Question

Suppose I have:

class Foo {
 public String Bar { get; set; }
}

and I have an Expression<Func<TIn,TOut>> where TOut is constrained to Foo. How can I create an Expression<Func<TIn, String>> for accessing the Bar property?

Edit:

The reason I want this is to write an HtmlHelper for ASP.NET MVC for a complex type – e.g. MyExtension(this HtmlHelper that, Expression<Func<TIn, TOut>> expression) that then calls other HtmlHelper extension methods for properties of the complex type – e.g EditorFor, etc)

Answer 1

You can use Body to access the body of the first expression, then Expression.Property() to access the property and finally Expression.Lambda() to combine it all into a lambda:

Expression<Func<TIn, string>> CreateExpression
    (Expression<Func<TIn, TOut>> expr)
{
    var barProperty = typeof(Foo).GetProperty("Bar");
    return Expression.Lambda<Func<TIn, string>>(
        Expression.Property(expr.Body, barProperty), expr.Parameters);
}

If you can use LINQKit this is somewhat simpler (don’ forget using LinqKit;):

public Expression<Func<TIn, string>> CreateExpression
    (Expression<Func<TIn, TOut>> expr)
{
    Expression<Func<TIn, string>> tmp = x => expr.Invoke(x).Bar;
    return tmp.Expand();
}