Question

I am trying to create a method that can accept multiple property names and a string value that will be use to research in entity possessing those properties

I want to a generic method to replace this one:

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

So I tried this:

    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);
    }

I do something wrong when I call Expression.Call Is it possible to build an expression like this or i must use concatenate expression?

Answer 1

我不确定你想要对每个参数做什么。假设所有字段都是string，并且您希望针对每个字段调用StartsWith，这将有效：

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);
}

Build an Expression Tree with multiple parameters

1 个答案: