表达式树复制或转换

Question

如何转换表单的ExpressionTree

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

至

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

其中POCO1和POCO2是C#对象,并且都具有Int32 Age属性

Answer 1

好吧,您可以创建将替换参数引用和修补程序成员访问表达式的自定义表达式访问者

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);