c# - Expression Tree Copy or Convert

Question

How to convert a ExpressionTree of form

Expression<Func<POCO1, bool>> exp = p => p.Age > 50;

to

Expression<Func<POCO2, bool>> exp2 = p => p.Age > 50;

where POCO1 and POCO2 are C# objects and both have Int32 Age property

Answer 1

well, you can make custom expression visitor that will replace parameter references and patch member access expressions

class Converter<TTo>
{
    class ConversionVisitor : ExpressionVisitor
    {
        private readonly ParameterExpression newParameter;
        private readonly ParameterExpression oldParameter;

        public ConversionVisitor(ParameterExpression newParameter, ParameterExpression oldParameter)
        {
            this.newParameter = newParameter;
            this.oldParameter = oldParameter;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return newParameter; // replace all old param references with new ones
        }

        protected override Expression VisitMember(MemberExpression node)
        {
            if (node.Expression != oldParameter) // if instance is not old parameter - do nothing
                return base.VisitMember(node);

            var newObj = Visit(node.Expression);
            var newMember = newParameter.Type.GetMember(node.Member.Name).First();
            return Expression.MakeMemberAccess(newObj, newMember);
        }
    }

    public static Expression<Func<TTo, TR>> Convert<TFrom, TR>(
        Expression<Func<TFrom, TR>> e
        )
    {
        var oldParameter = e.Parameters[0];
        var newParameter = Expression.Parameter(typeof(TTo), oldParameter.Name);
        var converter = new ConversionVisitor(newParameter, oldParameter);
        var newBody = converter.Visit(e.Body);
        return Expression.Lambda<Func<TTo, TR>>(newBody, newParameter);
    }
}

class A
{
    public int Value { get; set; }
}

class B
{
    public int Value { get; set; }
}

Expression<Func<A, int>> f = x => x.Value;
var f2 = Converter<B>.Convert(f);