Question

我有这样的方法：

private bool Method_1(Expression<Func<IPerson, bool>> expression)
{
    /* Some code that will call Method_2 */
}

在这种方法中，我想将IPerson类型更改为其他类型。我想调用另一个看起来像这样的方法：

private bool Method_2(Expression<Func<PersonData, bool>> expression)
{
    /* Some code */
}

因此，在method_1我需要将IPerson更改为PersonData。我怎么能这样做？

编辑：

当我致电：Method_1(p => p.Id == 1)我想'保存'条件（p.Id == 1），但我想在另一种类型上执行此条件，即IPerson。因此，我需要更改表达式或使用IPerson

创建新表达式

编辑II：

对于那些感兴趣的人，这是（现在）我的解决方案：

private class CustomExpressionVisitor<T> : ExpressionVisitor
{
    ParameterExpression _parameter;

    public CustomExpressionVisitor(ParameterExpression parameter)
    {
        _parameter = parameter;
    }

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

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Member.MemberType == System.Reflection.MemberTypes.Property)
        {
            MemberExpression memberExpression = null;
            var memberName = node.Member.Name;
            var otherMember = typeof(T).GetProperty(memberName);
            memberExpression = Expression.Property(Visit(node.Expression), otherMember);
            return memberExpression;
        }
        else 
        {
            return base.VisitMember(node);
        }
    }
}

这就是我使用它的方式：

public virtual bool Exists(Expression<Func<Dto, bool>> expression)
{
    var param = Expression.Parameter(typeof(I));
    var result = new CustomExpressionVisitor<I>(param).Visit(expression.Body);
    Expression<Func<I, bool>> lambda = Expression.Lambda<Func<I, bool>>(result, param);

    bool exists = _repository.Exists(lambda);
    return exists;
}

Answer 1

如果您使用.net 4（更新：如评论ExpressionVisitor中所述，在版本4而不是4.5中添加），这很容易。它需要对旧框架进行一些Google搜索：

有一些假设，但我认为它们对您的DTO和实体场景有效 - 访问的属性必须匹配。

class PersonData
{
    public bool Prop { get; set; }
}

interface IPerson 
{
    bool Prop { get; set; }
}

在.net 4中定义了ExpressionVisitor类，如果您使用较旧的类，则需要编写或查找它的实现，这样会更容易：

class Visitor<T> : ExpressionVisitor
{
    ParameterExpression _parameter;

    //there must be only one instance of parameter expression for each parameter 
    //there is one so one passed here
    public Visitor(ParameterExpression parameter)
    {
        _parameter = parameter;
    }

    //this method replaces original parameter with given in constructor
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return _parameter;
    }

    //this one is required because PersonData does not implement IPerson and it finds
    //property in PersonData with the same name as the one referenced in expression 
    //and declared on IPerson
    protected override Expression VisitMember(MemberExpression node)
    {
        //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);
    }
}

概念证明：

class Program
{
   static void Main()
    {
        //sample expression
        Expression<Func<IPerson, bool>> expression = x => x.Prop;

        //parameter that will be used in generated expression
        var param = Expression.Parameter(typeof(PersonData));
        //visiting body of original expression that gives us body of the new expression
        var body = new Visitor<PersonData>(param).Visit(expression.Body);
        //generating lambda expression form body and parameter 
        //notice that this is what you need to invoke the Method_2
        Expression<Func<PersonData, bool>> lambda = Expression.Lambda<Func<PersonData, bool>>(body, param);
        //compilation and execution of generated method just to prove that it works
        var boolValue = lambda.Compile()(new PersonData());
    }
}

请注意，这适用于简单表达式。如果您需要处理x.Prop.Prop1 < 3，则需要进一步扩展。

如何更改表达式树中的类型？

1 个答案: