Question

我已经搜索了SO的低点，为我的问题寻找解决方案。

我在

等简单表达方面找到了几个答案

var exp1 Expression<Func<T, bool>> x => x.Name == "MyName"

但是当表达式如下时我遇到了麻烦：

var exp1 Expression<Func<T, bool>> x => x.Category.Name == "Coupe"

对于简单的，我能够将任何表达式从一种类型（T）转换为另一种类型（TT），我还需要在其他情况下进行，更复杂......

任何可以提供帮助的人？这是我到目前为止所得到的：

private class CustomVisitor<T> : ExpressionVisitor
{
private readonly ParameterExpression mParameter;

public CustomVisitor(ParameterExpression parameter)
{
    mParameter = parameter;
}

//this method replaces original parameter with given in constructor
protected override Expression VisitParameter(ParameterExpression node)
{
    return mParameter;
}
private int counter = 0;

/// <summary>
/// Visits the children of the <see cref="T:System.Linq.Expressions.MemberExpression" />.
/// </summary>
/// <param name="node">The expression to visit.</param>
/// <returns>
/// The modified expression, if it or any subexpression was modified; otherwise, returns the original expression.
/// </returns>
/// <exception cref="System.NotImplementedException"></exception>
protected override Expression VisitMember(MemberExpression node)
{
    counter++;
    System.Diagnostics.Debug.WriteLine("{0} - {1}", node.ToString(), counter);
    try
    {
        //only properties are allowed if you use fields then you need to extend
        // this method to handle them
        if (node.Member.MemberType != System.Reflection.MemberTypes.Property)
            throw new NotImplementedException();

        //name of a member referenced in original expression in your 
        //sample Id in mine Prop
        var memberName = node.Member.Name;
        //find property on type T (=PersonData) by name
        var otherMember = typeof(T).GetProperty(memberName);
        //visit left side of this expression p.Id this would be p
        var inner = Visit(node.Expression);

        return Expression.Property(inner, otherMember);
    }
    catch (Exception ex)
    {
        return null;
    }
}
}

效用方法：

public static Expression<Func<TDestin, T>> ConvertTypesInExpression<TSource, TDestin, T>(Expression<Func<TSource, T>> source)
{
    var param = Expression.Parameter(typeof(TDestin));

    var body = new CustomVisitor<TDestin>(param).Visit(source.Body);

    Expression<Func<TDestin, T>> lambda = Expression.Lambda<Func<TDestin, T>>(body, param);

    return lambda;
}

它的使用方式如下：

var changedFilter = ConvertTypesInExpression<ClientNotificationRuleDto, ClientNotificationRule, bool>(filterExpression);

因此，如果有人可以帮助一些想法或指针，那就太棒了！

Answer 1

分析此测试：

class Replaced
{
    public Inner Inner { get; set; }
}

class Inner
{
    public string Name { get; set; }
}

class Replacing
{
    public Inner Inner { get; set; }
}

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        var parameter = Expression.Parameter(typeof(Replacing));
        var visitor = new CustomVisitor(parameter);
        Expression<Func<Replaced, bool>> expression = x => x.Inner.Name == "ss";
        var resultExpression = (Expression<Func<Replacing, bool>>)visitor.Visit(expression);

        var function = resultExpression.Compile();
        var result = function(new Replacing
         {
             Inner = new Inner
             {
                 Name = "ss"
             }
         });

        Assert.IsTrue(result);
    }
}

internal class CustomVisitor : ExpressionVisitor
{
    private readonly ParameterExpression mParameter;

    private int counter = 0;

    public CustomVisitor(ParameterExpression parameter)
    {
        mParameter = parameter;
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
       return Expression.Lambda(
          Visit(node.Body), 
          node.Parameters.Select(x => (ParameterExpression)Visit(x)).ToArray());
//or simpler but less generic        
//return Expression.Lambda(Visit(node.Body), mParameter);
    }

    //this method will be called twice first for Name and then for Inner
    protected override Expression VisitMember(MemberExpression node)
    {
        counter++;
        System.Diagnostics.Debug.WriteLine("{0} - {1}", node.ToString(), counter);

        if (node.Member.MemberType != System.Reflection.MemberTypes.Property)
            throw new NotImplementedException();

        var memberName = node.Member.Name;
        var inner = Visit(node.Expression);
        var otherMember = inner.Type.GetProperty(memberName);
        return Expression.Property(inner, otherMember);
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return mParameter;
    }
}

请注意，访问成员会被调用两次，并且必须对两个调用做出相应的反应。您还需要覆盖lambda创建，因为它将在参数替换中失败。

PS：永远不要捕获基类Exception，这只是一种不好的做法，而异常时的panic返回null是错误的。

Answer 2

在@Rafal的宝贵帮助和this的见解下，我设法为我的需求找到了解决方案

public static class EXpressionTreeTools
{
    #region ConvertTypesInExpression

    /// <summary>
    /// Converts the types in the expression.
    /// </summary>
    /// <typeparam name="TSource">The source type (the "replacee").</typeparam>
    /// <typeparam name="TDestin">The destiny type (the replacer).</typeparam>
    /// <typeparam name="T">The type of the result fo the expression evaluation</typeparam>
    /// <param name="source">The source expression.</param>
    /// <returns></returns>
    public static Expression<Func<TDestin, T>> ConvertTypesInExpression<TSource, TDestin, T>(Expression<Func<TSource, T>> source)
    {
        var parameter = Expression.Parameter(typeof(TDestin));
        var visitor = new CustomVisitor(parameter);
        //Expression<Func<TSource, bool>> expression = x => x.Inner.Name == "ss";
        Expression<Func<TDestin, T>> resultExpression = (Expression<Func<TDestin, T>>)visitor.Visit(source);

        return resultExpression;
    }

    #endregion

    #region CustomVisitor

    /// <summary>
    /// A custom "visitor" class to traverse expression trees
    /// </summary>
    /// <typeparam name="T"></typeparam>
    internal class CustomVisitor : ExpressionVisitor
    {
        private readonly ParameterExpression mParameter;

        public CustomVisitor(ParameterExpression parameter)
        {
            mParameter = parameter;
        }

        protected override Expression VisitLambda<T>(Expression<T> node)
        {
            return Expression.Lambda(
               Visit(node.Body),
               node.Parameters.Select(x => (ParameterExpression)Visit(x)).ToArray());
            //or simpler but less generic        
            //return Expression.Lambda(Visit(node.Body), mParameter);
        }

        //this method will be called twice first for Name and then for Inner
        protected override Expression VisitMember(MemberExpression node)
        {
            if (node.Member.MemberType != System.Reflection.MemberTypes.Property)
            //throw new NotImplementedException();
            {
                Expression exp = this.Visit(node.Expression);

                if (exp == null || exp is ConstantExpression) // null=static member
                {
                    object @object = exp == null ? null : ((ConstantExpression)exp).Value;
                    object value = null; Type type = null;
                    if (node.Member is FieldInfo)
                    {
                        FieldInfo fi = (FieldInfo)node.Member;
                        value = fi.GetValue(@object);
                        type = fi.FieldType;
                    }
                    else if (node.Member is PropertyInfo)
                    {
                        PropertyInfo pi = (PropertyInfo)node.Member;
                        if (pi.GetIndexParameters().Length != 0)
                            throw new ArgumentException("cannot eliminate closure references to indexed properties");
                        value = pi.GetValue(@object, null);
                        type = pi.PropertyType;
                    }
                    return Expression.Constant(value, type);
                }
                else // otherwise just pass it through
                {
                    return Expression.MakeMemberAccess(exp, node.Member);
                }
            }
            var memberName = node.Member.Name;
            var inner = Visit(node.Expression);
            var otherMember = inner.Type.GetProperty(memberName);
            return Expression.Property(inner, otherMember);
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return mParameter;
        }
    }

    #endregion
}

转换表达式树类型

2 个答案: