构建具有多个参数的表达式树

Question

我正在尝试创建一个可以接受多个属性名称和一个字符串值的方法，该值将用于研究拥有这些属性的实体

我想要一个通用方法来替换这个：

    public static Client[] GetClientByInfo(string info)
    {
        return Context.Db.Clients.Where(c =>
            c.LastName.Contains(info) ||
            c.FirstName.Contains(info)).ToArray();
    }

所以我尝试了这个：

    public static T[] FindByText<T>(string text, string[] properties)
        where T: class
    {
        return Context.Db.Set<T>().AsNoTracking().Where(PropertyEquals<T, string>(properties, text)).ToArray();
    }

    public static Expression<Func<TItem, bool>> PropertyEquals<TItem, TValue>(string[] properties, TValue value)
    {
        MethodInfo startWithMethod = typeof(string).GetMethod("StartsWith", BindingFlags.Public | BindingFlags.Instance, null, new Type[] { typeof(string) }, null);
        ParameterExpression parameter = Expression.Parameter(typeof(TItem));
        ConstantExpression constant = Expression.Constant(value, typeof(TValue));
        MemberExpression[] members = new MemberExpression[properties.Length];
        for (int i = 0; i < properties.Length; i++)
            members[i] = Expression.Property(parameter, properties[i]);

        MethodCallExpression callExp = Expression.Call(parameter, startWithMethod, members);

        return Expression.Lambda<Func<TItem, bool>>(callExp, parameter);
    }

我调用的时候出错了Expression.Call 是否可以构建这样的表达式，或者我必须使用连接表达式？

Answer 1

我不确定你想对每个参数做什么。假设所有字段都是 string，并且您想针对每个字段调用 StartsWith，这将起作用：

public static Expression<Func<TItem, bool>> PropertyEquals<TItem>(string[] properties, string value)
{
    MethodInfo startWithMethod = typeof(string).GetMethod("StartsWith", BindingFlags.Public | BindingFlags.Instance, null, new Type[] { typeof(string) }, null);
    ParameterExpression parameter = Expression.Parameter(typeof(TItem));
    ConstantExpression constant = Expression.Constant(value);
    MemberExpression[] members = new MemberExpression[properties.Length];
    for (int i = 0; i < properties.Length; i++)
        members[i] = Expression.Property(parameter, properties[i]);

    Expression predicate = null;
    foreach (var item in members)
    {
        MethodCallExpression callExp = Expression.Call(item, startWithMethod, constant);
        predicate = predicate == null 
            ? (Expression)callExp
            : Expression.OrElse(predicate, callExp);
    }

    return Expression.Lambda<Func<TItem, bool>>(predicate, parameter);
}

构建具有多个参数的表达式树

Build an Expression Tree with multiple parameters

c#

lambda

expression-trees