构造具有链式属性的表达式树？

Question

我有一个接受 Expression<Func<T, string>> 的方法，例如 x => x.Name 和一个术语，以及 returns x => x.Name.Contains(term):

给定模型；

class X
{
    public Y Y {get; set;}
}

class Y
{
    public string Z {get; set;}
}

它适用于 GenerateForMember<Y>(y => y.Z, "foobar")，但目前不适用于 GenerateForMember<X>(x => x.Y.Z, "foobar")。它给出了例外

'Z' is not a member of 'UserQuery+X'

如何更新我的方法以使用链式属性？

方法如下：

protected Expression<Func<T, bool>> GenerateForMember<T>(Expression<Func<T,string>> expression, string term)
{
    var type = typeof(T);

    var memberExpression = ((expression.Body.NodeType == ExpressionType.Convert) 
        ? ((UnaryExpression)expression.Body).Operand 
        : expression.Body) as MemberExpression;

    ParameterExpression parameter = Expression.Parameter(type, type.Name.ToLower());
    MemberExpression member = Expression.PropertyOrField(parameter, memberExpression.Member.Name);

    var propertyInfo = memberExpression.Member as PropertyInfo;

    var memberType = propertyInfo == null
        ? ((FieldInfo) memberExpression.Member).FieldType
        : propertyInfo.PropertyType;


    ConstantExpression constant = Expression.Constant(term, typeof(string));

    MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    var containsMethodExp = Expression.Call(member, method, constant);

    return Expression.Lambda<Func<T, bool>>(containsMethodExp, parameter);
}

Answer 1

您正在剖析原始表达式，然后重新构造它。这是没有必要的。您可以直接使用 expression.Body 来创建方法调用。像这样，它应该适用于任何 lambda 表达式。

var type = typeof(T);

ParameterExpression parameter = Expression.Parameter(type, type.Name.ToLower());

ConstantExpression constant = Expression.Constant(term, typeof(string));

MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var containsMethodExp = Expression.Call(expression.Body, method, constant);

return Expression.Lambda<Func<T, bool>>(containsMethodExp, parameter);

Answer 2

试试这个：

protected Expression<Func<T, bool>> GenerateForMember<T>(Expression<Func<T, string>> expression, string term)
{
    ConstantExpression constant = Expression.Constant(term, typeof(string));

    MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });

    var containsMethodExp = Expression.Call(expression.Body, method, constant);

    return Expression.Lambda<Func<T, bool>>(containsMethodExp, expression.Parameters[0]);
}

构造具有链式属性的表达式树？

Construct expression tree with chained properties?

c#

expression-trees