Convert Expression<Func<T, TProperty>> to Expression<Func<object, object>> and vice versa

Question

Is there a way to convert the property selector Expression<Func<T, TProperty>> to Expression<Func<object, object>> and vice versa? I already know how to convert to Expression<Func<T, object>> using...

Expression<Func<T, TProperty>> oldExp;
Expression.Lambda<Func<T, object>>(Expression.Convert(oldExp.Body, typeof(object)), oldExp.Parameters);

... but I need to effectively cast both, the argument and the result of the function, not just e.g replace them with a ExpressionVisitor because they need to be casted back later.

Answer 1

You were correct that you need to use an ExpressionVisitor and ExpressionConvert.

Here are the two methods that you asked for (as well as some support methods):

public Expression<Func<object, object>> ConvertToObject<TParm, TReturn>(Expression<Func<TParm, TReturn>> input)
{
    var parm = Expression.Parameter(typeof(object));
    var castParm = Expression.Convert(parm, typeof(TParm));
    var body = ReplaceExpression(input.Body, input.Parameters[0], castParm);
    body = Expression.Convert(body, typeof(object));
    return Expression.Lambda<Func<object, object>>(body, parm);
}

public Expression<Func<TParm, TReturn>> ConvertBack<TParm, TReturn>(Expression<Func<object, object>> input)
{
    var parm = Expression.Parameter(typeof(TParm));
    var castParm = Expression.Convert(parm, typeof(object));
    var body = ReplaceExpression(input.Body, input.Parameters[0], castParm);
    body = Expression.Convert(body, typeof(TReturn));
    return Expression.Lambda<Func<TParm, TReturn>>(body, parm);
}

Expression ReplaceExpression(Expression body, Expression source, Expression dest)
{
    var replacer = new ExpressionReplacer(source, dest);
    return replacer.Visit(body);
}

public class ExpressionReplacer : ExpressionVisitor
{
    Expression _source;
    Expression _dest;

    public ExpressionReplacer(Expression source, Expression dest)
    {
        _source = source;
        _dest = dest;
    }

    public override Expression Visit(Expression node)
    {
        if (node == _source)
            return _dest;

        return base.Visit(node);
    }
}

Sample usage:

Expression<Func<Customer, string>> expression = c => c.Name;
var convertedExpression = ConvertToObject<Customer, string>(expression);
var backExpression = ConvertBack<Customer, string>(convertedExpression);

Of course we can replace the original two methods with one method with more type parameters:

public Expression<Func<TTargetParm, TTargetReturn>> ConvertGeneric<TParm, TReturn, TTargetParm, TTargetReturn>(Expression<Func<TParm, TReturn>> input)
{
    var parm = Expression.Parameter(typeof(TTargetParm));
    var castParm = Expression.Convert(parm, typeof(TParm));
    var body = ReplaceExpression(input.Body, input.Parameters[0], castParm);
    body = Expression.Convert(body, typeof(TTargetReturn));
    return Expression.Lambda<Func<TTargetParm, TTargetReturn>>(body, parm);
}

Sample usage:

Expression<Func<Customer, string>> expression = c => c.Name;
var convertedExpression = ConvertGeneric<Customer, string, object, object>(expression);
var backExpression = ConvertGeneric<object, object, Customer, string>(convertedExpression);