用链式属性构造表达式树

本文关键字：表达式 属性 | 更新日期: 2023-09-27 18:16:04

我有一个方法，它接受Expression<Func<T, string>>(例如x => x.Name)和一个术语，并返回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GenerateForMember<X>(x => x.Y.Z, "foobar")。它给出了异常

'Z'不是'UserQuery+X'的成员

我如何更新我的方法与连锁属性工作?

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);
}

用链式属性构造表达式树

您正在剖析原始表达式，然后再重新构造它。这是不必要的。您可以直接使用expression.Body来创建方法调用。像这样，它应该可以与任何lambda表达式一起工作。

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);

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]);
}
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