            // as this is way one any type of property is ok
            control.DataBindings.Add("Text", dataObject, Name(dataMember));
        }

        public static void BindEnabled<T>(this Control control, T dataObject, Expression<Func<T, bool>> dataMember)
        {       
           control.Bind(c => c.Enabled, dataObject, dataMember);
        }
    }
}

This makes use of a lot of the new stuff in C# 3.5 and shows just what is possible. Now if only we had hygienic macros lisp programmer may stop calling us second class citizens)